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952cc209 | 1 | **************************** |

2 | * * | |

3 | * PYTHIA version 6.1 * | |

4 | * * | |

5 | **************************** | |

6 | ||

7 | (Last updated 17 August 2000) | |

8 | ||

9 | The new PYTHIA version is a logical continuation of previous versions. | |

10 | Therefore a user should not feel lost. However, many details have been | |

11 | changed. The major changes (so far) are: | |

12 | - The supersymmetric process machinery of SPYTHIA has been included. | |

13 | - PYTHIA and JETSET have been merged. | |

14 | - All real variables are declared in double precision. | |

15 | - The internal mapping of particle codes has changed. | |

16 | - Extended capabilities to handle reactions of virtual photons. | |

17 | - Baryon production according to advanced popcorn scheme (new option as | |

18 | of version 6.110, with some consequences also for default behaviour). | |

19 | ||

20 | Below follows a more extensive list of main changes, performed to move | |

21 | from Pythia 5.7 and Jetset 7.4 to Pythia 6.1. Eventually this file | |

22 | will be complemented by a completely updated manual. However, based | |

23 | on the information here and some common sense it should be possible | |

24 | to use the program already now, if you are familiar with previous | |

25 | versions. | |

26 | ||

27 | ----------------------------------------------------------------------- | |

28 | ||

29 | PYTHIA/JETSET CODE MERGING | |

30 | ||

31 | * The PYTHIA and JETSET routines have been joined into one single file. | |

32 | - LUDATA and PYDATA have been joined to a single BLOCK DATA. | |

33 | - LUTEST and PYTEST have been joined to a single test program. | |

34 | - SAVE statements are common for former JETSET and PYTHIA routines. | |

35 | - version information (for the title page) is based on MSTP(181-185) | |

36 | while MSTP(186) and MSTU(181-186) are not used any longer. | |

37 | ||

38 | * All JETSET routines and commonblocks have been renamed to begin | |

39 | with PY. | |

40 | - In most cases just by letting LU -> PY or UL -> PY. | |

41 | - Special cases: RLU -> PYR (also PYRGET, PYRSET), KLU -> PYK, | |

42 | PLU -> PYP. Also commonblock and internal variables of the form | |

43 | *RLU* are replaced by *RPY* (where * represents "wildcard" | |

44 | characters). | |

45 | - To declare integer functions, a line INTEGER PYK,PYCHGE,PYCOMP | |

46 | has been added at the beginning of all routines. | |

47 | - To avoid a name clash, LUXTOT becomes PYXTEE. | |

48 | - Some comment lines in code have been modified; also the name | |

49 | JETSET becomes PYTHIA. | |

50 | ||

51 | * Persons who have code that relies on the /LUJETS/ single precision | |

52 | commonblock could easily write a translation routine to copy the | |

53 | /PYJETS/ double precision information to /LUJETS/. In fact, only | |

54 | the LUGIVE and LULOGO routines of JETSET have access to some PYTHIA | |

55 | commonblocks, and therefore these are the only ones that need to | |

56 | be modified if one, for some reason, would like to combine the | |

57 | new PYTHIA with the old JETSET routines. Similarly, it would only | |

58 | require minor changes in the PYHEPC routine code to allow the | |

59 | /HEPEVT/ commonblock to be in single precision, as before. | |

60 | ||

61 | ----------------------------------------------------------------------- | |

62 | ||

63 | DOUBLE PRECISION | |

64 | ||

65 | * Conversion from single to double precision. | |

66 | - All real constants by explicitly exchanging E for D where present | |

67 | and else adding D0. | |

68 | - All real variables with an IMPLICIT DOUBLE PRECISION(A-H, O-Z) | |

69 | at beginning of all routines. | |

70 | - Commonblocks with an odd number of integers before real variables | |

71 | have been padded with a dummy integer variable or reordered: | |

72 | COMMON/PYJETS/N,NPAD,K(4000,5),P(4000,5),V(4000,5) | |

73 | COMMON/PYSUBS/MSEL,MSELPD,MSUB(200),KFIN(2,-40:40),CKIN(200) | |

74 | COMMON/PYUPPR/NUP,KUP(20,7),NFUP,IFUP(10,2),PUP(20,5),Q2UP(0:10) | |

75 | COMMON/PYINT5/NGENPD,NGEN(0:200,3),XSEC(0:200,3) | |

76 | (In the HARD PROCESSES section below is described further changes; | |

77 | specifically MSUB, NGEN and XSEC are expanded to 500 processes.) | |

78 | - Obsolete conversions with DBLE(), SNGL() etc. have been removed. | |

79 | - pi, 2pi and GeV <-> fm conversion given with more decimals. | |

80 | - Some blanks have been removed and lines reformatted or split when | |

81 | lines have become too long. | |

82 | - PYUPDA(3,..) writes values with D0 added. | |

83 | - Commonblock /PYINT9/ with differential cross section sum in double | |

84 | precision is superfluous and has been removed. | |

85 | - Note that in the Fortran 77 standard COMPLEX cannot be defined | |

86 | as double precision. COMPLEX is used only very sparingly, and only | |

87 | in the PYRESD, PYRAND and PYSIGH routines. Some small pieces of code | |

88 | therefore still use single precision. If you have a compiler | |

89 | option for automatic promotion of single to double you are | |

90 | welcome to use it to handle also these parts, but otherwise | |

91 | they should not be harmful. | |

92 | ||

93 | ----------------------------------------------------------------------- | |

94 | ||

95 | PARTICLE CODES AND DATA | |

96 | ||

97 | * Some particles have been renamed: | |

98 | 7 from l to b' | |

99 | 8 from h to t' | |

100 | 17 from chi to tau' | |

101 | 18 from nu_chi to nu'_tau | |

102 | 25 from H to h | |

103 | 35 from H' to H | |

104 | Furthermore, in all names where a tilde was previously used to | |

105 | indicate an antiparticle, the previous alternative 'bar' is now | |

106 | used throughout, both in particle names and process titles | |

107 | (to avoid confusion with supersymmetry, see below). | |

108 | ||

109 | * Some particle codes have been changed according to | |

110 | the "LEP 2 standard" (see LEP 2 workshop proceedings): | |

111 | psi' from 30443 to 100443 | |

112 | Upsilon' from 30553 to 100553 | |

113 | d* from 7 to 4000001 | |

114 | u* from 8 to 4000002 | |

115 | e*- from 17 to 4000011 | |

116 | nu*_e from 18 to 4000012 | |

117 | The codes 7, 8, 17 and 18 are now exclusively used for fourth | |

118 | generation fermions. The switch MSTP(6) is then superfluous. | |

119 | PYRESD and other code pieces have been rewritten to take into | |

120 | account the change. | |

121 | ||

122 | * Supersymmetric particle codes have been introduced according to | |

123 | the "LEP 2 standard" (see LEP 2 workshop proceedings): | |

124 | 1000001 ~d_L 2000001 ~d_R 1000021 ~g | |

125 | 1000002 ~u_L 2000002 ~u_R 1000022 ~chi_10 | |

126 | 1000003 ~s_L 2000003 ~s_R 1000023 ~chi_20 | |

127 | 1000004 ~c_L 2000004 ~c_R 1000024 ~chi_1+ | |

128 | 1000005 ~b_1 2000005 ~b_2 1000025 ~chi_30 | |

129 | 1000006 ~t_1 2000006 ~t_2 1000035 ~chi_40 | |

130 | 1000011 ~e_L- 2000011 ~e_R- 1000037 ~chi_2+ | |

131 | 1000012 ~nu_eL 2000012 ~nu_eR 1000039 ~G | |

132 | 1000013 ~mu_L- 2000013 ~mu_R- | |

133 | 1000014 ~nu_muL 2000014 ~nu_muR | |

134 | 1000015 ~tau_1- 2000015 ~tau_2- | |

135 | 1000016 ~nu_tauL 2000016 ~nu_tauR | |

136 | In the third generation the left and right states are assumed | |

137 | to mix to nontrivial mass eigenstates, while mixing is not included | |

138 | in the first two. Note that all sparticle names begin with a tilde. | |

139 | Default masses are arbitrary and branching ratios not set at all. | |

140 | This is taken care of at initialization if IMSS(1) is positive | |

141 | (see below). | |

142 | ||

143 | * A hint on large particle numbers: if you want to avoid mistyping | |

144 | the number of zeros, it may pay off to define a line like | |

145 | PARAMETER (KSUSY1=1000000,KSUSY2=2000000,KEXCIT=4000000) | |

146 | at the beginning of your program and then refer to particles as | |

147 | KSUSY1+1 = ~d_L and so on. This then also agrees with the internal | |

148 | notation. | |

149 | ||

150 | * A number of technicolour particle codes have been added: | |

151 | 51 pi_tech0 54 rho_tech0 | |

152 | 52 pi_tech+ 55 rho_tech+ | |

153 | 53 pi'_tech0 56 omega_tech0 | |

154 | ||

155 | * Some new particle codes for doubly charged Higgs production in | |

156 | left-right-symmetric scenarios. | |

157 | 61 H_L++ 64 nu_Re | |

158 | 62 H_R++ 65 nu_Rmu | |

159 | 63 W_R+ 66 nu_Rtau | |

160 | The indices _L and _R indicate belonging to left or right SU(2) | |

161 | gauge group. | |

162 | ||

163 | * Top and fourth generation hadrons are gone. Henceforth the t, b' and | |

164 | t' quarks are always assumed to decay before they would have time to | |

165 | hadronize. | |

166 | - All t, b' and t' hadron codes are unknown to the program. | |

167 | (In a pinch, such a hadron could be represented e.g. by a string | |

168 | with a top quark and an antiquark or diquark, with string mass | |

169 | equated to the expected hadron mass.) | |

170 | - The MSTP(48) and MSTP(49) switches are removed; decay treatment is | |

171 | as the old default option 2. | |

172 | - Extra code has been inserted in PYEVNT and PYEXEC to decay any | |

173 | leftover resonances (including these quarks) before fragmentation | |

174 | routines are called. | |

175 | - The particles codes 86 - 89, previously used for generic t, b' and | |

176 | t' hadron decays are gone. | |

177 | - The decay channel of particle 17 to 89 is removed, and PYRESD | |

178 | changed accordingly. | |

179 | - The PYLIST(11) listing is changed. | |

180 | - Also the PYLIST(12) listing is changed. The MSTU(14) switch is gone; | |

181 | restrictions on which codes are listed can only be applied with | |

182 | MSTU(1) and MSTU(2). | |

183 | - In the decay description, matrix element codes 45 and 46 are now | |

184 | superfluous. MSTJ(25) and MSTJ(27) are no longer used, and | |

185 | MSTJ(23) is less used. | |

186 | - PYTEST is reduced so it does not involve these hadrons. | |

187 | ||

188 | * There is a new scheme to relate the standard KF codes with the | |

189 | compressed KC codes. | |

190 | - We remind that KF codes essentially follow the PDG standard for | |

191 | particle numbering, and with the introduction of SUSY now range up | |

192 | to seven-digit codes (plus a sign). They therefore cannot be used to | |

193 | directly access information in particle data tables. The compressed | |

194 | KC codes range between 1 and 500, and give the index to the KCHG, | |

195 | PMAS, MDCY, CHAF and MWID arrays. | |

196 | - Each KF code known to the program is now one-to-one associated | |

197 | with a KC code; the only doublevaluedness left is that both | |

198 | the particle KF and its antiparticle -KF (if existing) is mapped | |

199 | to the same KC. This specifically means that all charm and bottom | |

200 | hadrons and all diquarks now are separately defined. | |

201 | - Whereas KF codes below 100 still obey KC=KF, the mapping of codes | |

202 | above 100 is completely changed. The mapping is no longer hardcoded | |

203 | in PYCOMP, but defined by the fourth component of the KCHG array | |

204 | (see below). Therefore it can be changed or expanded during the | |

205 | course of a run, either by PYUPDA calls or by direct user | |

206 | intervention. | |

207 | ||

208 | * The KCHG array in /PYDAT2/ has been expanded with a fourth component, | |

209 | where KCHG(KC,4)=KF. It thus gives the inverse mapping from KC codes | |

210 | to KF ones (see above). | |

211 | ||

212 | * The PYCOMP code has been completely rewritten. It gives the direct | |

213 | mapping from the KF codes to the KC ones (see above). | |

214 | - Internally the PYCOMP uses a binary search in a table, with KF codes | |

215 | arranged in increasing order, based on the KCHG(KC,4) array. This | |

216 | table is constructed the first time PYCOMP is called, at which time | |

217 | MSTU(20) is set to 1. In case of a user change of the KCHG(KC,4) | |

218 | array one should reset MSTU(20)=0 to force a reinitialization at the | |

219 | next PYCOMP call (this is automatically done in PYUPDA calls). | |

220 | To speed up execution, the latest (KF,KC) pair is kept in memory | |

221 | and checked before the standard binary search. | |

222 | - Code has been changed thoughout the program to be compatible with | |

223 | this new mode of PYCOMP operation. | |

224 | ||

225 | * Particle data is now stored and read out for each particle separately. | |

226 | - PYMASS uses tables of charm and bottom hadron masses rather than | |

227 | mass formulae. | |

228 | - The array CHAF(500) has been expanded to CHAF(500,2), where the | |

229 | first component gives the particle name and the second the | |

230 | antiparticle one (where existing). | |

231 | - PYNAME accesses ready-constructed names rather than constructs | |

232 | the names from scratch. | |

233 | - PYCHGE accesses charges directly from the KCHG(KC,1) array. | |

234 | ||

235 | * PYUPDA has been changed. | |

236 | - Option 1 writes out a table of all particle codes defined. | |

237 | For each particle is given its KF code, particle and antiparticle | |

238 | names in CHAF , the three first KCHG components, the PMAS components, | |

239 | MWID (see below) and the first MDCY component. The information on | |

240 | decay channels is unchanged, but the format expanded. | |

241 | - Option 2 reads in the table written in the,form described above, | |

242 | and replaces all existing data, including the KF<->KC mapping, | |

243 | with the new ones. | |

244 | - Option 3 reads in a table, like option 2, but uses it as a | |

245 | complement to rather than a replacement of existing data. | |

246 | The input file should therefore only contain new particles and | |

247 | particles with changed data. New particles are added to the | |

248 | bottom of the KC and decay channel tables. Changed particles | |

249 | retain their KC codes and hence the position of particle data, but | |

250 | their old decay channels are removed, this space is recuperated, | |

251 | and new decay channels are added at the end. | |

252 | - Option 4 corresponds to the old option 3, i.e. writes existing | |

253 | data to DATA statements for inclusion in the default program | |

254 | code. | |

255 | ||

256 | * The maximum number of decay channels has been expanded from 2000 | |

257 | to 4000; this affects the arrays MDME, BRAT and KFDP in PYDAT3, | |

258 | and MSTU(7). | |

259 | ||

260 | * PYLIST, PYSTAT and PYUPDA are changed to allow for the larger | |

261 | particle codes that may now appear. | |

262 | ||

263 | ----------------------------------------------------------------------- | |

264 | ||

265 | RESONANCE DECAYS | |

266 | ||

267 | * The dimensions of the WDTP and WDTE return arrays of PYWIDT have | |

268 | been expanded from a maximum of 40 to 200 decay channels. | |

269 | ||

270 | * PYWIDT has been modified so that it returns total and partial widths | |

271 | in units of GeV. Previously most widths were given in dimensionless | |

272 | units, with an extra multiplicative factor added elsewhere, e.g. in | |

273 | PYINRE or PYSIGH. Therefore also these routines are modified. Also | |

274 | VINT(117) is now in dimensions of GeV. | |

275 | ||

276 | * Commonblock PYINT4 is completely reorganized as | |

277 | COMMON/PYINT4/MWID(500),WIDS(500,5) | |

278 | The WIDP and WIDE arrays were essentially only used by PYSTAT(2) | |

279 | and have been eliminated. | |

280 | Where before the resonances could only be found in the range | |

281 | 21:40, in the new description any compressed code KC between | |

282 | 1 and 500 can be used to represent a resonance. | |

283 | ||

284 | MWID(KC) gives the character of particle with compressed code KC, | |

285 | mainly as used in PYWIDT to calculate widths of resonances | |

286 | (not necessarily at the nominal mass). | |

287 | = 0 : an ordinary particle; not to be treated as resonance. | |

288 | = 1 : a resonance for which the partial and total widths | |

289 | (and hence branching ratios) are dynamically calculated | |

290 | in PYWIDT calls; i.e. special code has to exist for each | |

291 | such particle. The effects of allowed/unallowed secondary | |

292 | decays are included, both in the relative composition | |

293 | of decays and in the process cross section. | |

294 | = 2 : The total width is taken to be the one stored in PMAS(KC,2) | |

295 | and the relative branching ratios the ones in BRAT(IDC) for | |

296 | decay channels IDC. There is then no need for any special | |

297 | code in PYWIDT to handle a resonance. During the run, | |

298 | the stored PMAS(KC,2) and BRAT(IDC) values are used to | |

299 | calculate the total and partial widths of the decay channels. | |

300 | Some extra information and assumptions are then used. | |

301 | Firstly, the stored BRAT values are assumed to be the full | |

302 | branching ratios, including all possible channels and | |

303 | all secondary decays. The actual relative branching fractions | |

304 | are modified to take into account that the simulation of some | |

305 | channels may be switched off (even selectively for a particle | |

306 | and an antiparticle), as given by MDME(IDC,1), and that | |

307 | some secondary channels may not be allowed, as expressed by | |

308 | the WIDS factors. This also goes into process cross sections. | |

309 | Secondly, it is assumed that all widths scale like sqrt(shat)/m, | |

310 | the ratio of the actual to the nominal mass. A further nontrivial | |

311 | change as a function of the actual mass can be set for each | |

312 | channel by the MDME(IDC,2) value, see below. | |

313 | = 3 : a hybrid version of options 1 and 2 above. At initialization | |

314 | the PYWIDT code is used to calculate PMAS(KC,2) and BRAT(IDC) | |

315 | at the nominal mass of the resonance. Special code must then | |

316 | exist in PYWIDT for the particle. The PMAS(KC,2) and BRAT(IDC) | |

317 | values overwrite the default ones. In the subsequent generation | |

318 | of events, the simpler scheme of option 2 is used, thus saving | |

319 | some execution time. | |

320 | Note: the Z and Z' cannot be used with options 2 and 3, since the | |

321 | more complicated interference structure implemented for those | |

322 | particles is only handled correctly for option 1. | |

323 | ||

324 | WIDS(KC,J) : gives the suppression factor to cross sections caused | |

325 | by the closing of some secondary decays, as calculated in PYWIDT. | |

326 | Is built up recursively from the lightest particle to the | |

327 | heaviest one at initialization, with the exception that W and Z | |

328 | are done already from the beginning (since these often are | |

329 | forced off the mass shell). WIDS can go wrong in case you | |

330 | have perverse situations where the branching ratios vary | |

331 | rapidly as a function of energy, across the resonance shape. | |

332 | This then influences process cross sections. | |

333 | The J components store information according to | |

334 | = 1 : a (matched) resonance-antiresonance pair or two identical | |

335 | resonances (e.g. W+W- or Z0Z0). | |

336 | = 2 : a single resonance (e.g. W+ or Z0). | |

337 | = 3 : a single antiresonance (e.g. W-). | |

338 | = 4 : a (matched) resonance-resonance pair, for particle which | |

339 | has a nonidentical antiparticle (e.g. W+W+). | |

340 | = 5 : a (matched) antiresonance-antiresonance pair (e.g. W-W-). | |

341 | ||

342 | * The MDME(IDC,2) matrix element codes for a specific decay channel have | |

343 | been expanded with further values that can be used for decay channels | |

344 | treated by the PYRESD/PYWIDT decay machinery. These codes have no | |

345 | meaning in the framework of ordinary particle decays in PYDECY. | |

346 | = 50 : (default behaviour, also obtained for any other code value | |

347 | apart from the ones listed below) do not include any special | |

348 | threshold factors. That is, a decay channel is left open even | |

349 | if the sum of daughter nominal masses is above the mother | |

350 | actual mass, which is possible if at least one of the daughters | |

351 | can be pushed off the mass shell. | |

352 | = 51 : a step threshold, i.e. a channel is switched off when | |

353 | the sum of daughter nominal masses is above the mother actual | |

354 | mass. | |

355 | = 52 : a beta-factor threshold, i.e. | |

356 | sqrt( (1-m1**2/m**2-m2**2/m**2)**2 - 4*m1**2*m2**2/m**4), | |

357 | assuming that the values stored in PMAS(KC,2) and BRAT(IDC) | |

358 | did not include any threshold effects at all. | |

359 | = 53 : as =52, but assuming that PMAS(KC,2) and BRAT(IDC) did | |

360 | include the threshold effects, so that the weight should be | |

361 | beta(at the actual mass)/beta(at the nominal mass). | |

362 | = 54 - 59 : free for further options. | |

363 | ||

364 | * VINT(91), VINT(92) are obsolete and replaced by WIDS(24,4), WIDS(24,5). | |

365 | ||

366 | * The decay angles in H -> Z0 Z0 -> 4 fermions were previously selected | |

367 | in the same way as for H -> W+ W- -> 4 fermions. Now the correct angular | |

368 | correlations are included also for this case. Reference: | |

369 | O. Linossier and R. Zitoun, internal ATLAS note and private communication. | |

370 | ||

371 | * PYRESD now takes an argument IRES. The standard call from PYEVNT, | |

372 | for the hard process, has IRES=0, and then finds resonances to be | |

373 | treated based on the subprocess number ISUB. In case of a nonzero | |

374 | IRES only the resonance in position IRES of the event record is | |

375 | considered. This is used by PYEVNT and PYEXEC to decay leftover | |

376 | resonances. (Example: a b -> W + t branching may give a t quark as | |

377 | beam remnant.) | |

378 | ||

379 | * Now also three decay products can be handled by PYRESD. | |

380 | ||

381 | * CKIN(49) and CKIN(50) have been introduced to allow minimum mass | |

382 | limits to be passed from PYRESD to PYOFSH. They are used for | |

383 | tertiary and higher resonances, i.e. those not controlled by | |

384 | CKIN(41)-CKIN(48). They need not be touched by the user. | |

385 | ||

386 | * An approximate 1 - 2.5 alpha_s/pi QCD correction factor has been | |

387 | introduced for the width of the top decay t -> b + W. | |

388 | ||

389 | * New default behaviour of the Higgs resonance shape. | |

390 | MSTP(49) : (D=1) assumed variation of the Higgs width as a function | |

391 | of the actual mass mhat = sqrt(shat) and the nominal mass m_H. | |

392 | = 0 : the width is proportional to mhat**3; thus the high-mass | |

393 | tail of the Breit-Wigner is enhanced. | |

394 | = 1 : the width is proportional to m_H**2 * mhat. For a fixed | |

395 | Higgs mass m_H this means a width variation across the | |

396 | Breit-Wigner more in accord with other resonances (such as | |

397 | the Z0). This alternative gives more emphasis to the | |

398 | low-mass tail, where the parton distributions are peaked | |

399 | (for hadron colliders). This option is favoured by | |

400 | resummation studies [M. Seymour, Phys. Lett. B354 (1995) | |

401 | 409]. | |

402 | Note : this switch does not affect processes 71 - 77, where a | |

403 | fixed Higgs width is used in order to control cancellation | |

404 | of divergences. | |

405 | ||

406 | ----------------------------------------------------------------------- | |

407 | ||

408 | HARD PROCESSES | |

409 | ||

410 | * The maximum number of processes has been expanded from 200 to 500; | |

411 | this affects MSUB, ISET, KFPR, COEF, NGEN, XSEC and PROC in | |

412 | several commonblocks. | |

413 | ||

414 | * SUSY processes have been introduced according to the SPYTHIA program. | |

415 | - Look in the publication | |

416 | SPYTHIA: A Supersymmetric Extension of PYTHIA 5.7 | |

417 | S. Mrenna, Computer Physics Commun. 101 (1997) 232 | |

418 | (hep-ph/9609360) | |

419 | for a description of the physics that has been implemented. | |

420 | - The list of new processes and process numbers is according to | |

421 | tables 2 and 3 in the SPYTHIA manual. Also the MSEL values | |

422 | of table 4 can be used. | |

423 | - Switches and free parameters that can be used to select a wide | |

424 | variety of SUSY scenarios are accessed in | |

425 | COMMON/PYMSSM/IMSS(0:99),RMSS(0:99) | |

426 | according to the description in SPYTHIA manual section 3.1. | |

427 | - The notation for sparticles follows the LEP 2 standard outlined | |

428 | above, and thus disagrees with the one in the SPYTHIA manual. | |

429 | - The supersymmetric code has largely been taken over unchanged | |

430 | from SPYTHIA, but a number of minor changes and bug fixes have | |

431 | been introduced. As examples, the sparticle mass selection has | |

432 | been improved, as has the selection of showering parton system. | |

433 | The strategy for the selection of slepton and squark flavour, in | |

434 | processes with several flavours allowed, has also been changed. | |

435 | - Some routine and commonblock names have been changed, but none | |

436 | of the major ones listed in the SPYTHIA manual. The dependence | |

437 | on CERN library routines has been eliminated. | |

438 | - A major administrative change is that it is now possible to set | |

439 | allowed decay channels of sparticles using the MDME array, | |

440 | as for ordinary resonances, and have this reflected in the | |

441 | process cross sections. | |

442 | - One difference between the SUSY simulation and the other parts of | |

443 | the program is that it is not beforehand known which sparticles | |

444 | may be stable. Normally this would mean either the chi_1 or the | |

445 | gravitino, but in principle also other sparticles could be | |

446 | stable. The ones found to be stable have their MWID(KC) and | |

447 | MDCY(KC,1) values set zero at initialization. If several | |

448 | PYINIT calls are made in the same run, with different SUSY | |

449 | parameters, the ones set zero above are not necessarily set | |

450 | back to nonzero values (the exception is chi_1), since the | |

451 | original values are not saved anywhere. This may then have to | |

452 | be done by hand, or else some particles that ought to decay will | |

453 | not do that. | |

454 | - Bottom squark production is now treated separately as for | |

455 | the top squark. However, there are more processes because bottom | |

456 | is in the PDF. The new processes are: | |

457 | 281 b q -> ~b_1 ~q_L (q not b) | |

458 | 282 b q -> ~b_2 ~q_R | |

459 | 283 b q -> ~b_1 ~q_R + ~b_2 ~q_L | |

460 | 284 b qbar -> ~b_1 ~q_Lbar | |

461 | 285 b qbar -> ~b_2 ~q_Rbar | |

462 | 286 b qbar -> ~b_1 ~q_Rbar + ~b_2 ~q_Lbar | |

463 | 287 q qbar -> ~b_1 ~b_1bar | |

464 | 288 2 2 | |

465 | 289 g g -> ~b_1 ~b_1bar | |

466 | 290 2 2 | |

467 | 291 b b -> ~b_1 ~b_1 | |

468 | 292 2 2 | |

469 | 293 1 2 | |

470 | 294 b g -> ~b_1 ~g | |

471 | 295 2 | |

472 | 296 b bbar -> ~b_1 ~b_2bar + ~b_1bar ~b_2 | |

473 | MSEL = 45 has been added specifically to switch on these processes. | |

474 | - New parameter | |

475 | IMMS(5) : (D=0) allows the user to set the stop, sbottom, and stau | |

476 | masses and mixings by hand. | |

477 | = 0 : no, the program calculates itself. | |

478 | = 1 : Yes, calculate from given input. In that case, | |

479 | RMMS(10) = lightest stop, RMSS(12) = heaviest stop, | |

480 | RMSS(11) = lightest sbottom, RMSS(13) = lightest stau, | |

481 | RMSS(14) = heaviest stau, and RMSS(26,27,28) are the | |

482 | (1,1) elements of the (2x2) mixing matrix for sbottom, | |

483 | stop, and stau. | |

484 | ||

485 | * Higgs pair production now added as explicit processes. (Since before | |

486 | some processes exist as Z' decay modes, where the Z' part can be | |

487 | switched off to simulate the expected behaviour within the MSSM.) | |

488 | 297 q qbar' -> H+/- h0 | |

489 | 298 q qbar' -> H+/- H0 | |

490 | 299 q qbar -> A0 h0 | |

491 | 300 q qbar -> A0 H0 | |

492 | 301 q qbar -> H+ H- | |

493 | ||

494 | * A new machinery has been introduced to generate the spectrum of | |

495 | transverse and longitudinal photons in a lepton beam, and to | |

496 | convolute that with the appropriate matrix elements, including | |

497 | the virtuality of the photons, see C. Friberg and T. Sjostrand, | |

498 | Eur. Phys. J. C 13 (2000) 151. | |

499 | - In order to obtain it, the PYINIT beam or target code should | |

500 | be given in the form 'gamma/lepton', where lepton can be either | |

501 | of e-, e+, mu-, mu+, tau- or tau+. Thus, | |

502 | for HERA : BEAM,TARGET = 'gamma/e-','p' | |

503 | for LEP : = 'gamma/e-','gamma/e+' | |

504 | Kinematics information in the PYINIT call should refer to the | |

505 | full energy available, with the program itself generating the | |

506 | fraction given to the photon(s). | |

507 | - The documentation section at the beginning of the event record | |

508 | has been expanded to reflect the new layer of administration. | |

509 | Positions 1 and 2 contain the original beam particles, e.g. | |

510 | e and p (or e+ and e-). In position 3 (and 4 for e+e-) | |

511 | is (are) the scattered outgoing lepton(s). Thereafter comes | |

512 | the normal documentation, but starting from the photon rather | |

513 | than a lepton. For ep, this means 4 and 5 are the gamma* and p, | |

514 | 6 and 7 the shower initiators, 8 and 9 the incoming partons to | |

515 | the hard interaction, and 10 and 11 the outgoing ones. Thus the | |

516 | documentation is 3 lines longer (4 for e+e-) than normally. | |

517 | - A number of new CKIN cuts have been introduced to restrict | |

518 | the range of kinematics for the photons generated off the | |

519 | lepton beams. In each quartet of numbers, the first two corresponds | |

520 | to the range allowed on incoming side 1 (beam) and the last two | |

521 | to side 2 (target). The cuts are only applicable for a lepton | |

522 | beam. Note that the x and Q2 (P2) variables are the basis | |

523 | for the generation, and so can be restricted with no loss of | |

524 | efficiency. For leptoproduction the W is uniquely given by the | |

525 | one x value of the problem, so here also W cuts are fully efficient. | |

526 | The other cuts may imply a slowdown of the program, but not as much | |

527 | as if equivalent cuts only are introduced after events are fully | |

528 | generated. | |

529 | CKIN(61) - CKIN(64) : (D=0.0001,0.99,0.0001,0.99) allowed range for | |

530 | the energy fractions x that the photon take of the respective | |

531 | incoming lepton energy. These fractions are defined in the | |

532 | cm frame of the collision, and differ from energy fractions | |

533 | as defined in another frame. (Watch out at HERA!) In order to | |

534 | avoid some technical problems, absolute lower and upper limits | |

535 | are set internally at 0.0001 and 0.9999. | |

536 | CKIN(65) - CKIN(68) : (D=0.,-1.,0.,-1. GeV^2) allowed range for the | |

537 | spacelike virtuality of the photon, conventionally called either | |

538 | Q2 or P2, depending on process. A negative number means that the | |

539 | upper limit is inactive, i.e. purely given by kinematics. A nonzero | |

540 | lower limit is implicitly given by kinematics constraints. | |

541 | CKIN(69) - CKIN(72) : (D=0.,-1.,0.,-1.) allowed range of the | |

542 | scattering angle theta of the lepton, defined in the cm frame | |

543 | of the event. (Watch out at HERA!) A negative number means that | |

544 | the upper limit is inactive, i.e. equal to pi. | |

545 | CKIN(73) - CKIN(76) : (D=0.0001,0.99,0.0001,0.99) allowed range for | |

546 | the lightcone fraction y that the photon take of the respective | |

547 | incoming lepton energy. The lightcone is defined by the | |

548 | four-momentum of the lepton or hadron on the other side of the | |

549 | event (and thus deviates from true lightcone fraction by mass | |

550 | effects that normally are negligible). The y value is related to | |

551 | the x and Q2 (P2) values by y = x + Q2/s if mass terms are | |

552 | neglected. | |

553 | CKIN(77), CKIN(78) : (D=2.,-1. GeV) allowed range for W, i.e. either | |

554 | the photon-hadron or photon-photon invariant mass. A negative | |

555 | number means that the upper limit is inactive. | |

556 | - This machinery cannot be combined with the variable-energy option | |

557 | obtainable for MSTP(171)=1. The reason is that a variable-energy | |

558 | machinery is now used internally for the gamma-hadron or gamma-gamma | |

559 | subsystem, with some information saved at initialization for the full | |

560 | energy. | |

561 | - Internally, some new variables are used: | |

562 | MINT(141), MINT(142) : KF code for incoming lepton beam or target | |

563 | particles, while MINT(11) and MINT(12) is then the photon code. | |

564 | A nonzero value is the main check whether the photon emission | |

565 | machinery should be called at all. | |

566 | MINT(143) : the number of tries before a successful kinematics | |

567 | configuration is found in PYGAGA. Used for the cross section | |

568 | updating in PYRAND. | |

569 | VINT(301) : cm energy for the full collision, while VINT(1) | |

570 | gives the gamma-hadron or gamma-gamma subsystem energy. | |

571 | VINT(302) : full squared cm energy, while VINT(2) gives the subsystem | |

572 | squared energy. | |

573 | VINT(303), VINT(304) : mass of beam or target lepton, while VINT(3) | |

574 | or VINT(4) give the mass of a photon emitted off it. | |

575 | VINT(305), VINT(306) : x values, i.e. respective photon energy | |

576 | fractions of the incoming lepton in the cm frame of the event. | |

577 | VINT(307), VINT(308) : Q2 or P2, virtuality of the respective photon | |

578 | (thus the square of VINT(3), VINT(4)). | |

579 | VINT(309), VINT(310) : y values, i.e. respective photon lightcone | |

580 | energy fraction of the lepton. | |

581 | VINT(311), VINT(312) : theta, scattering angle of the respective | |

582 | lepton in the cm frame of the event. | |

583 | VINT(313), VINT(314) : phi, azimuthal angle of the respective | |

584 | scattered lepton in the cm frame of the event. | |

585 | VINT(319) : photon flux factor in PYGAGA for current event. | |

586 | VINT(320) : photon flux factor in PYGAGA at initialization. | |

587 | - Some of these values are also saved in the MSTI and PARI arrays at | |

588 | the end of the event generation. (In the case of pileup events, | |

589 | values stored here refer to the first event, while the MINT/VINT | |

590 | ones are for the latest one, as usual.) | |

591 | MSTI(71), MSTI(72) : KF code for incoming lepton beam or target | |

592 | particles, while MSTI(11) and MSTI(12) is then the photon code. | |

593 | PARI(101) : cm energy for the full collision, while PARI(11) | |

594 | gives the gamma-hadron or gamma-gamma subsystem energy. | |

595 | PARI(102) : full squared cm energy, while PARI(12) gives the subsystem | |

596 | squared energy. | |

597 | PARI(103), PARI(104) : x values, i.e. respective photon energy | |

598 | fractions of the incoming lepton in the cm frame of the event. | |

599 | PARI(105), PARI(106) : Q2 or P2, virtuality of the respective photon | |

600 | (thus the square of PARI(3), PARI(4)). | |

601 | PARI(107), PARI(108) : y values, i.e. respective photon lightcone | |

602 | energy fraction of the lepton. | |

603 | PARI(109), PARI(110) : theta, scattering angle of the respective | |

604 | lepton in the cm frame of the event. | |

605 | PARI(111), PARI(112) : phi, azimuthal angle of the respective | |

606 | scattered lepton in the cm frame of the event. | |

607 | - A new routine has been added for internal use: | |

608 | SUBROUTINE PYGAGA(IGA) | |

609 | IGA = 1 : call at initialization to set up x and Q2 limits etc. | |

610 | = 2 : call at maximization step to give estimate of maximal | |

611 | photon flux factor. | |

612 | = 3 : call at the beginning of the event generation to select | |

613 | the kinematics of the photon emission and to give the | |

614 | flux factor. | |

615 | = 4 : call at the end of the event generation to set up the | |

616 | full kinematics of the photon emission. | |

617 | - Since there are currently no processes associated with resolved | |

618 | longitudinal photons, the effect of these can be approximated by | |

619 | some nonzero MSTP(17) and PARP(165). (Additionally, PARP(167) or | |

620 | PARP(168) may need to be set.) | |

621 | MSTP(17) : (D=4) possibility of a extrafactor for resolved processes, | |

622 | to approximately take into accound the effects of longitudinal | |

623 | photons. Given on the form | |

624 | R = 1 + PARP(165) * r(Q^2,mu^2) * f_L(y,Q^2)/f_T(y,Q^2). | |

625 | Here the 1 represents the basic transverse contribution, | |

626 | PARP(165) is some arbitrary overall factor, and f_L/f_T | |

627 | the (known) ratio of longitudinal to transverse photon | |

628 | flux factors. The arbitrary function r depends on the photon | |

629 | virtuality Q^2 and the hard scale mu^2 of the process. | |

630 | = 0 : No contribution, i.e. r=0. | |

631 | = 1 : r = 4 * mu^2 * Q^2 / (mu^2 + Q^2)^2. | |

632 | = 2 : r = 4 * Q^2 / (mu^2 + Q^2). | |

633 | = 3 : r = 4 * Q^2 / (m_{rho}^2 + Q^2). | |

634 | = 4 : r = 4 * m_V^2 * Q^2 / (m_V^2 + Q^2)^2. | |

635 | = 5 : r = 4 * Q^2 / (m_V^2 + Q^2). | |

636 | In options 4 and 5 m_V is the vector meson mass for VMD | |

637 | and 2 * k_T for GVMD states. Since there is no mu dependence | |

638 | for these options (as well as for =3) they also affect | |

639 | minimum-bias cross sections, where mu would be vanishing. | |

640 | Currently the rho mass is used also in options 4 and 5, for | |

641 | simplicity. | |

642 | NOTE: For a photon given by the gamma/e option in the PYINIT call, | |

643 | the y spectrum is dynamically generated and y is thus known | |

644 | from event to event. For a photon beam in the PYINIT call, | |

645 | y is unknown from the onset, and has to be provided by the | |

646 | user if any longitudinal factor is to be included. So long | |

647 | as these values, in PARP(167) and PARP(168), are at their | |

648 | default values, 0, it is assumed they have not been set and | |

649 | thus the MSTP(17) and PARP(165) values are inactive. | |

650 | PARP(165) : (D=0.5) a simple multiplicative factor applied to the | |

651 | cross section for the transverse resolved photons, see above | |

652 | in MSTP(17). No preferred value, but typically one could use | |

653 | PARP(165)=1 as main contrast to the no-effect =0, with the | |

654 | default arbitrarily chosen in the middle. | |

655 | PARP(167), PARP(168): (D=2*0) the longitudinal energy fraction | |

656 | y of an incoming photon, side 1 or 2, used in the R expression | |

657 | to evaluate f_L(y,Q^2)/f_T(y,Q^2). Need not be supplied when | |

658 | a photon spectrum is generated inside a lepton beam, but only | |

659 | when a photon is directly given as argument in the PYINIT call. | |

660 | VINT(315), VINT(316): internal storage of the R factor above, for | |

661 | each of the two sides. | |

662 | PARI(113), PARI(114); values of the R factors above, for each of | |

663 | the two sides. | |

664 | ||

665 | * New total cross sections have been introduced into PYXTOT for | |

666 | Generalized Vector Meson Dominance (GVMD) states, and both VMD and | |

667 | GVMD parameterizations have been extended also to include virtual | |

668 | photons. Further details in C. Friberg and T. Sjostrand, in | |

669 | preparation. | |

670 | - The GVMD states are seen as a continuous spectrum, characterized | |

671 | by the k_T scale of the gamma -> q + qbar branching, with | |

672 | k_T stretching between k_0 and p_Tmin(W^2). The rate of fluctuation | |

673 | into such states is given by perturbative QED, while the | |

674 | hadronic cross section for a given state is assumed to obey geometric | |

675 | scaling, i.e. fall off like k_rho^2/k_T^2 relative to a VMD state | |

676 | for a real photon, where k_rho is a reference scale. | |

677 | - The jet cross sections for these GVMD states are associated with | |

678 | the anomalous part of the photon structure function, just like | |

679 | the homogeneous part is associated with the VMD states. | |

680 | - GVMD state also have "elastic" and diffractive cross sections | |

681 | obtained by the same scaling of VMD cross sections as indicated | |

682 | above for the total cross section. The mass selection of the | |

683 | GVMD state is according to dm^2/(m^2+Q^2)^2 between limits | |

684 | 2 k_0 < m < 2 p_Tmin(W^2), i.e. the mass is associated with | |

685 | 2 k_T of the state. See VINT(69), VINT(79) below. A GVMD state | |

686 | is bookkept as a diffractive state in event listing, even when | |

687 | it scatters "elastically", since the subsequent hadronization | |

688 | descriptions are very similar. | |

689 | - Whether or not minimum bias events are simulated depends on the | |

690 | CKIN(3) value. For a low CKIN(3), CKIN(3) < p_Tmin(W_init^2), | |

691 | like the default value CKIN(3) = 0, low-pT physics is switched | |

692 | on together with jet production, with the latter properly | |

693 | eikonalized to be lower than the total one. For a high CKIN(3), | |

694 | CKIN(3) > p_Tmin(W_init^2), only jet production is included. | |

695 | This is just like for hadron-hadron collisions, except that the | |

696 | initialization energy scale W_init is selected in the allowed | |

697 | W range rather than to be the full CM energy. When MSEL=2, also | |

698 | elastic and diffractive events are simulated. | |

699 | - Multiple interactions become possible in both the VMD and GVMD | |

700 | sector, with the average number of interactions given by the | |

701 | ratio of the jet to the total cross section. Currently only | |

702 | the simpler default scenario MSTP(82)=1 is implemented, i.e. | |

703 | the more sophisticated variable-impact-parameter ones need further | |

704 | physics studies and model development. | |

705 | - For a virtual photon of virtuality Q^2, the total cross section is | |

706 | reduced by a dipole factor (m_rho^2/(m_rho^2 + Q^2))^2 for a VMD | |

707 | state and by (4 k_T^2/(4 k_T^2 + Q^2))^2 for a GVMD one. That | |

708 | is, the "mass" of a GVMD state is taken to be 2 k_T.Properly | |

709 | each VMD state should have own mass, but so far this has not been | |

710 | implemented. This would mainly be of relevance for J/psi, where | |

711 | however also other complications enter. | |

712 | - gamma* gamma* cross sections are obtained by simple multiplicative | |

713 | factors as above, one for each photon, relative to rho rho events | |

714 | (and other vector mesons). | |

715 | - The primordial kT selection is described in the section on MSTP(66). | |

716 | For clarity, we point out that elastic and diffractive events are | |

717 | characterized by the mass of the diffractive states but without | |

718 | any primordial kT, while jet production involves a primordial kT | |

719 | but no mass selection. Both are thus not used at the same time, | |

720 | but implicitly they are associated as m = 2 k_T. | |

721 | - New or modified commonblock variables: | |

722 | MSTP(15) : (D=0) modified default, to give same pT cutoff procedure | |

723 | as for VMD jet cross sections. | |

724 | PARP(15) : (D=0.5 GeV) k_0 scale where GVMD k_T spectrum begins. | |

725 | MINT(50) : now set = 1 also for anomalous states, to indicate that | |

726 | total cross sections are defined for them. | |

727 | VINT(67), VINT(68) : the mass of a VMD state; for GVMD photons | |

728 | the VMD state with the equivalent flavour content. | |

729 | VINT(69), VINT(70) : the actual mass of a VMD or GVMD state; | |

730 | agrees with the above for VMD but is selected as a larger | |

731 | number for GVMD. Required for elastic and diffractive events. | |

732 | VINT(63), VINT(64) : the squared (!) mass of the outgoing states; | |

733 | for elastic events equal to VINT(69)^2 and VINT(70)^2 and for | |

734 | diffractive events above that. | |

735 | VINT(154) : current p_Tmin(W^2) value; see section on UNDERLYING | |

736 | EVENTS for details. | |

737 | VINT(149) : the scaled value 4 p_Tmin(W^2)^2/W^2; denominator | |

738 | changed from s and therefore needs to be recalculated for each | |

739 | new event (like VINT(154)). | |

740 | PARP(18) : (D=0.4 GeV) scale k_rho, such that the cross sections | |

741 | of a GVMD state of scale k_T is suppressed by a factor | |

742 | k_rho^2/k_T^2 relative to those of a VMD state. Order should be | |

743 | m_rho/2, with some finetuning to fit data. | |

744 | VINT(317) : dipole suppression factor in PYXTOT for current event. | |

745 | VINT(318) : dipole suppression factor in PYXTOT at initialization. | |

746 | MSTP(66) : (D=5) see separate note below. | |

747 | - The MSTP(84) and MSTP(85) switches have been made obsolete by these | |

748 | changes and no longer exist. | |

749 | ||

750 | * An additional suppression of resolved (VMD or GVMD) cross sections is | |

751 | introduced to compensate for an overlap with DIS processes in the | |

752 | region of intermediate Q^2 and rather small W^2. | |

753 | - MSTP(20) : (D=3) suppression of resolved cross sections. | |

754 | = 0 : no; used as is. | |

755 | > 0 : yes, by a factor (W^2/(W^2 + Q_1^2 + Q_2^2))^MSTP(20). | |

756 | (where Q_i^2 = 0 for an incoming hadron). | |

757 | - The suppression factor is joined with the dipole suppression | |

758 | stored in VINT(317) and VINT(318). | |

759 | ||

760 | * New processes have been introduced for incoming virtual (spacelike) | |

761 | photons, as obtained e.g. in ep and e+e- collisions. These are | |

762 | thus extensions of processes previously encoded for real photons. | |

763 | - 131 f_i + gamma*_T -> f_i + g (cf. proc 33) | |

764 | 132 f_i + gamma*_L -> f_i + g | |

765 | 133 f_i + gamma*_T -> f_i + gamma (cf. proc 34) | |

766 | 134 f_i + gamma*_L -> f_i + gamma | |

767 | 135 g + gamma*_T -> f_i + fbar_i (cf. proc 54) | |

768 | 136 g + gamma*_L -> f_i + fbar_i | |

769 | 137 gamma*_T + gamma*_T -> f_i + fbar_i (cf. proc 58) | |

770 | 138 gamma*_T + gamma*_L -> f_i + fbar_i | |

771 | 139 gamma*_L + gamma*_T -> f_i + fbar_i | |

772 | 140 gamma*_L + gamma*_L -> f_i + fbar_i | |

773 | - Here indices _T and _L represent transverse and longitudinal | |

774 | photons, respectively. In the limit of vanishing virtuality, | |

775 | the _T photon cross section approaches that for a real photon, | |

776 | while the _L one vanishes. | |

777 | - The virtuality of the photon or photons can be stored in P(1,5) | |

778 | and P(2,5), respectively, provided PYINIT is called with the | |

779 | 'FIVE' option. A spacelike photon of virtuality Q**2 (or P**2, | |

780 | depending on notational convention followed) would thus have | |

781 | P(i,5) = -Q (or -P). The virtuality could be varied from one | |

782 | event to the next, but then it is convenient to initialize | |

783 | for the lowest virtuality likely to be encountered. | |

784 | - In several of the standard MSEL options, processes selected for | |

785 | real photons have been replaced by the corresponding processes | |

786 | for virtual ones. | |

787 | ||

788 | * Direct processes in the range of k_T values stretching between k_0 and | |

789 | p_Tmin(W^2) are, by an eikonalization process, associated with the | |

790 | low-pT part of the GVMD states above. Further details in C. Friberg | |

791 | and T. Sjostrand, in preparation. | |

792 | - As a consequence, the minimum pT for direct processes should be | |

793 | increased from k_0 to p_Tmin(W^2). | |

794 | - New variable: | |

795 | MSTP(18) : (D=3) choice of pTmin for direct processes: | |

796 | = 1 : same as for VMD and GVMD states, as explained above.. | |

797 | = 2 : pTmin is chosen to be PARP(15), i.e. the old behaviour. | |

798 | In this case, also parton distributions, jet cross sections | |

799 | and alpha_strong values were dampened for small pT. | |

800 | = 3 : as =1, but if the Q scale of the virtual photon is | |

801 | above the VMD/GVMD p_Tmin(W^2), pTmin is chosen equal to Q. | |

802 | This is part of the strategy to mix in DIS processes at | |

803 | pT below Q, e.g. in MSTP(14)=30. | |

804 | ||

805 | * New process 99 for DIS scattering, by photon exchange only. Thus, in | |

806 | this sense less powerful than process 10, but allows the use of the | |

807 | same photon flux machinery as for other gamma*-p and gamma*-gamma* | |

808 | processes, and thus offers a unified description in the region of | |

809 | intermediate Q2 values. | |

810 | - Notice that it counts as a "total cross section" process, in the | |

811 | sense that the hard subprocess in itself contains no high-pT | |

812 | scale. Therefore, it will be switched off in event class mixes | |

813 | such as MSTP(14)=30 if CKIN(3) is above pTmin(W^2) and MSEL | |

814 | is not 2. | |

815 | - 99 f_i + gamma* -> f_i. | |

816 | - Since the standard 2 -> 1 kinematics machinery is not relevant for | |

817 | this process - shat = 0 - a new code ISET(ISUB)=8 is introduced | |

818 | for the kinematics selection machinery in PYRAND, and a new routine | |

819 | PYDISG for setting up the kinematics, beam remnants and showers. | |

820 | - New variable to select DIS cross section. | |

821 | MSTP(19) : (D=4) choice of partonic cross section in process 99. | |

822 | = 0 : QPM answer 4 pi^2 alpha_em/Q^2 * | |

823 | \sum_q e_q^2 (x q(x,Q^2) + x qbar(x,Q^2)) | |

824 | (with parton distributions frozen below the lowest Q | |

825 | allowed in the parameterization). Note that this answer | |

826 | is divergent for Q^2 -> 0 and thus violates gauge | |

827 | invariance. | |

828 | = 1 : QPM answer is modified by a factor Q^2/(Q^2 + m_rho^2) | |

829 | to provide a finite cross section in the Q^2 -> 0 limit. | |

830 | A minimal regularization recipe. | |

831 | = 2 : QPM answer is modified by a factor Q^4/(Q^2 + m_rho^2)^2 | |

832 | to provide a vanishing cross section in the Q^2 -> 0 limit. | |

833 | Appropriate if one assumes that the normal photoproduction | |

834 | description gives the total cross section for Q^2 = 0, | |

835 | without any DIS contribution. | |

836 | = 3 : as = 2, but additionally suppression by a parameterized | |

837 | factor f(W^2,Q^2) (different for gamma*-p and gamma*-gamma*) | |

838 | that avoids doublecounting the direct-process region where | |

839 | p_T > Q. Shower evolution for DIS events is then also | |

840 | restricted to be at scales below Q, whereas evolution all | |

841 | the way up to W is allowed in the other options above. | |

842 | = 4 : as = 3, but additionally include factor 1/(1-x) for | |

843 | conversion from F_2 to sigma. This is formally required, | |

844 | but is only relevant for small W2 and therefore often | |

845 | neglected. | |

846 | - MINT(107),MINT(108) = 4 denotes DIS photon on respective side. | |

847 | MINT(123) = 8 denotes DIS*VMD/p or vice verse, = 9 DIS*anomalous | |

848 | or vice versa. | |

849 | In MINT(41)-MINT(46), a DIS photon is treated same way as a direct | |

850 | one. | |

851 | - Many of the normal kinematical variables for 2 -> 2 processes are | |

852 | not defined for this process. The pT in PARI(17) is explicitly set | |

853 | =0, but some others may well contain irrelevant junk. | |

854 | ||

855 | * MSTP(14) is extended with new possibilities to select the nature | |

856 | of incoming virtual photons. The reason is that the existing | |

857 | options specify e.g. direct * VMD, summing over the possibilities | |

858 | of which photon is direct and which anomalous. This is allowed | |

859 | when the situation is symmetric, i.e. for two incoming real photons, | |

860 | but not if one is virtual. Some of the new options agree with | |

861 | previous ones, but are included to allow a more consistent pattern. | |

862 | MSTP(14): (D=30) structure of incoming photon beam or target. | |

863 | = 11 : direct * direct (see note 4). | |

864 | = 12 : direct * VMD (i.e. first photon direct, second VMD). | |

865 | = 13 : direct * anomalous. | |

866 | = 14 : VMD * direct. | |

867 | = 15 : VMD * VMD. | |

868 | = 16 : VMD * anomalous. | |

869 | = 17 : anomalous * direct. | |

870 | = 18 : anomalous * VDM. | |

871 | = 19 : anomalous * anomalous. | |

872 | = 20 : a mixture of the nine above components, in the same | |

873 | spirit as =10 provides a mixture for real gammas (or | |

874 | a virtual gamma on a hadron). For gamma-hadron, this | |

875 | option coincides with =10. | |

876 | = 21 : direct * direct (see note 4). | |

877 | = 22 : direct * resolved. | |

878 | = 23 : resolved * direct. | |

879 | = 24 : resolved * resolved. | |

880 | = 25 : a mixture of the four above components, offering a | |

881 | simpler alternative to =20 in cases where the parton | |

882 | distributions of the photon have not been split into VMD | |

883 | and anomalous components. For gamma-hadron, only two | |

884 | components need be mixed. | |

885 | = 26 : DIS * VMD/p. | |

886 | = 27 : DIS * anomalous. | |

887 | = 28 : VMD/p * DIS. | |

888 | = 29 : anomalous * DIS. | |

889 | = 30 : a mixture of all the 4 (for gamma*-p) or 13 (for | |

890 | gamma*-gamma*) that are available, is as = 20 with the | |

891 | DIS processes 26-29 mixed in. | |

892 | Note 1: The MSTP(14) options apply for a photon defined by a 'gamma' | |

893 | or 'gamma/lepton' beam in the PYINIT call, but not to those | |

894 | photons implicitly obtained in a 'lepton' beam with the | |

895 | MSTP(12)=1 option. This latter approach to resolved photons is | |

896 | more primitive and is no longer recommended. | |

897 | Note 2: these new options are not needed and therefore not defined | |

898 | for e-p collisions. The recommended 'best' values thus are | |

899 | MSTP(14)=30, which also is the new default value. | |

900 | Note 3: as a consequence of the appearance of new event classes, | |

901 | the MINT(122) and MSTI(9) code is not the same for gamma* gamma* | |

902 | events as for gamma p, gamma* p or gamma gamma ones. | |

903 | Instead the code is 3*(icode_1 - 1) + icode_2, where icode is | |

904 | 1 for direct, 2 for VMD and 3 for anomalous/GVMD and indices | |

905 | refer to the two incoming photons. For gamma* p code 4 is DIS, | |

906 | and for gamma* gamma* codes 10-13 corresponds to the MSTP(14) | |

907 | codes 26-29. As before, MINT(122) and MSTI(9) are only defined | |

908 | when several processes are to be mixed, not when generating one | |

909 | at a time. Also the MINT(123) code is modified (not shown here). | |

910 | Note 4: The direct * direct event class excludes lepton pair | |

911 | production when run with the default MSEL=1 option (or MSEL=2), | |

912 | in order not to confuse users. You can obtain lepton pairs as well, | |

913 | e.g. by running with MSEL=0 and switching on the desired processes | |

914 | by hand. | |

915 | ||

916 | * MSTP(16) is new variable to select momentum variable of | |

917 | e -> gamma branching. | |

918 | MSTP(16) (D=1) choice of definition of the fractional momentum | |

919 | taken by a photon radiated off a lepton. Enters in the flux | |

920 | factor for the photon rate, and thereby in cross sections. | |

921 | = 0 ; x, i.e. energy fraction in the rest frame of the event. | |

922 | = 1 ; y, i.e. lightcone fraction. | |

923 | ||

924 | * MSTP(32) : (D=8) has been expanded with new options for the choice | |

925 | of Q2 scale, specifically intended for processes with incoming | |

926 | virtual photons. The new options are ordered from a "minimal" | |

927 | dependence on the virtualities to a "maximal" one, based on | |

928 | reasonable kinematics considerations. The old default value | |

929 | MSTP(32)=2 forms the starting point, with no dependence at | |

930 | all, and the new default is some intermediate choice. | |

931 | Notation is that P1**2 and P2**2 are the virtualities of the | |

932 | two incoming particles, pT the transverse momentum of the | |

933 | scattering process, and m3 and m4 the masses of the two | |

934 | outgoing partons. For a direct photon, P**2 is the photon | |

935 | virtuality and x=1. For a resolved photon, P**2 still refers | |

936 | to the photon, rather than the unknown virtuality of the | |

937 | reacting parton in the photon, and x is the momentum fraction | |

938 | taken by this parton. | |

939 | = 6 : Q2 = (1 + x1*P1**2/shat + x2*P2**2/shat)* | |

940 | (pT**2 + m3**2/2 + m4**2/2). | |

941 | = 7 : Q2 = (1 + P1**2/shat + P2**2/shat)* | |

942 | (pT**2 + m3**2/2 + m4**2/2). | |

943 | = 8 : Q2 = pT**2 + (P1**2 + P2**2 +m3**2 + m4**2)/2. | |

944 | = 9 : Q2 = pT**2 + P1**2 + P2**2 +m3**2 + m4**2. | |

945 | = 10 : s (the full energy-squared of the process). | |

946 | Note: options 6 and 7 are motivated by assuming that one | |

947 | wants a scale that interpolates between that for small | |

948 | that and uhat for small uhat, such as | |

949 | Q2 = - that*uhat/(that+uhat). When kinematics for | |

950 | the 2 -> 2 process is constructed as if an incoming | |

951 | photon is massless when it is not, it gives rise to a | |

952 | mismatch factor 1 + P**2/shat (neglecting the other | |

953 | masses) in this Q2 definition, which is then what is | |

954 | used in option 7 (with the neglect of some small | |

955 | cross-terms when both photons are virtual). When a | |

956 | virtual photon is resolved, the virtuality of the | |

957 | incoming parton can be anything from x*P**2 and upwards. | |

958 | So option 6 uses the smallest kinematically possible | |

959 | value, while 7 is more representative of the typical | |

960 | scale. Option 8 and 9 are more handwaving extensions of | |

961 | the default option, with 9 specially constructed to | |

962 | ensure that the Q2 scale is always bigger than P**2. | |

963 | ||

964 | * MSTP(66) has been expanded with new default option for the | |

965 | selection of lower parton-shower cut-off (and primordial kT). | |

966 | MSTP(66) : (D=5) choice of lower cut-off for initial-state QCD | |

967 | radiation in VMD or anomalous photoproduction events. | |

968 | = 0 : the lower Q2 cut-off is the standard one in PARP(62)^2. | |

969 | = 1 : for anomalous photons, the lower Q2 cut-off is the | |

970 | larger of PARP(62)^2 and VINT(283) or VINT(284), | |

971 | where the latter is the virtuality scale for the | |

972 | gamma -> q qbar vertex on the appropriate side of | |

973 | the event. The VINT values are selected logarithmically | |

974 | even between PARP(15)^2 and the Q2 scale of the | |

975 | parton distributions of the hard process. | |

976 | = 2 : extended option of the above, intended for virtual | |

977 | photons. For VMD photons, the lower Q2 cut-off is the | |

978 | larger of PARP(62)^2 and the P^2_{int} scale of the | |

979 | SaS parton distributions. For anomalous photons, | |

980 | the lower cut-off is chosen as for =1, but the | |

981 | VINT(283) and VINT(284) are here selected logarithmically | |

982 | even between P^2_{int} and the Q2 scale of the | |

983 | parton distributions of the hard process. | |

984 | = 3 : simplified option, default in versions 6.143 - 6.147. | |

985 | The k_T of the anomalous/GVMD component is distributed | |

986 | like 1/k_T^2 between k_0 and p_Tmin(W^2). Apart from | |

987 | the change of the upper limit, this option works just | |

988 | like = 1. | |

989 | = 4 : a stronger damping at large k_T, like | |

990 | dk_T^2/(k_T^2 + Q^2/4)^2 with | |

991 | k_0 < k_T < p_Tmin(W^2). Apart from this, | |

992 | it works like = 1. | |

993 | = 5 : a k_T generated as in =4 is added vectorially with a | |

994 | standard Gaussian k_T generated like for VMD states. | |

995 | Ensures that GVMD has typical k_T's above those of VMD, | |

996 | in spite of the large primordial k_T's implied by hadronic | |

997 | physics. (Probably attributable to a lack of soft QCD | |

998 | radiation in parton showers.) | |

999 | ||

1000 | * New processes | |

1001 | - 146 e + gamma -> e* | |

1002 | 169 q + qbar -> e + e* | |

1003 | - similar to existing processes 147,148 or 167,168 for q*. | |

1004 | ||

1005 | * Several new processes for technicolour production. | |

1006 | NOTE: as of version 6.126 changes/additions appear according | |

1007 | to the next section. | |

1008 | - 191 f_i + fbar_i -> rho_techni0 | |

1009 | 192 f_i + fbar_j -> rho_techni+- | |

1010 | 193 f_i + fbar_i -> omega_techni0 | |

1011 | 194 f_i + fbar_i -> f_k + fbar_k | |

1012 | - The first three processes are based on s-channel production of | |

1013 | the respective resonance. All decay modes implemented can be | |

1014 | simulated separately or in combination, in the standard fashion. | |

1015 | These include pairs of fermions, of gauge bosons, of technipions, | |

1016 | and of mixtures gauge bosons + technipions. | |

1017 | - Process 194 includes full interference between rho_techni0 and | |

1018 | omega_techni0. It can only be used for one final-state flavour | |

1019 | at a time. This flavour is set in KFPR(194,1). | |

1020 | - The physics parameters of the technicolour scenario are: | |

1021 | PARP(140) : (D=0.0) multiplicative fudge factor, entering | |

1022 | quadratically in the width for pi_tech+ -> W+ b bbar. | |

1023 | PARP(141) : (D=0.33333) sin(chi), sinus of mixing angle between | |

1024 | gague bosons and technipions in the decay of technirhos; | |

1025 | if 0 the decay is entirely to technipions and if 1 entirely | |

1026 | to gauge bosons. | |

1027 | PARP(142) : (D=82 GeV) F_T, decay constant of the technipion | |

1028 | states; the technipion widths are proportional to 1/F_T^2. | |

1029 | PARP(143) : (D=1.0) Q_U, charge of the up-type technifermions; | |

1030 | the down-type ones have Q_D = Q_U - 1 and thus do not | |

1031 | require a separate parameter. | |

1032 | PARP(144) : (D=4.0) N_TC, the number of technicolours, that | |

1033 | enters in several cross sections and decay rates. | |

1034 | PARP(145) : (D=200 GeV) M_T, mass parameter for the decay | |

1035 | omega_techni0 -> gamma + pi_techni0; the partial width | |

1036 | is proportional to 1/M_T^2. | |

1037 | PARP(146) - PARP(148) : (D=1.0, 1.0, 1.0) multiplicative fudge | |

1038 | factors, entering quadratically in the widths of technipions | |

1039 | to a fermion pair. The three numbers are for pi_tech0, | |

1040 | pi_tech+ and pi_tech'0, respectively. | |

1041 | PARP(149) - PARP(150) : (D=1.0, 0.0) multiplicative fudge factors, | |

1042 | entering linearly in the widths of technipions to a gluon pair. | |

1043 | The two numbers are for pi_tech0 and pi_tech'0, respectively. | |

1044 | - The main references are | |

1045 | E. Eichten and K. Lane, Phys. Lett. B388 (1996) 803 | |

1046 | E. Eichten, K. Lane and J. Womersley, in preparation | |

1047 | ||

1048 | * Starting with version 6.126, the simulation of the production and | |

1049 | decays of technicolor particles has been substantially upgraded. | |

1050 | - The processes 149, 191, 192, and 193 are to be considered obsolete, | |

1051 | and are temporarily retained to allow cross checking with the new | |

1052 | processes. | |

1053 | - Process 194 has been changed to more accurately represent the | |

1054 | mixing between the photon, Z, techni_rho0, and techni_omega | |

1055 | particles in the Drell-Yan process. Process 195 is the analogous | |

1056 | process including W and techni_rho+/- mixing. By default, the final | |

1057 | state fermions are e+ e- and e+/- nu_e, respectively. These can be | |

1058 | changed through the parameters KFPR(194,1) and KFPR(195,1), | |

1059 | respectively (the KFPR value should represent a charged fermion). | |

1060 | - The full set of recommended processes are: | |

1061 | Drell--Yan (ETC == Extended TechniColor) | |

1062 | 194 f+fbar -> f'+fbar' (ETC) | |

1063 | 195 f+fbar' -> f"+fbar"' (ETC) | |

1064 | techni_rho0/omega | |

1065 | 361 f + fbar -> W_L+ W_L- | |

1066 | 362 f + fbar -> W_L+/- pi_T-/+ | |

1067 | 363 f + fbar -> pi_T+ pi_T- | |

1068 | 364 f + fbar -> gamma pi_T0 | |

1069 | 365 f + fbar -> gamma pi_T0' | |

1070 | 366 f + fbar -> Z0 pi_T0 | |

1071 | 367 f + fbar -> Z0 pi_T0' | |

1072 | 368 f + fbar -> W+/- pi_T-/+ | |

1073 | charged techni_rho | |

1074 | 370 f + fbar' -> W_L+/- Z_L0 | |

1075 | 371 f + fbar' -> W_L+/- pi_T0 | |

1076 | 372 f + fbar' -> pi_T+/- Z_L0 | |

1077 | 373 f + fbar' -> pi_T+/- pi_T0 | |

1078 | 374 f + fbar' -> gamma pi_T+/- | |

1079 | 375 f + fbar' -> Z0 pi_T+/- | |

1080 | 376 f + fbar' -> W+/- pi_T0 | |

1081 | 377 f + fbar' -> W+/- pi_T0' | |

1082 | - All of the processes from 361 to 377 can be accessed at once | |

1083 | by setting MSEL=50. | |

1084 | - The production and decay rates depend on several "Straw Man" | |

1085 | technicolor parameters: | |

1086 | Techniparticle masses | |

1087 | PMAS(51,1) : (D=110.0 GeV) neutral techni_pi mass | |

1088 | PMAS(52,1) : (D=110.0 GeV) charged techni_pi mass | |

1089 | PMAS(53,1) : (D=110.0 GeV) neutral techni_pi' mass | |

1090 | PMAS(54,1) : (D=210.0 GeV) neutral techni_rho mass | |

1091 | PMAS(55,1) : (D=210.0 GeV) charged techni_rho mass | |

1092 | PMAS(56,1) : (D=210.0 GeV) techni_omega mass | |

1093 | Note: the rho and omega masses are not pole masses | |

1094 | Lagrangian parameters | |

1095 | PARP(141) : (D= 0.33333) sine of chi, the mixing angle between | |

1096 | technipion interaction and mass eigenstates | |

1097 | PARP(142) : (D=82.0000 GeV) F_T, the technipion decay constant | |

1098 | PARP(143) : (D= 1.3333) Q_U, charge of up-type technifermion; | |

1099 | the down-type technifermion has a charge Q_D=Q_U-1 | |

1100 | PARP(144) : (D= 4.0000) N_TC, number of technicolors; fixes the | |

1101 | relative values of g_em and g_etc | |

1102 | PARP(145) : (D= 1.0000) C_c, coefficient of the technipion decays | |

1103 | to charm; appears squared in the decay rate | |

1104 | PARP(146) : (D= 1.0000) C_b, coefficient of the technipion decays | |

1105 | to bottom; appears squared in the decay rate | |

1106 | PARP(147) : (D= 0.0182) C_t, coefficient of the technipion decays | |

1107 | to top, estimated to be m_b/m_t; appears squared in the decay rate | |

1108 | PARP(148) : (D= 1.0000) C_tau, coefficient of the technipion decays | |

1109 | to tau; appears squared in the decay rate | |

1110 | PARP(149) : (D=0.00000) C_pi, coefficient of technipion decays | |

1111 | to gg | |

1112 | PARP(150) : (D=1.33333) C_pi', coefficient of technipion' decays | |

1113 | to gg | |

1114 | ****Note the switch from PARP to PARJ**** | |

1115 | PARJ(172) : (D=200.000 GeV) M_V, vector mass parameter for technivector | |

1116 | decays to transverse gauge bosons and technipions | |

1117 | PARJ(173) : (D=200.000 GeV) M_A, axial mass parameter for technivector | |

1118 | decays to transverse gauge bosons and technipions | |

1119 | PARJ(174) : (D=0.33300) sine of chi', the mixing angle between | |

1120 | the technipion' interaction and mass eigenstates | |

1121 | PARJ(175) : (D=0.05000) isospin violating technirho/techniomega | |

1122 | mixing amplitude | |

1123 | - As a final comment, it is worth mentioning that the decays products | |

1124 | of the W and Z bosons are distributed according to phase space, | |

1125 | regardless of their designation as W_L/Z_L or transverse gauge bosons. | |

1126 | The exact meaning of longitudinal or transverse polarizations in this | |

1127 | case requires more thought. | |

1128 | - References: | |

1129 | K. Lane, hep-ph/9903369 | |

1130 | K. Lane, hep-ph/9903372 | |

1131 | ||

1132 | * Several new processes for doubly charged Higgs production in | |

1133 | left-right-symmetric models, with an additional righthanded SU(2) | |

1134 | gauge group. | |

1135 | - 341 l + l -> H_L++/-- | |

1136 | 342 l + l -> H_R++/-- | |

1137 | 343 l + gamma -> H_L++/-- + e-/+ | |

1138 | 344 l + gamma -> H_R++/-- + e-/+ | |

1139 | 345 l + gamma -> H_L++/-- + mu-/+ | |

1140 | 346 l + gamma -> H_R++/-- + mu-/+ | |

1141 | 347 l + gamma -> H_L++/-- + tau-/+ | |

1142 | 348 l + gamma -> H_R++/-- + tau-/+ | |

1143 | 349 f + fbar -> H_L++ + H_L-- | |

1144 | 350 f + fbar -> H_R++ + H_R-- | |

1145 | 351 f_i + f_j -> f_k + f_l + H_L++/-- | |

1146 | 352 f_i + f_j -> f_k + f_l + H_R++/-- | |

1147 | - Default model masses are | |

1148 | code name mass (GeV) | |

1149 | 61 H_L++ 200 | |

1150 | 62 H_R++ 200 | |

1151 | 63 W_R+ 750 | |

1152 | 64 nu_Re 750 | |

1153 | 65 nu_Rmu 750 | |

1154 | 66 nu_Rtau 750 | |

1155 | - Main decays implemented are | |

1156 | H_L++ -> l_i+ l_j+ (i, j generation index) | |

1157 | -> W_L+ W_L+ | |

1158 | H_R++ -> l_i+ l_j+ | |

1159 | -> W_R+ W_R+ | |

1160 | W_R+ -> q_i qbar_j (assuming standard CKM matrix) | |

1161 | -> l_i+ nu_Ri (if kinematically allowed) | |

1162 | - The physics parameters of this scenario are | |

1163 | PARP(181) - PARP(189) : (D = 0.1, 0.01, 0.01, 0.01, 0.1, 0.01, | |

1164 | 0.01, 0.01, 0.3) Yukawa couplings of leptons to H++, assumed | |

1165 | same for H_L++ and H_R++. Is a symmetric 3*3 array, where | |

1166 | PARP(177+3*i+j) gives the coupling to a lepton pair with | |

1167 | generation indices i and j. Thus the default matrix is | |

1168 | dominated by the diagonal elements and especially by the | |

1169 | tau-tau one. | |

1170 | PARP(190) : (D=0.64) g_L = e/sin(theta_W). | |

1171 | PARP(191) : (D=0.64) g_R, assumed same as g_L. | |

1172 | PARP(192) : (D=5 GeV) v_L vacuum expectation value of the | |

1173 | left-triplet. The corresponding v_R is assumed given by | |

1174 | v_R = sqrt(2) M_W_R / g_R and is not stored explicitly. | |

1175 | - The main references are | |

1176 | K. Huitu, J. Maalampi, A. Pietil\"a and M. Raidal, | |

1177 | Nucl. Phys. B487 (1997) 27 and private communication; | |

1178 | G. Barenboim, K. Huitu, J. Maalampi and M. Raidal, | |

1179 | Phys. Lett. B394 (1997) 132. | |

1180 | ||

1181 | * Two new processes for chi_c production. | |

1182 | - 104 g + g -> chi_0c | |

1183 | 105 g + g -> chi_2c | |

1184 | - These are the lowest-order equivalents of processes 87 and 89. | |

1185 | Note that g + g -> chi_1c is forbidden, and so not included as | |

1186 | a match to process 88. | |

1187 | - Reference Bai83. | |

1188 | ||

1189 | * Three new processes for J/psi production. | |

1190 | - 106 g + g -> J/psi + gamma | |

1191 | 107 g + gamma -> J/psi + g | |

1192 | 108 gamma + gamma -> J/psi + gamma | |

1193 | - All of these are closely related to the existing process 86, | |

1194 | g + g -> J/psi + g; only the colour- and charge-related | |

1195 | prefactors differ in the matrix element expressions. | |

1196 | - References: | |

1197 | 106: M. Drees and C.S. Kim, Z. Phys. C53 (1991) 673 | |

1198 | 107: E.L. Berger and D. Jones, Phys. Rev. D23 (1981) 1521 | |

1199 | 108: H. Jung, private communication; | |

1200 | H. Kharraziha, private communication | |

1201 | ||

1202 | * Process 1 has been modified so that masses are included in the | |

1203 | expression for the decay polar angle distribution. | |

1204 | ||

1205 | * Processes 15, 19, 22, 30 and 35 have been corrected for an inconsistent | |

1206 | use of width definition in the Breit-Wigner shape, which affected | |

1207 | the low-mass part of the gamma*/Z0 spectrum. | |

1208 | ||

1209 | * The previous process 131, g + g -> Z + b + bbar, has been removed, | |

1210 | for reasons of inefficient phase space generation. This implies that | |

1211 | - all the RK... routines are gone | |

1212 | - all code related to ISET(ISUB)=6 is removed | |

1213 | - variables MINT(35) and VINT(81-84) are unused | |

1214 | - cuts CKIN(51-56) are restricted in usage | |

1215 | - options 5 and 6 of PYOFSH are removed (with old option 7 moved to 5) | |

1216 | ||

1217 | * Processes 147, 148, 167 and 168 have been modified to take into | |

1218 | account changes in d*, u*, e* and nu*_e codes. | |

1219 | ||

1220 | * New parameter and new default behaviour of the program. | |

1221 | MSTP(9) : (D=0) inclusion of top (and fourth generation quarks) as | |

1222 | allowed remnant flavours q' in processes that involve q -> q' + W | |

1223 | branchings and where the matrix elements have been calculated | |

1224 | under the assumption that q' is massless. | |

1225 | = 0 : no. | |

1226 | = 1 : yes, but it is possible, as before, to switch off individual | |

1227 | channels by the setting of MDME switches. Mass effects are | |

1228 | taken into account, in a crude fashion, by rejecting events | |

1229 | where kinematics becomes inconsistent when the q' mass is | |

1230 | included. | |

1231 | ||

1232 | * Many processes proceed via an s-channel resonance, described by a | |

1233 | Breit-Wigner. In some instances this description is not really valid | |

1234 | far away from the resonance position, e.g. because interference with | |

1235 | other graphs should then be included. The wings of the Breit-Wigner | |

1236 | are therefore routinely cut out in most processes, though not all. | |

1237 | This cut has been modified and can now be set by the user. | |

1238 | PARP(48) : (D=50.) the Breit-Wigner factor in the cross section is | |

1239 | set to vanish for masses that deviate from the nominal one by | |

1240 | more than PARP(48) times the nominal resonance width (i.e. the | |

1241 | width evaluated at the nominal mass). Is used in most processes | |

1242 | with a single s-channel resonance, but there are some exceptions, | |

1243 | notably gamma/Z0 and W+-. | |

1244 | ||

1245 | * The PYSTAT routine has been expanded with a new option 6. | |

1246 | A CALL PYSTAT(6) will produce a list of all subprocesses implemented | |

1247 | in the program. | |

1248 | ||

1249 | * New parameter. | |

1250 | PARP(104) : (D=0.8 GeV) minimum energy above threshold for the | |

1251 | evaluation of total, elastic and diffractive cross sections. | |

1252 | Below this lower cut, the cross section is made to vanish. | |

1253 | ||

1254 | ----------------------------------------------------------------------- | |

1255 | ||

1256 | THE E+E- ROUTINES | |

1257 | ||

1258 | * The e+e- routines PYEEVT and PYONIA (formerly LUEEVT and LUONIA) | |

1259 | have been kept in this version, but may disappear in the future. | |

1260 | The functionality of PYEEVT is obtained with PYTHIA subprocess 1 and | |

1261 | that of PYONIA by the decay of Upsilon in PYDECY. Some differences | |

1262 | exist between the respective alternatives. | |

1263 | - The PYEEVT flavour selection and resonance shape handling is not as | |

1264 | good as the subprocess 1 one. | |

1265 | - The PYEEVT initial-state photon radiation is based on exact first | |

1266 | order rather than exponentiated structure functions, and is inferior | |

1267 | in terms of total photon energy radiated but may be better for | |

1268 | high-angle photons (here subprecess 19 can also be used, however). | |

1269 | - Further examples could be given. | |

1270 | ||

1271 | * Formerly, the main difference was that LUEEVT also had a number of | |

1272 | matrix-element options, in addition to the default parton-shower one. | |

1273 | These options are now available also from PYTHIA subprocess 1, | |

1274 | as follows. | |

1275 | - MSTP(48) : (D=0) switch for the treatment of gamma*/Z0 decay for | |

1276 | process 1 in e+e- events. | |

1277 | = 0 : normal PYTHIA machinery. | |

1278 | = 1 : if the decay of the Z0 is to either of the five lighter | |

1279 | quarks, d, u, s, c or b, the special treatment of Z0 | |

1280 | decay is accessed, including matrix element options. | |

1281 | - This option is based on the machinery of the PYEEVT and associated | |

1282 | routines when it comes to the description of QCD multijet structure | |

1283 | and the angular orientation of jets, but relies on the normal | |

1284 | PYEVNT machinery for everything else: cross section calculation, | |

1285 | initial state photon radiation, flavour composition of decays | |

1286 | (i.e. information on channels allowed), etc. | |

1287 | - The initial state has to be e+e; forward-backward asymmetries would | |

1288 | not come out right for quark-annihilation production of the gamma*/Z0 | |

1289 | and therefore the machinery defaults to the standard one in such | |

1290 | cases. | |

1291 | - You can set the behaviour for the MSTP(48) option using the normal | |

1292 | matrix element related switches. This especially means MSTJ(101) for | |

1293 | the selection of first- or second-order matrix elements (=1 and =2, | |

1294 | respectively). Further selectivity is obtained with the switches | |

1295 | and parameters MSTJ(102) (for the angular orientation part only), | |

1296 | MSTJ(103) (except the production threshold factor part), MSTJ(106), | |

1297 | MSTJ(108) - MSTJ(111), PARJ(121), PARJ(122), PARJ(125) - PARJ(129). | |

1298 | Information can be read from MSTJ(120), MSTJ(121), PARJ(150), | |

1299 | PARJ(152) - PARJ(156), PARJ(168), PARJ(169), PARJ(171). | |

1300 | - The other e+e- switches and parameters should not be touched. In most | |

1301 | cases they are simply not accessed, since the related part is handled | |

1302 | by the PYEVNT machinery instead. In other cases they could give | |

1303 | incorrect or misleading results. Beam polarization as set by | |

1304 | PARJ(131) - PARJ(134), for instance, is only included for the | |

1305 | angular orientation, but is missing for the cross section information. | |

1306 | PARJ(123) and PARJ(124) for the Z0 mass and width are set in the | |

1307 | PYINIT call based on the input mass and calculated widths. | |

1308 | - The cross section calculation is unaffected by the matrix element | |

1309 | machinery. Thus also for negative MSTJ(101) values, where only specific | |

1310 | jet multiplicities are generated, the PYSTAT cross section is the | |

1311 | full one. | |

1312 | ||

1313 | ----------------------------------------------------------------------- | |

1314 | ||

1315 | PARTON DISTRIBUTIONS | |

1316 | ||

1317 | * The "structure function" expression is replaced by "parton distribution". | |

1318 | Hence routines PYST** are renamed PYPD**. | |

1319 | ||

1320 | * A number of old proton distributions have been removed, and newer ones | |

1321 | inserted. The current list of distributions available in MSTP(51) is | |

1322 | - MSTP(51) : (D=4) | |

1323 | = 1 : CTEQ 3L (leading order). | |

1324 | = 2 : CTEQ 3M (MSbar). | |

1325 | = 3 : CTEQ 3D (DIS). | |

1326 | = 4 : GRV 94L (leading order). | |

1327 | = 5 : GRV 94M (MSbar). | |

1328 | = 6 : GRV 94D (DIS). | |

1329 | = 7 : CTEQ 5L (leading order). | |

1330 | = 8 : CTEQ 5M1 (MSbar; slightly updated version of CTEQ 5M). | |

1331 | = 11: GRV 92L (leading order). | |

1332 | = 12: EHLQ 1 (leading order). | |

1333 | = 13: EHLQ 1 (leading order). | |

1334 | = 14: DO 1 (leading order). | |

1335 | = 15: DO 2 (leading order). | |

1336 | - References: CTEQ Collaboration, H.L. Lai et al., Phys. Rev. | |

1337 | D51 (1995) 4763, hep-ph/9903282; | |

1338 | M. Gluck, E. Reya and A. Vogt, Z. Phys. C67 (1995) 433 | |

1339 | - New routines: PYCTEQ, PYGRVL, PYGRVM, PYGRVD, PYGRVV, PYGRVW, PYGRVS | |

1340 | PYCT5L, PYCT5M and PYPDPO. | |

1341 | - Note 1: distributions 11-15 are obsolete and should not be used for | |

1342 | current physics studies. They are only implemented to have some sets | |

1343 | in common between Pythia 5 and 6, for crosschecks. | |

1344 | - Note 2: distribution 16 is an undocumented toy model with all parton | |

1345 | distributions like 1/x; see code for details. | |

1346 | ||

1347 | * The SaS parton distributions (accessed with MSTP(55) = 5 - 12) have | |

1348 | been upgraded from version 1 to version 2 of the SaSgam library (as | |

1349 | before, with routines and commonblocks renamed). This is no change for | |

1350 | real photons, as have been studied so far, but allows in the future | |

1351 | several new alternatives to extend the distributions to virtual photons. | |

1352 | - MSTP(60) : (D=7) extension of the SaS real-photon distributions to | |

1353 | off-shell photons, especially for the anomalous component. For | |

1354 | details, see G.A. Schuler and T. Sjostrand, Phys. Lett. B376 (1996) | |

1355 | 193. | |

1356 | = 1 : dipole dampening by integration; very time-consuming. | |

1357 | = 2 : P_0^2 = max( Q_0^2, P^2 ) | |

1358 | = 3 : P'_0^2 = Q_0^2 + P^2. | |

1359 | = 4 : P_{eff} that preserves momentum sum. | |

1360 | = 5 : P_{int} that preserves momentum and average evolution range. | |

1361 | = 6 : P_{eff}, matched to P_0 in P2 -> Q2 limit. | |

1362 | = 7 : P_{int}, matched to P_0 in P2 -> Q2 limit. | |

1363 | - The PYGGAM argument list is expanded with one further input parameter IP2 | |

1364 | SUBROUTINE PYGGAM(ISET,X,Q2,P2,IP2,F2GM,XPDFGM) | |

1365 | where MSTP(60) is used as IP2 value in internal calls. | |

1366 | - PYGVMD and PYGANO has VXPGA(-6:6) as new last argument. The array contains | |

1367 | the valence part ofd the distributions at return. | |

1368 | - COMMON/PYINT9/VXPVMD(-6:6),VXPANL(-6:6),VXPANH(-6:6),VXPDGM(-6:6) | |

1369 | Purpose: to give the valence parts of the photon parton distributions | |

1370 | (x-weighted, as usual) when the PYGGAM routine is called. Companion to | |

1371 | /PYINT8/, which gives the total parton distributions. | |

1372 | VXPVMD(KFL) : valence distributions of the VMD part; matches | |

1373 | XPVMD in /PYINT8/. | |

1374 | VXPANL(KFL) : valence distributions of the anomalous part of light | |

1375 | quarks; matches XPANL in /PYINT8/. | |

1376 | VXPANH(KFL) : valence distributions of the anomalous part of heavy | |

1377 | quarks; matches XPANH in /PYINT8/. | |

1378 | VXPDGM(KFL) : gives the sum of valence distributions parts; | |

1379 | matches XPDFGM in the PYGGAM call. | |

1380 | Note 1: the Bethe-Heitler and direct contributions in XPBEH(KFL) and | |

1381 | XPDIR(KFL) in /PYINT8/ are pure valence-like, andtherefore are not | |

1382 | duplicated here. | |

1383 | Note 2: the sea parts of the distributions can be obtained by taking the | |

1384 | appropriate differences between the total distributions and the | |

1385 | valence distributions. | |

1386 | ||

1387 | * The default set for the parton distributions of the pion has been changed: | |

1388 | MSTP(53) (D=3) choice of pion parton distribution set; default is | |

1389 | GRV LO (updated version). | |

1390 | ||

1391 | * New argument KFA for PYPDEL routine, which now also can handle muons and | |

1392 | taus. | |

1393 | ||

1394 | ----------------------------------------------------------------------- | |

1395 | ||

1396 | PARTON SHOWERS | |

1397 | ||

1398 | * PYSHOW allows the photon emission cutoff parameter to be set | |

1399 | separately for quarks and leptons. The former function remains with | |

1400 | PARJ(83), while the latter introduces the new parameter PARJ(90), | |

1401 | with default 0.0001 GeV. Thus photon emission off leptons becomes | |

1402 | more realistic, covering a larger part of the phase space. Since | |

1403 | the lepton mass is not explicitly included in the shower formalism, | |

1404 | the emission rate is still not well reproduced (underestimated!) for | |

1405 | lepton-photon invariant masses smaller than roughly twice the | |

1406 | lepton mass itself. | |

1407 | ||

1408 | * PYSHOW has been modified and expanded with new options related to | |

1409 | mass effects in the shower. | |

1410 | - First, the emission of gluons off primary quarks in gamma*/Z0 decays | |

1411 | has been modified. Specifically, the matrix-element correction factor | |

1412 | obtained for MSTJ(47)=2 or 3 (default) has been modified, to better | |

1413 | take into account how the shower populates the phase space for | |

1414 | massive quarks (which is used as denominator if the corrective | |

1415 | weight). This increases the amount of gluon radiation, so that e.g. | |

1416 | the amount of b bbar g three-jet events at LEP1 (within some | |

1417 | reasonable 3-jet region) goes up by about 5%. Light quarks are not | |

1418 | affected. | |

1419 | - Second, the description of g -> q qbar branchings has been expanded | |

1420 | with several new options, in order to explore a larger range of | |

1421 | uncertainty in predictions. | |

1422 | MSTJ(42) (D=2) coherence level in shower. | |

1423 | = 3 : in the definition of the angle in a g -> q qbar | |

1424 | branchings, the naive massless expression is reduced by a | |

1425 | factor sqrt(1 - 4 m_q^2/m_g^2), which can be motivated | |

1426 | by a corresponding actual reduction in the p_T by mass | |

1427 | effects. The requirement of angular ordering then kills | |

1428 | fewer potential g -> q qbar branchings, i.e. the rate of | |

1429 | such comes up. The q -> q g and g -> g g branchings are not | |

1430 | changed from =2. This option is fully within the range of | |

1431 | uncertainty that exists. | |

1432 | = 4 : no angular ordering requirement conditions at all are | |

1433 | imposed on g -> q qbar branchings, while angular ordering | |

1434 | still is required for q -> q g and g -> gg. This is an | |

1435 | unrealistic extreme, and results obtained with it should | |

1436 | not be overstressed. However, for some studies it is of | |

1437 | interest. For instance, it not only gives a much higher | |

1438 | rate of charm and bottom production in showers, but also | |

1439 | affects the kinematical distributions of such pairs. | |

1440 | MSTJ(44) (D=2) alpha-strong argument in shower. | |

1441 | = 3 : While pT^2 is used as alpha_strong argument in q -> q g | |

1442 | and g -> gg branchings, as in =2, instead m^2/4 is used as | |

1443 | argument for g -> q qbar ones. The argument is that the | |

1444 | soft-gluon resummation results suggesting the pT^2 scale | |

1445 | in the former processes is not valid for the latter one, | |

1446 | so that any multiple of the mass of the branching parton | |

1447 | is a perfectly valid alternative. The m^2/4 ones then gives | |

1448 | continuity with pT^2 for z=1/2. Furthermore, with this | |

1449 | choice,it is no longer necessary to have the requirement | |

1450 | of a minimum pT in branchings, else required in order to | |

1451 | avoid having alpha_strong blow up. Therefore, in this option, | |

1452 | that cut has been removed for g -> gg branchings. | |

1453 | Specifically, when combined with MSTJ(42)=4, it is possible | |

1454 | to reproduce the naive 1 + cos^2(theta) angular distribution | |

1455 | of g -> gg branchings, which is not possible in any other | |

1456 | approach. (However, as noted above, it may give too high | |

1457 | a charm and bottom production rate in showers.) | |

1458 | ||

1459 | * PYSSPA is improved by new scheme for merging matrix-element and | |

1460 | parton-shower descriptions of initial-state radiation in the | |

1461 | production of a single gauge-boson resonance, i.e. Z/gamma*, W, | |

1462 | Z', W' and R. Also for other processes the possibility of changing | |

1463 | the maximal scale of shower radiation is introduced, although here | |

1464 | without the complete matrix-element correction machinery. The scheme | |

1465 | is based on the study by | |

1466 | G. Miu and T. Sjostrand, LU TP 98-30 and hep-ph/9812455; | |

1467 | see also G. Miu, LU TP 98-9, hep-ph/9804317. | |

1468 | As a consequence, the MSTP(68) variable takes on a new meaning. | |

1469 | MSTP(68) : (D=1) choice of maximum virtuality scale and matrix-element | |

1470 | matching scheme for initial-state radiation. | |

1471 | = 0 : maximum shower virtuality is the same as the Q2 choice for | |

1472 | the parton distributions, see MSTP(32). (Except that the | |

1473 | multiplicative extra factor PARP(34) is absent and instead | |

1474 | PARP(67) can be used for this purpose. No matrix-element | |

1475 | correction. | |

1476 | = 1 : as =0 for most processes, but new scheme for processes 1, 2, | |

1477 | 141, 142 and 144, i.e. single s-channel colourless gauge boson | |

1478 | production: gamma*/Z0, W+-, Z', W'+- and R. Here the maximum | |

1479 | scale of shower evolution is s, the total squared energy. | |

1480 | The nearest branching on either side of the hard scattering, | |

1481 | of the types q -> q + g, f -> f + gamma, g -> q + qbar or | |

1482 | gamma -> f + fbar, are corrected by the ratio of the | |

1483 | first-order matrix-element weight to the parton-shower one, | |

1484 | so as to obtain an improved description. See the references | |

1485 | above for a detailed description. Note that the improvements | |

1486 | apply both for incoming hadron and lepton beams. | |

1487 | = 2 : the maximum scale for initial-state shower evolution is always | |

1488 | selected to be s, except for the 2 -> 2 QCD processes 11, 12, | |

1489 | 13, 28, 53 and 68. Based on the experience in the references | |

1490 | above, there is reason to assume that this does give an | |

1491 | improved qualitative description of the high-pT tail, although | |

1492 | the quantitative agreement is currently beyond our control. | |

1493 | No matrix-element corrections, even for the processes in =1. | |

1494 | The QCD exception is to avoid the doublecounting issues that | |

1495 | could easily arise here. | |

1496 | = -1 : as =0, except that there is no requirement on uhat being | |

1497 | negative, see point 2 below. | |

1498 | Note that the default behaviour of PYSSPA is changed, in several | |

1499 | respects. | |

1500 | 1) For the processes listed in MSTP(68)=1, as a consequence of the new | |

1501 | default MSP(68) value (the old one was =0). This clearly is very | |

1502 | important for the high-pT tail and hence implies a significant | |

1503 | improvement here. | |

1504 | 2) A new cut is imposed on the combination of z and Q2 values | |

1505 | in a branching, | |

1506 | uhat = Q2 - shat_old * (1-z)/z = Q2 - shat_new * (1-z) < 0, | |

1507 | where the association with the uhat variable is relevant if the branching | |

1508 | is reinterpreted in terms of a 2 -> 2 scattering. Usually such a | |

1509 | requirement comes out of the kinematics, and therefore is imposed | |

1510 | eventually anyway. The corner of emissions that do not respect this | |

1511 | requirement is that where the Q2 value of the spacelike emitting | |

1512 | parton is little changed and the z value of the branching is close | |

1513 | to unity. (That is, such branchings are kinematically allowed, but | |

1514 | since the mapping to matrix-element variables would assume the first | |

1515 | parton to have Q2=0, this mapping gives an unphysical uhat, and hence | |

1516 | no possibility to impose a matrix-element correction factor.) The | |

1517 | correct behaviour in this region is beyond leading-log preditivity. | |

1518 | It is mainly important for the hardest emission, i.e. with largest Q2. | |

1519 | The effect of this change is to reduce the total amount of emission | |

1520 | by a non-negligible amount when no matrix-element correction is applied. | |

1521 | (Witness results with the special option MSTP(68)=-1.) For matrix-element | |

1522 | corrections to be applied, this requirement must be used for the hardest | |

1523 | branching, and then whether it is used or not for the softer ones is | |

1524 | less relevant. | |

1525 | 3) The PARP(65) parameter for minimum gluon energy emitted in a | |

1526 | spacelike showers is modified by an extra factor roughly corresponding | |

1527 | to the 1/gamma factor for the boost to the hard subprocess frame. | |

1528 | Earlier, when a subsystem was strongly boosted, i.e. at large rapidities | |

1529 | in the cm frame of the collision, the PARP(65) requirement became quite | |

1530 | stringent on the low-energy incoming side. Therefore much radiation | |

1531 | could be cut out. Since this point gives changes of opposite sign to | |

1532 | point 2, the net result of both changes gives a small net result. | |

1533 | 4) The angular-ordering requirement is based on ordering p_T/p rather | |

1534 | than p_T/p_L, i.e. replacing tan(theta) by sin(theta). Earlier the | |

1535 | starting value tan(theta)_max = 10 could actually be violated by some | |

1536 | bona fide emissions for strongly boosted subsystems, where one side has | |

1537 | small p_L. Therefore some emissions were incorrectly removed when | |

1538 | MSTP(62)=3, i.e. default. | |

1539 | 5) For incoming muon (and tau) beams the kinematical variables are | |

1540 | better selected to represent the differences in lepton mass. | |

1541 | ||

1542 | * PARP(67): (D=1) new default value to better represent matching between | |

1543 | hard-process scale and initial-statee-radiation scale in QCD processes. | |

1544 | ||

1545 | * New variable MSTP(69), replacing some of the functionality previously | |

1546 | provided by MSTP(68) but removed with the change in PYSSPA with | |

1547 | Pythia 6.119: | |

1548 | MSTP(69) (D=0) possibility to change Q2 scale for parton distributions | |

1549 | from the MSTP(32) choice, especially for e+e-. | |

1550 | = 0 : use MSTP(32) scale. | |

1551 | = 1 : in lepton-lepton collisions, the QED lepton-inside-lepton | |

1552 | parton distributions are evaluated with s, the full squared CM | |

1553 | energy, as scale. | |

1554 | = 2 : s is used as parton distribution scale also in other | |

1555 | processes. | |

1556 | ||

1557 | ----------------------------------------------------------------------- | |

1558 | ||

1559 | UNDERLYING EVENTS | |

1560 | ||

1561 | * The assumption of a (more or less) energy-independent pTmin or pT0 | |

1562 | lower cut-off of jet production in multiple interactions was developed | |

1563 | in the days when parton distributions were normally assumed flat at | |

1564 | small x (i.e. x*f_i(x,Q2) -> constant for x -> 0 at small Q2). In view | |

1565 | of the HERA data this is no longer a valid assumption, and parton | |

1566 | distributions have evolved to reflect this. The consequence is that | |

1567 | the jet rate above some fixed small pTmin is increasing much faster | |

1568 | than originally assumed. If unchecked, it leads to a much too fast | |

1569 | increase in the multiple interaction rate, based on comparisons with | |

1570 | data or with physics intuition. While we have no understanding of the | |

1571 | detailed physics mechanisms, it appears sensible to introduce an | |

1572 | explicit energy-dependence of pTmin and pT0 that closely matches | |

1573 | this small-x dependence or, alternatively, the increase of the total | |

1574 | cross section by the Pomeron term. Therefore the form used internally | |

1575 | is now | |

1576 | pTmin = PARP(81) * (ECM/PARP(89))**PARP(90), alternatively | |

1577 | pT0 = PARP(82) * (ECM/PARP(89))**PARP(90), | |

1578 | with ECM the current center of mass energy. Two new parameters are | |

1579 | introduced, PARP(89) and PARP(90), and the default values of PARP(81) | |

1580 | and PARP(82) are changed. | |

1581 | PARP(81) : (D=1.9 GeV) effective mimimum transverse momentum pTmin | |

1582 | for multiple interactions with MSTP(82) = 1, at the reference | |

1583 | energy scale PARP(89), with the degree of energy rescaling | |

1584 | given by PARP(90). | |

1585 | PARP(82) : (D=2.1 GeV) regularization scale pT0 of the transverse | |

1586 | momentum spectrum for multiple interactions with MSTP(82) >= 2, | |

1587 | at the reference energy scale PARP(89), with the degree of energy | |

1588 | rescaling given by PARP(90). (Current default based on MSTP(82)=4 | |

1589 | option, without any change of MSTP(2) or MSTP(33).) | |

1590 | PARP(89) : (D=1000 GeV) reference energy scale, at which PARP(81) and | |

1591 | PARP(82) give the pTmin and pT0 values directly. Has no physical | |

1592 | meaning in itself, but is used for convenience only. (A form | |

1593 | pTmin = PARP(81) * ECM**PARP(90) would have been equally possible, | |

1594 | but then with a less transparent meaning of PARP(81).) For studies | |

1595 | of the pTmin dependence at some specific energy it is convenient to | |

1596 | choose PARP(89) equal to this energy. | |

1597 | PARP(90) : (D=0.16) power of the energy-rescaling term of the pTmin and | |

1598 | pT0 parameters, which are assumed proportional to ECM**PARP(90). | |

1599 | The default value is inspired by the rise of the total cross | |

1600 | section by the Pomeron term, s**epsilon = ECM**(2*epsilon) = | |

1601 | ECM**(2*0.08), which is not inconsistent with the small-x behaviour. | |

1602 | It is also reasonably consistent with the energy-dependence | |

1603 | implied with a comparison with the UA5 multiplicity distributions | |

1604 | at 200 and 900 GeV. PARP(90) = 0 is an allowed value, i.e. it is | |

1605 | possible to have energy-independent parameters. | |

1606 | ||

1607 | ----------------------------------------------------------------------- | |

1608 | ||

1609 | HADRONIZATION | |

1610 | ||

1611 | * Additions and changes to the baryon production models: | |

1612 | see separate section below. | |

1613 | ||

1614 | * The possibility of colour rearrangement has been introduced for | |

1615 | subprocess 25, W+W- pair production. | |

1616 | - For a description of the basic physics ideas and the scenarios | |

1617 | implemented, see | |

1618 | T. Sjostrand and V.A. Khoze, Z. Phys. C62 (1994) 281. | |

1619 | Available is also an alternative (the GH one) loosely based on | |

1620 | G. Gustafson and J. Hakkinen, Z. Phys. C64 (1994) 659. | |

1621 | - Only events where both W's decay hadronically (and not to top) | |

1622 | are affected. At most one reconnection is allowed per event. | |

1623 | - The code is based on the one available since long for | |

1624 | the PYTHIA 5.7 program as a freestanding extension. The former | |

1625 | version provided further information on where in an event a | |

1626 | reconnection occurs, but was less well integrated in the PYTHIA | |

1627 | framework than is the current implementation. | |

1628 | - A new subroutine | |

1629 | SUBROUTINE PYRECO(IW1,IW2,NSD1,NAFT1) | |

1630 | has been added with the different scenarios. It is called from | |

1631 | PYRESD where appropriate. | |

1632 | - MSTP(115) : (D=0) (C) choice of colour rearrangement scenario. | |

1633 | = 0 : no reconnection. | |

1634 | = 1 : scenario I, reconnection inspired by a type I | |

1635 | superconductor, with the reconnection probability related | |

1636 | to overlap volume in space and time between the W+ and W- | |

1637 | strings. Related parameters are found in PARP(115) - | |

1638 | PARP(119), with PARP(117) of special interest. | |

1639 | = 2 : scenario II, reconnection inspired by a type II | |

1640 | superconductor, with reconnection possible when two string | |

1641 | cores cross. Related parameter in PARP(115). | |

1642 | = 3 : scenario II', as model II but with the additional | |

1643 | requirement that a reconnection will only occur if the | |

1644 | total string length is reduced by it. | |

1645 | = 5 : the GH scenario, where the reconnection can occur that | |

1646 | reduces the total string length (Lambda measure) most. | |

1647 | PARP(120) gives the fraction of such event where a | |

1648 | reconnection is actually made; since almost all events | |

1649 | could allow a reconnection that would reduce the string | |

1650 | length, PARP(120) is almost the same as the reconnection | |

1651 | probability. | |

1652 | = 11 : the intermediate scenario, where a reconnection is | |

1653 | made at the "origin" of events, based on the subdivision | |

1654 | of all radiation of a q-qbar system as coming either from | |

1655 | the q or the qbar. PARP(120) gives the assumed probability | |

1656 | that a reconnection will occur. A somewhat simpleminded | |

1657 | model, but not quite unrealistic. | |

1658 | = 12 : the instantaneous scenario, where a reconnection is | |

1659 | allowed to occur before the parton showers, and showering | |

1660 | is performed inside the reconnected systems with maximum | |

1661 | virtuality set by the mass of the reconnected systems. | |

1662 | PARP(120) gives the assumed probability that a reconnection | |

1663 | will occur. Is completely unrealistic, but useful as an | |

1664 | extreme example with very large effects. | |

1665 | - PARP(115) : (D=1.5 fm) (C) the average fragmentation time of a | |

1666 | string, giving the exponential suppression that a reconnection | |

1667 | cannot occur if strings decayed before crossing. Is implicitly | |

1668 | fixed by the string constant and the fragmentation function | |

1669 | parameters, and so a significant change is not recommended. | |

1670 | - PARP(116) : (D=0.5 fm) (C) width of the type I string, giving the | |

1671 | radius of the Gaussian distribution in x and y separately. | |

1672 | - PARP(117) : (D=0.6) (C) k_I, the main free parameter in the | |

1673 | reconnection probability for scenario I; the probability is | |

1674 | given by PARP(117) times the overlap volume, up to saturation | |

1675 | effects. | |

1676 | - PARP(118), PARP(119) : (D=2.5,2.0) (C) f_r and f_t, respectively, | |

1677 | used in the Monte Carlo sampling of the phase space volume | |

1678 | in scenario I. There is no real reason to change these numbers. | |

1679 | - PARP(120) : (D=1.0) (D) (C) fraction of events in the GH, intermediate | |

1680 | and instantaneous scenarios where a reconnection is allowed to | |

1681 | occur. For the GH one a further suppression of the reconnection | |

1682 | rate occurs from the requirement of reduced string length in a | |

1683 | reconnection. | |

1684 | - MSTI(32) : information on whether a reconnection occured in the | |

1685 | current event; is 0 normally but 1 in case of reconnection. | |

1686 | - MINT(32) : information on whether a reconnection occured in the | |

1687 | current event; is 0 normally but 1 in case of reconnection. | |

1688 | ||

1689 | * The Bose-Einstein description is expanded with several new options. | |

1690 | The earlier global method to ensure energy conservation | |

1691 | (MSTJ(54)=0) has been replaced by a local one, i.e. the default | |

1692 | behaviour haschanged. | |

1693 | - For a description of the basic physics ideas and the scenarios | |

1694 | implemented, see | |

1695 | L. Lonnblad and T. Sjostrand, Eur. Phys. J. C2 (1998) 165. | |

1696 | - Some new switches and parameters have been added. | |

1697 | - MSTJ(53) (D=0) In e+e- -> W+W-, apply BE algorithm | |

1698 | = 0 : on all pion pairs. | |

1699 | = 1 : only on pairs were both pions come from the same W. | |

1700 | = 2 : only on pairs were the pions come from different Ws. | |

1701 | = -1 : on all pairs except unequal pions coming from | |

1702 | different Ws. | |

1703 | = -2 : when calculating balancing shifts for pions from same W, | |

1704 | only consider pairs from this W. | |

1705 | - MSTJ(54) (D=2) Alternative local energy compensation. (Notation | |

1706 | in brackets refer to the one used in the above paper.) | |

1707 | = 0 : global energy compensation (BE_0). | |

1708 | = 1 : compensate with identical pairs by negative BE | |

1709 | enhancement with a third of the radius (BE_3). | |

1710 | = 2 : ditto, but with the compensation constrained to vanish | |

1711 | at Q=0, by an addional 1-exp(-Q2*R2/4) factor (BE_32). | |

1712 | = -1 : compensate with pair giving the smallest invariant mass | |

1713 | (BE_m). | |

1714 | = -2 : compansate with pair giving the smallest string length | |

1715 | (BE_lambda). | |

1716 | - MSTJ(55) (D=0) Calculation of difference vector. | |

1717 | = 0 : in the lab frame. | |

1718 | = 1 : in the CMS of the given pair. | |

1719 | - MSTJ(56) (D=0) In e+e- -> W+W-, include distance between W's. | |

1720 | = 0 : radius is the same for all pairs. | |

1721 | = 1 : radius for pairs from different W's is R+deltaR_WW. | |

1722 | (When considering W pairs with an energy well above threshold, | |

1723 | this should give more realistic results.) | |

1724 | - MSTJ(57) (D=1) Penalty for shifting particles with close-by | |

1725 | identical neighbors in local energy compensation, MSTJ(54) < 0. | |

1726 | = 0 : no penalty. | |

1727 | = 1 : penalty. | |

1728 | - PARJ(95) :(R) Set to the energy imbalance after the BE algorithm, | |

1729 | before rescaling of momenta. | |

1730 | - PARJ(96) : (R) Set to the alpha needed to retain energy-momentum | |

1731 | conservation in each event for relevant models. | |

1732 | ||

1733 | * Particle data has in part been updated to the PDG 1996 edition | |

1734 | (Particle Data Group, R.M. Barnett et al.,Phys. Rev. D54 (1996) 1. | |

1735 | Not yet updated are the weakly decaying charm and bottom hadron | |

1736 | branching ratios - here new information is added continuously, but | |

1737 | still without a coherent picture. (In the PDG attempts at constrained | |

1738 | fits, i.e. the numbers relevent for generators, 24% of D0 decays | |

1739 | are left unexplained, 36% of D+, 82% of D_s, 93% of Lambda_c, | |

1740 | and no attempt at all is made in the bottom sector.) | |

1741 | ||

1742 | * A new decay channel may be selected in PYDECY after 200 failures. | |

1743 | ||

1744 | ----------------------------------------------------------------------- | |

1745 | ||

1746 | BARYON PRODUCTION MODELS | |

1747 | ||

1748 | * New advanced scheme for baryon production with the popcorn mechanism, | |

1749 | plus some minor changes in the default older popcorn scheme. | |

1750 | - New code written by Patrik Eden, patrik@thep.lu.se. | |

1751 | - For a description of the new popcorn scheme, see | |

1752 | P. Eden and G. Gustafson, Z. Phys. C75 (1997) 41. | |

1753 | ||

1754 | * The default baryon production option, MSTJ(12)=2, is not changed in | |

1755 | any significant way. Advance warning is given, however, that the | |

1756 | default may be changed in future versions, at least to MSTJ(12)=3 | |

1757 | and possibly to MSTJ(12)=5. | |

1758 | ||

1759 | * Three new features for the baryon production are introduced. | |

1760 | - Improved treatment of SU(6) symmetry requirements. | |

1761 | + If q -> B + (qq)bar is SU(6)-rejected it may now change to | |

1762 | q -> M + q'. | |

1763 | + If qq -> M + qq', SU(6) symmetry is included in the weights for qq'; | |

1764 | qq is kept with unit probability. | |

1765 | + As before, qq is kept and only q is reselected when qq -> B + qbar | |

1766 | is SU(6)-rejected. | |

1767 | + As before, the joining qq + q -> B (when joining the two string | |

1768 | sides) suffers no SU(6) suppression. | |

1769 | The arguments for this procedure are presented below. It should not | |

1770 | be regarded as a new model, rather a more correct implementation of | |

1771 | the old. However, in order to enable the user to see the effects of | |

1772 | the SU(6) weighting separately, both procedures are available as | |

1773 | different options. | |

1774 | - Suppression of diquark vertices with small Gamma values. This is | |

1775 | based on a study of the production dynamics of the three quarks that | |

1776 | form a baryon. The main experimental consequence is a suppression | |

1777 | of the baryon production rate at large momentum fraction. | |

1778 | - New flavour algorithm for baryons and popcorn (also using the | |

1779 | small-Gamma suppression). While the old popcorn alternative allowed | |

1780 | at most one meson to be produced in between the baryon and the | |

1781 | antibaryon, the new model allows an arbitrary number. The new | |

1782 | flavour model makes explicit use of the popcorn suppression | |

1783 | exp(-2*m_T*M_T/k), where m_T is the transverse mass of the quark | |

1784 | creating the colour fluctuation, M_T is the total invariant | |

1785 | transverse mass of the popcorn meson system, and k is the string | |

1786 | tension constant. Thus two parameters, representing the mean | |

1787 | 2*m_T/k for light quarks and s-quarks, respectively, governs both | |

1788 | diquark and popcorn meson production. A corresponding parameter is | |

1789 | introduced for the fragmentation of strings that contain diquarks | |

1790 | already from the beginning, i.e. baryon remnants. | |

1791 | ||

1792 | * The arguments for the the new flavour SU(6) rules are as follows. | |

1793 | - In case of rejection due to SU(6) when q -> B + (qq)bar, one again | |

1794 | chooses between a diquark or a quark. If choosing diquark, a new one | |

1795 | is selected and tested, etc. In earlier versions of JETSET and PYTHIA, | |

1796 | the algorithm was instead to always produce a new diquark if the | |

1797 | previous one had been rejected. This leads to a slightly faster | |

1798 | algorithm and a better interpretation of the input parameter for the | |

1799 | diquark-to-quark production rate. However, the probability that a | |

1800 | quark will produce a baryon and a antidiquark is then flavour | |

1801 | independent, which is not in agreement with the model. With | |

1802 | JETSET 7.4 default values, this leads e.g. to an enhancement of the | |

1803 | Omega- relative to primary proton production with approximately a | |

1804 | factor 1.2. | |

1805 | - When selecting flavours according to the popcorn model for | |

1806 | qq -> M + qq', the quark coming from the accepted qq is kept, and the | |

1807 | other member of qq', as well as the spin of qq', is chosen with weights | |

1808 | taking SU(6) symmetry into account. Thus the flavour of qq is not | |

1809 | influenced by SU(6) factors for qq', but the popcorn meson is. | |

1810 | - When a diquark has been fitted into a symmetrical three-particle | |

1811 | state, it should not suffer any further SU(6) suppressions. Thus the | |

1812 | accompanying antidiquark should "survive" with unit probability. When | |

1813 | producing a quark to go with a previously produced diquark, this is | |

1814 | achieved by testing the configuration against the proper SU(6) factor, | |

1815 | and in case of rejection keep the diquark and pick a new quark, which | |

1816 | then is tested, etc. | |

1817 | - There is no obvious corresponding algorithm available when a quark | |

1818 | from one side and a diquark from the other are joined to form the | |

1819 | last hadron of the string. In this case the quark is a member of a | |

1820 | pair, in which the antiquark already has formed a specific hadron. | |

1821 | Thus the quark flavour cannot be reselected. One could consider the | |

1822 | SU(6) rejection as a major joining failure, and restart the | |

1823 | fragmentation of the original string, but then the the already accepted | |

1824 | diquark DOES suffer extra SU(6) suppression. In the program the joining | |

1825 | of a quark and a diquark is always accepted. | |

1826 | ||

1827 | * While the default behaviour of the older diquark and popcorn | |

1828 | scenarios is essentially unchanged, the new implementation of baryon | |

1829 | production does imply some minor differences also here. | |

1830 | - The "brute force" suppression of rank 1 baryons by a paramter PARJ(19), | |

1831 | is no longer a special option (previously MSTJ(12)=3). For backward | |

1832 | compatibility, it is however not removed. Instead it is in fact always | |

1833 | on, but is effectively off by keeping PARJ(19)=1, its default value. | |

1834 | - New, but fairly equivalent treatment of baryon production in closed | |

1835 | strings. (Calling the probability for diquark production x, the | |

1836 | probability for baryon production is changed from x at 1st vertex | |

1837 | and (1-x)x at 2nd, to 0 at 1st and x+(1-x)x at 2nd.) | |

1838 | - New treatment of random flavour selection - the 'rndmflav' decay | |

1839 | product - in hadron decays. If the production of daughters fails, | |

1840 | a new rndmflav is now selected. Previously the same one was used | |

1841 | until successful. (This is changed for the following reason: if | |

1842 | rndmflav is a diquark, at least one BB~ pair is produced, which | |

1843 | makes it more difficult to fulfill energy conservation, especially | |

1844 | if the decaying hadron is light.) | |

1845 | - New treatment of a low-mass closed string = cluster -> 2 hadrons. | |

1846 | (If splitting the cluster by a diquark, the old model approximation | |

1847 | of only one popcorn meson means that only one member of the | |

1848 | diquark-antidiquark pair should be allowed to split to a popcorn | |

1849 | meson. This is accounted for when splitting larger closed strings | |

1850 | in PYSTRF, and when selecting rndmflav's in PYDECY. However, | |

1851 | it was previously not done in PYPREP.) | |

1852 | ||

1853 | * How to use the baryon production options. | |

1854 | - Use of the old diquark and popcorn models, MSTJ(12) = 1 and 2, is | |

1855 | essentially unchanged. Note, however, that PARJ(19) is available | |

1856 | for an ad-hoc suppression of first-rank baryon production. | |

1857 | - Use of the old popcorn model with new SU(6) weighting: | |

1858 | + Set MSTJ(12)=3. | |

1859 | + Increase PARJ(1) by approximately a factor 1.2 to retain about the | |

1860 | same effective baryon production rate as in MSTJ(12)=2. | |

1861 | + Note: the new SU(6) weighting e.g. implies that the total | |

1862 | production rate of charm and bottom baryons is reduced. | |

1863 | - Use of the old flavour model with new SU(6) treatment and modified | |

1864 | fragmentation function for diquark vertices (which softens baryon | |

1865 | spectra): | |

1866 | + Set MSTJ(12)=4. | |

1867 | + Increase PARJ(1) by about a factor 1.7 and PARJ(5) by about a | |

1868 | factor 1.2 to restore the baryon and popcorn rates of the | |

1869 | MSTJ(12)=2 default. | |

1870 | - Use of the new flavour model (automatically with modified diquark | |

1871 | fragmentation function.) | |

1872 | + Set MSTJ(12)=5. | |

1873 | + Increase PARJ(1) by approximately a factor 2. | |

1874 | + Change PARJ(18) from 1 to approx. 0.19. | |

1875 | + Instead of PARJ(3-7), tune PARJ(8-10,18). (Here PARJ(10) is used | |

1876 | only in collisions having remnants of baryon beam particles.) | |

1877 | + Note: the proposed parameter values are based on a global fit to | |

1878 | all baryon production rates. This e.g. means that the proton rate | |

1879 | is lower than in the MSTJ(12)=2 option, with current data | |

1880 | somewhere in between. The PARJ(1) value would have to be about | |

1881 | 3 times higher in MSTJ(12)=5 than in =2 to have the same total | |

1882 | baryon production rate (=proton+neutron), but then other baryon | |

1883 | rates would not match at all. | |

1884 | - The new options MSTJ(12)=4 and =5 (and, to some extent, =3) soften | |

1885 | baryon spectra in such a way that PARJ(45) (delta-a for diquarks in | |

1886 | the Lund symmetric fragmentation function) is available for a retune. | |

1887 | It affects i.e. baryon-antibaryon rapidity correlations and the | |

1888 | baryon excess over antibaryons in quark jets. | |

1889 | ||

1890 | * Changes in and additions to the commonblocks. | |

1891 | MSTU(121-125) : Internal flags and counters; only MSTU(123) may be | |

1892 | touched by user. | |

1893 | MSTU(121) : Popcorn meson counter. | |

1894 | MSTU(122) : Points at the proper diquark production weights, to | |

1895 | distinguish between ordinary popcorn and rank 0 diquark | |

1896 | systems. Only needed if MSTJ(12)=5. | |

1897 | MSTU(123) : Initalization flag. If MSTU(123) is 0 in a PYKFDI call, | |

1898 | PYKFIN is called and MSTU(123) set to 1. Would need to be | |

1899 | reset by the user if flavour parameters are changed in the | |

1900 | middle of a run. | |

1901 | MSTU(124) : First parton flavour in decay call, stored to easily | |

1902 | find random flavour partner in a popcorn system. | |

1903 | MSTU(125) : Maximum number of popcorn mesons allowed in decay flavour | |

1904 | generation. If a larger popcorn system passes the fake string | |

1905 | suppressions, the error KF=0 is returned and the flavour | |

1906 | generation for the decay is restarted. | |

1907 | MSTU(131-140) : Store of popcorn meson flavour codes in decay algorithm. | |

1908 | Purely internal. | |

1909 | MSTJ(12) : (D=2) Main switch for choice of baryon production model. | |

1910 | Suppression of rank 1 baryons by a parameter PARJ(19) is no longer | |

1911 | governed by the MSTJ(12) switch, but instead turned on by setting | |

1912 | PARJ(19)<1. | |

1913 | Three new options are available: | |

1914 | = 3 : as =2, but with improved SU(6) treatment. | |

1915 | = 4 : as =3, but also suppressing diquark vertices with low Gamma | |

1916 | values. | |

1917 | = 5 : Revised popcorn model. Independent of PARJ(3-7). Depending | |

1918 | on PARJ(8-10). Including the same kind of suppression as =4. | |

1919 | PARJ(8), PARJ(9) : (D=0.6,1.2 GeV^-1) The new popcorn parameters B_u | |

1920 | and dB = B_s - B_u. Used to suppress popcorn mesons of total | |

1921 | invariant mass M_T by exp(-B_q*M_T). | |

1922 | PARJ(10) : (D=0.6 GeV^-1) Corresponding parameter for suppression of | |

1923 | leading rank mesons of transverse mass M_T in the fragmentation of | |

1924 | diquark jets. | |

1925 | PARF(131-187) : Different diquark and popcorn weights, calculated in | |

1926 | PYKFIN. | |

1927 | PARF(191) : (D=0.2 GeV) Non-constituent mass of ud_0 diquark, which has | |

1928 | a slight influence on the weights in the new algorithm. | |

1929 | PARF(192) : (D=0.5) Gamma suppression parameter. The suppression factor | |

1930 | is 1 - PARF(192)**Gamma, with Gamma in GeV^2. | |

1931 | PARF(193,194) : Store of some parameters used by the present popcorn | |

1932 | system. | |

1933 | PARF(201-1400) : Weights for every possible popcorn meson construction | |

1934 | in the MSTJ(12)=5 option. Thus MSTJ(12)=5 is forbidden to be | |

1935 | combined with MSTJ(15)=1. | |

1936 | ||

1937 | * In summary, all commonblock variables are completely internal, except | |

1938 | MSTU(123), MSTJ(12), PARJ(8-10) and PARF(191-192). | |

1939 | - PARF(191-192) should not need to be changed. | |

1940 | - MSTU(123) should be 0 when starting, and reset to 0 whenever changing | |

1941 | a switch or parameter which influences flavour weights. | |

1942 | - With MSTJ(12)=4, PARJ(5) may need to increase. | |

1943 | - With MSTJ(12)=5, a preliminary tune suggests | |

1944 | PARJ(8,9,10) = 0.6, 1.2, 0.6, PARJ(1)=0.20 and PARJ(18)=0.19. | |

1945 | ||

1946 | * Three new subroutines are added, but are only needed for internal use. | |

1947 | SUBROUTINE PYKFIN : Precalculates a set of diquark and popcorn weights. | |

1948 | Called by PYKFDI if MSTU(123)=0. Sets MSTU(123) to 1. | |

1949 | SUBROUTINE PYNMES(KFDIQ) : If KFDIQ=0, it generates the number of | |

1950 | popcorn mesons and stores some relevant parameters. If KFDIQ not 0 | |

1951 | it generates number of leading rank mesons in the fragmentation of | |

1952 | a diquark string with original diquark KFDIQ. Called by PYKFDI. | |

1953 | SUBROUTINE PYDCYK(KFL1,KFL2,KFL3,KF) : Handles flavour production in | |

1954 | the decay of unstable particles and small string clusters. Is | |

1955 | essentially the same as PYKFDI, but takes into acount the effects | |

1956 | of string dynamics on flavour selection in the MSTJ(12)>3 options. | |

1957 | KFL1,KFL2,KFL3 and KF are the same as for PYKFDI. Called by PYDECY | |

1958 | and PYPREP. | |

1959 | ||

1960 | * The complete list of subprogram changes is as follows. | |

1961 | PYCOMP : Taking internal popcorn flags on diquarks into account. | |

1962 | PYDECY, PYMASS : No longer checking diquarks for popcorn flags | |

1963 | before calling PYCOMP. | |

1964 | PYKFDI : Quite differently formulated, but equivalent algorithm | |

1965 | introduced. Improved treatment of SU(6) symmetry requirements. | |

1966 | New flavour algorithm, based on advanced popcorn model. | |

1967 | PYNMES : New function. Selects number of popcorns mesons in a | |

1968 | popcorn system. (Also used in the reformulated algorithm of | |

1969 | the old model, when it always returns 0 or 1 popcorn meson.) | |

1970 | PYKFIN : New subroutine. Precalculates a large set of flavour | |

1971 | production weights from the input parameters. | |

1972 | PYSTRF : The rank 1 baryon suppression no longer depends on any switch, | |

1973 | but merely on the suppression parameter. Default is no suppression. | |

1974 | New option, suppressing diquark vertices at small Gamma, | |

1975 | introduced. New (but corresponding) treatment of baryon production | |

1976 | at first and second vertex of closed string. Suppression factors of | |

1977 | popcorn meson system due to its transverse mass in new flavour | |

1978 | algorithm introduced. Junction strings forbidden to be combined | |

1979 | with new popcorn options. | |

1980 | PYINDF : The rank 1 baryon suppression no longer depends on any switch, | |

1981 | but merely on the suppression parameter. Default is no suppression. | |

1982 | Warning message if trying to combine with new popcorn options. | |

1983 | (No new options implemented.) | |

1984 | PYDCYK : New subroutine, handles flavour selection in new popcorn model | |

1985 | for the case of cluster and hadron decays, where no dynamical | |

1986 | string variables are present. Generalized to take care of old | |

1987 | flavour models as well. | |

1988 | PYDECY : Uses PYDCYK instead of PYKFDI to a large extent. Reselects | |

1989 | random flavour which failed. | |

1990 | PYPREP : Uses PYDCYK instead of PYKFDI in cluster decays. This implies | |

1991 | a better treatment of closed string clusters, where previously both | |

1992 | a random flavour diquark and its antidiquark partner was tested for | |

1993 | popcorn. | |

1994 | PYDATA : PARJ(8-10) given default values for new flavour algorithm. | |

1995 | Old model kept as default in MSTJ(12), PARJ(1) and PARJ(18). | |

1996 | PARF(131-194,201-1400) and MSTU(121-140) used internally. | |

1997 | ||

1998 | * Internally the diquark codes have been extended to store the necessary | |

1999 | further popcorn information. As before, an initially existing diquark | |

2000 | has a code of the type 1000*q_a + 100*q_b + 2s+1, where q_a > q_b. | |

2001 | Diquarks created in the fragmentation process now have the longer code | |

2002 | 10000*q_c + 1000*q_a + 100*q_b + 2s+1, i.e. one further digit is set. | |

2003 | Here q_c is the curtain quark, i.e. the flavour of the quark-antiquark | |

2004 | pair that is shared between the baryon and the antibaryon, either | |

2005 | q_a or q_b. The non-curtain quark, the other of q_a and q_b, may have | |

2006 | its antiquark partner in a popcorn meson. In case there are no popcorn | |

2007 | mesons this information is not needed, but is still set at random to be | |

2008 | either of q_a and q_b. The extended code is used internally in PYSTRF | |

2009 | and PYDECY and in some routines called by them, but is not visible in | |

2010 | any event listings. | |

2011 | ||

2012 | ----------------------------------------------------------------------- | |

2013 | ||

2014 | INTERFACES TO OTHER GENERATORS | |

2015 | ||

2016 | * In e+e- annihilation events, a convenient classification of electroweak | |

2017 | physics is by the number of fermions in the final state. Two fermions | |

2018 | from Z0 decay is LEP1 physics, four fermions can come e.g. from W+W- | |

2019 | or Z0Z0 events at LEP2, and at higher energies six fermions are produced | |

2020 | by three-gauge-boson production or top-antitop. Often interference terms | |

2021 | are non-negligible, requiring much more complex matrix-element expressions | |

2022 | than are normally provided in PYTHIA. Dedicated electroweak generators | |

2023 | often exist, however, and the task is therefore to interface them to | |

2024 | the generic parton showering and hadronization machinery available in | |

2025 | PYTHIA. In the LEP2 workshop (I.G. Knowles et al., in Physics at LEP2, | |

2026 | CERN 96-01, eds. G.Altarelli, T. Sjostrand and F. Zwirner, p. 103) one | |

2027 | possible strategy was outline to allow reasonably standardized | |

2028 | interfaces between the electroweak and the QCD generators. The LU4FRM | |

2029 | routine was provided for the key four-fermion case. This routine is now | |

2030 | included here, in slightly modified form, together with two new siblings | |

2031 | for two and six fermions. The former is trivial and included mainly for | |

2032 | completeness, while the latter is rather more delicate. | |

2033 | - CALL PY2FRM(IRAD,ITAU,ICOM) | |

2034 | Purpose: to allow a parton shower to develop and partons to hadronize | |

2035 | from a two-fermion starting point. The initial list is supposed to | |

2036 | be ordered such that the fermion precedes the antifermion. In | |

2037 | addition, an arbitrary number of photons may be included, e.g. from | |

2038 | initial-state radiation; these will not be affected by the operation | |

2039 | and can be put anywhere. The scale for QCD (and QED) radiation is | |

2040 | automatically set to be the mass of the fermion-antifermion pair. | |

2041 | (It is thus not suited for Bhabha scattering.) | |

2042 | IRAD : final-state QED radiation. | |

2043 | = 0 : no final-state photon radiation, only QCD showers. | |

2044 | = 1 : photon radiation inside each final fermion pair, also leptons, | |

2045 | in addition to the QCD one for quarks. | |

2046 | ITAU : handling of tau lepton decay (where PYTHIA does not include | |

2047 | spin effects, although some generators provide the helicity | |

2048 | information that would allow a more sophisticated modelling). | |

2049 | = 0 : taus are considered stable. | |

2050 | = 1 : taus are allowed to decay. | |

2051 | ICOM : place where information about the event (flavours, momenta etc.) | |

2052 | is stored at input and output. | |

2053 | = 0 : in the HEPEVT commonblock (meaning that information is | |

2054 | automatically translated to PYJETS before treatment and back | |

2055 | afterwards). | |

2056 | = 1 : in the PYJETS commonblock. All fermions and photons can e.g. | |

2057 | be given with status code K(I,1)=1, flavour code in K(I,2) | |

2058 | and five-momentum (momentum, energy, mass) in P(I,J). The | |

2059 | V vector and remaining components in the K one are best put | |

2060 | to zero. Also remember to set the total number of entries N. | |

2061 | - CALL PY4FRM(ATOTSQ,A1SQ,A2SQ,ISTRAT,IRAD,ITAU,ICOM) | |

2062 | Purpose: to allow a parton shower to develop and partons to hadronize | |

2063 | from a four-fermion starting point. The initial list of fermions | |

2064 | is supposed to be ordered in the sequence fermion (1) - | |

2065 | antifermion (2) - fermion (3) - antifermion (4). The flavour pairs | |

2066 | should be arranged so that, if possible, the first two could come | |

2067 | from a W+ and the second two from a W-; else each pair should have | |

2068 | flavours consistent with a Z0. In addition, an arbitrary number of | |

2069 | photons may be included, e.g. from initial-state radiation; these | |

2070 | will not be affected by the operation and can be put anywhere. | |

2071 | Since the colour flow need not be unique, three real and one | |

2072 | integer numbers are providing further input. Once the colour | |

2073 | pairing is determined, the scale for QCD (and QED) radiation is | |

2074 | automatically set to be the mass of the fermion-antifermion pair. | |

2075 | (This is the relevant choice for normal fermion pair production | |

2076 | from resonance decay, but is not suited e.g. for 2-gamma processes | |

2077 | dominated by small-t propagators.) The pairing is also meaningful | |

2078 | for QED radiation, in the sense that a four-lepton final state is | |

2079 | subdivided into two radiating subsystems in the same way. Only if | |

2080 | the event consists of one lepton pair and one quark pair is the | |

2081 | information superfluous. | |

2082 | ATOTSQ : total squared amplitude for the event, irrespective of | |

2083 | colour flow. | |

2084 | A1SQ : squared amplitude for the configuration with fermions 1 + 2 and | |

2085 | 3 + 4 as the two colour singlets. | |

2086 | A2SQ : squared amplitude for the configuration with fermions 1 + 4 and | |

2087 | 3 + 2 as the two colour singlets. | |

2088 | ISTRAT : the choice of strategy to select either of the two possible | |

2089 | colour configurations. Here 0 is supposed to represent a reasonable | |

2090 | compromize, while 1 and 2 are selected so as to give the largest | |

2091 | reasonable spread one could imagine. | |

2092 | = 0 : pick configurations according to relative probabilities | |

2093 | A1SQ : A2SQ. | |

2094 | = 1 : assign the interference contribution to maximize the 1 + 2 | |

2095 | and 3 + 4 pairing of fermions. | |

2096 | = 2 : assign the interference contribution to maximize the 1 + 4 | |

2097 | and 3 + 2 pairing of fermions. | |

2098 | IRAD : final-state QED radiation. | |

2099 | = 0 : no final-state photon radiation, only QCD showers. | |

2100 | = 1 : photon radiation inside each final fermion pair, also leptons, | |

2101 | in addition to the QCD one for quarks. | |

2102 | ITAU : handling of tau lepton decay (where PYTHIA does not include | |

2103 | spin effects, although some generators provide the helicity | |

2104 | information that would allow a more sophisticated modelling). | |

2105 | = 0 : taus are considered stable. | |

2106 | = 1 : taus are allowed to decay. | |

2107 | ICOM : place where information about the event (flavours, momenta etc.) | |

2108 | is stored at input and output. | |

2109 | = 0 : in the HEPEVT commonblock (meaning that information is | |

2110 | automatically translated to PYJETS before treatment and back | |

2111 | afterwards). | |

2112 | = 1 : in the PYJETS commonblock. All fermions and photons can e.g. | |

2113 | be given with status code K(I,1)=1, flavour code in K(I,2) | |

2114 | and five-momentum (momentum, energy, mass) in P(I,J). The | |

2115 | V vector and remaining components in the K one are best put | |

2116 | to zero. Also remember to set the total number of entries N. | |

2117 | - CALL PY6FRM(P12,P13,P21,P23,P31,P32,PTOP,IRAD,ITAU,ICOM) | |

2118 | Purpose: to allow a parton shower to develop and partons to hadronize | |

2119 | from a six-fermion starting point. The initial list of fermions is | |

2120 | supposed to be ordered in the sequence fermion (1) - antifermion (2) - | |

2121 | fermion (3) - antifermion (4) - fermion (5) - antifermion (6). The | |

2122 | flavour pairs should be arranged so that, if possible, the first two | |

2123 | could come from a Z0, the middle two from a W+ and the last two from | |

2124 | a W-; else each pair should have flavours consistent with a Z0. | |

2125 | Specifically, this means that in a t-tbar event, the t decay products | |

2126 | would be found in 1 (b) and 3 and 4 (from the W+ decay) and the tbar | |

2127 | ones in 2 (bbar) and 5 and 6 (from the W- decay). In addition, an | |

2128 | arbitrary number of photons may be included, e.g. from initial-state | |

2129 | radiation; these will not be affected by the operation and can be put | |

2130 | anywhere. Since the colour flow need not be unique, further input is | |

2131 | needed to specify this. The number of possible interference | |

2132 | contributions being much larger than for the four-fermion case, we | |

2133 | have not tried to implement different strategies. Instead six | |

2134 | probabilities may be input for the different pairings, that the user | |

2135 | e.g. could pick at the six possible squared amplitudes, or according | |

2136 | to some more complicated scheme for how to handle the interference | |

2137 | terms. The treatment of cascades must be quite different for top | |

2138 | events and the rest. For a normal three-boson event, each fermion | |

2139 | pair would form one radiating system, with scale set equal to the | |

2140 | fermion-antifermion invariant mass. (This is the relevant choice for | |

2141 | normal fermion pair production from resonance decay, but is not | |

2142 | suited e.g. for 2-gamma processes dominated by small-t propagators.) | |

2143 | In the top case, on the other hand, the b (bbar) would be radiating | |

2144 | with a recoil taken by the W+ (W-) in such a way that the t (tbar) | |

2145 | mass is preserved, while the W dipoles would radiate as normal. | |

2146 | Therefore the user need also supply a probability for the event to | |

2147 | be a top one, again e.g. based on some squared amplitude. | |

2148 | P12, P13, P21, P23, P31, P32 : relative probabilities for the six possible | |

2149 | pairings of fermions with antifermions. The first (second) digit tells | |

2150 | which antifermion the first (second) fermion is paired with, with the | |

2151 | third pairing given by elimination. Thus e.g. P23 means the first | |

2152 | fermion is paired with the second antifermion, the second fermion | |

2153 | with the third antifermion and the third fermion with the first | |

2154 | antifermion. Pairings are only possible between quarks and leptons | |

2155 | separately. The sum of probabilities for allowed pairings is | |

2156 | automatically normalized to unity. | |

2157 | PTOP : the probability that the configuration is a top one; a number | |

2158 | between 0 and one. In this case, it is important that the order | |

2159 | described above is respected, with the b and bbar coming first. | |

2160 | No colour ambiguity exists if the top interpretation is selected, | |

2161 | so then the P12 - P32 numbers are not used. (One could imagine | |

2162 | colour reconnection at later stages of the process, e.g. between | |

2163 | the two W's. However, we are then no longer speaking of ambiguities | |

2164 | related to the hard process itself but rather to the possibility of | |

2165 | subsquent reconnection, e.g. at the nonperturbative level. This is | |

2166 | an interesting topic in itself, but not the one addressed here, | |

2167 | where the colour assignment is used for the full cascade evolution.) | |

2168 | IRAD : final-state QED radiation. | |

2169 | = 0 : no final-state photon radiation, only QCD showers. | |

2170 | = 1 : photon radiation inside each final fermion pair, also leptons, | |

2171 | in addition to the QCD one for quarks. | |

2172 | ITAU : handling of tau lepton decay (where PYTHIA does not include | |

2173 | spin effects, although some generators provide the helicity | |

2174 | information that would allow a more sophisticated modelling). | |

2175 | = 0 : taus are considered stable. | |

2176 | = 1 : taus are allowed to decay. | |

2177 | ICOM : place where information about the event (flavours, momenta etc.) | |

2178 | is stored at input and output. | |

2179 | = 0 : in the HEPEVT commonblock (meaning that information is | |

2180 | automatically translated to PYJETS before treatment and back | |

2181 | afterwards). | |

2182 | = 1 : in the PYJETS commonblock. All fermions and photons can e.g. | |

2183 | be given with status code K(I,1)=1, flavour code in K(I,2) | |

2184 | and five-momentum (momentum, energy, mass) in P(I,J). The | |

2185 | V vector and remaining components in the K one are best put | |

2186 | to zero. Also remember to set the total number of entries N. | |

2187 | ||

2188 | * The above routines are not set up to handle QCD four-jet events, i.e. | |

2189 | events of the types q qbar g g and q qbar q' qbar' (with q' qbar' coming | |

2190 | from a gluon branching). Such events are generated in normal parton | |

2191 | showers, but not necessarily at the right rate (a problem that may be | |

2192 | especially interesting for massive quarks like b). Therefore one would | |

2193 | like to start a QCD parton shower from a given four-parton configuration. | |

2194 | Already some time ago, a machinery was developed to handle this kind of | |

2195 | occurences, see J. Andre and T. Sjostrand, Phys. Rev. D57 (1998) 5767. | |

2196 | This approach has now been adapted to Pythia 6.1, in a somewhat modified | |

2197 | form. The main change is that, in the original work, the colour flow was | |

2198 | picked in a separate first step (not discussed in the publication, since | |

2199 | it is part of the standard Jetset 4-parton configuration machinery), | |

2200 | which reduces the number of allowed q qbar g g parton-shower histories. | |

2201 | In the current implementation, more geared towards completely external | |

2202 | generators, no colour flow assumptions are made, meaning a few more | |

2203 | possible shower histories to pick between. Another change is that mass | |

2204 | effects are better respected by the z definition. In its structure, the | |

2205 | new code is rather different from the original Jetset 7.4 based one. | |

2206 | The code contains one new user routime, PY4JET, two new auxiliary ones, | |

2207 | PY4JTW and PY4JTS, and significant additions to the PYSHOW showering | |

2208 | routine. | |

2209 | - CALL PY4JET(PMAX,IRAD,ICOM) | |

2210 | Purpose: to allow a parton shower to develop and partons to hadronize | |

2211 | from a q qbar g g or q qbar q' qbar' original configuration. The | |

2212 | partons should be ordered exactly as indicated above, with the | |

2213 | primary q qbar pair first and thereafter the two gluons or the | |

2214 | secondary q' qbar' pair. (Strictly speaking, the definition of | |

2215 | primary and secondary fermion pair is ambiguous. In practice, | |

2216 | however, differences in topological variables like the pair mass | |

2217 | should make it feasible to have some sensible criterion on an event | |

2218 | by event basis.) Within each pair, fermion should precede antifermion. | |

2219 | In addition, an arbitrary number of photons may be included, e.g. from | |

2220 | initial-state radiation; these will not be affected by the operation | |

2221 | and can be put anywhere. The program will select a possible | |

2222 | parton shower history from the given parton configuration, and then | |

2223 | continue the shower from there on. The history selected is displayed | |

2224 | in lines Nold+1 to Nold+6, where Nold is the N value before the | |

2225 | routine is called. Here the masses and energies of intermediate | |

2226 | partons are clearly displayed. The lines Nold+7 and Nold+8 contain | |

2227 | the equivalent on-mass-shell parton pair from which the shower is | |

2228 | started. | |

2229 | PMAX : the maximum mass scale (in GeV) from which the shower is started | |

2230 | in those branches that are not already fixed by the matrix-element | |

2231 | history. If PMAX is set zero (actually below PARJ(82), the shower | |

2232 | cutoff scale), the shower starting scale is instead set to be equal | |

2233 | to the smallest mass of the virtual partons in the reconstructed | |

2234 | shower history. A fixed PMAX can thus be used to obtain a reasonably | |

2235 | exclusive set of four-jet events (to that PMAX scale), with little | |

2236 | five-jet contamination, while the PMAX=0 option gives a more | |

2237 | inclusive interpretation, with five- or more-jet events possible. | |

2238 | Note that the shower is based on evolution in mass, meaning the cut | |

2239 | is really one of mass, not of pT, and that it may therefore be | |

2240 | advantageous to set up the matrix elements cuts accordingly if one | |

2241 | wishes to mix different event classes. This is not a requirement, | |

2242 | however. | |

2243 | IRAD : final-state QED radiation. | |

2244 | = 0 : no final-state photon radiation, only QCD showers. | |

2245 | = 1 : photon radiation is allowed in the QCD shower (but currently | |

2246 | a photon cannot be one of the four original partons). | |

2247 | ICOM : place where information about the event (flavours, momenta etc.) | |

2248 | is stored at input and output. | |

2249 | = 0 : in the HEPEVT commonblock (meaning that information is | |

2250 | automatically translated to PYJETS before treatment and back | |

2251 | afterwards). | |

2252 | = 1 : in the PYJETS commonblock. All fermions and photons can e.g. | |

2253 | be given with status code K(I,1)=1, flavour code in K(I,2) | |

2254 | and five-momentum (momentum, energy, mass) in P(I,J). The | |

2255 | V vector and remaining components in the K one are best put | |

2256 | to zero. Also remember to set the total number of entries N. | |

2257 | ||

2258 | ----------------------------------------------------------------------- | |

2259 | ||

2260 | HISTOGRAMS | |

2261 | ||

2262 | * The GBOOK package was written in 1979, at a time when HBOOK was not | |

2263 | available in Fortran 77. It has been used since as a small and simple | |

2264 | histogramming program. For this new version of PYTHIA the program has | |

2265 | been updated to run together with PYTHIA in double precision. Only the | |

2266 | one-dimensional histogram part has been retained, and subroutine names | |

2267 | have been changed to fit PYTHIA conventions. These modified routines | |

2268 | are now distributed together with PYTHIA. They would not be used for | |

2269 | final graphics, but may be handy for simple checks. | |

2270 | ||

2271 | * Basic principles. | |

2272 | - There is a maximum of 1000 histograms at the disposal of the user, | |

2273 | numbered in the range 1 to 1000. Before a histogram can be filled, | |

2274 | space must be reserved (booked) for it, and histogram information | |

2275 | provided. | |

2276 | - Histogram contents are stored in a commonblock of dimension 20000, | |

2277 | in the order they are booked. Each booked histogram requires NX+28 | |

2278 | numbers, where NX is the number of x bins and the 28 include limits, | |

2279 | under/overflow and the title. If you run out of space, the program | |

2280 | can be recompiled with larger dimensions. | |

2281 | - Histograms can be manipulated with a few routines. | |

2282 | - Histogram output is "line printer" style, i.e. no graphics. | |

2283 | ||

2284 | * CALL PYBOOK(ID,TITLE,NX,XL,XU) | |

2285 | Purpose: to book a one-dimensional histogram. | |

2286 | ID : histogram number, integer between 1 and 1000. | |

2287 | TITLE : histogram title, at most 60 characters. | |

2288 | NX : number of bins (in x direction) in histogram, integer between | |

2289 | 1 and 100. | |

2290 | XL, XU : lower and upper bound, respectively, on the x range | |

2291 | covered by the histogram. | |

2292 | ||

2293 | * CALL PYFILL(ID,X,W) | |

2294 | Purpose: to fill a one-dimensional histogram. | |

2295 | ID : histogram number. | |

2296 | X : x coordinate of point. | |

2297 | W : weight to be added in this point. | |

2298 | ||

2299 | * CALL PYFACT(ID,F) | |

2300 | Purpose: to rescale the contents of a histogram. | |

2301 | ID : histogram number. | |

2302 | F : rescaling factor, i.e. a factor that all bin contents (including | |

2303 | overflow etc.) are multiplied by. | |

2304 | Remark: a typical rescaling factor could be f = | |

2305 | 1/(bin size * number of events) = NX/(XU-XL) * 1/(number of events). | |

2306 | ||

2307 | * CALL PYOPER(ID1,OPER,ID2,ID3,F1,F2) | |

2308 | Purpose: this is a general purpose routine for editing one or several | |

2309 | histograms, which all are assumed to have the same number of | |

2310 | bins. Operations are carried out bin by bin, including overflow | |

2311 | bins etc. | |

2312 | OPER: gives the type of operation to be carried out, a one-character | |

2313 | string or a CHARACTER*1 variable. | |

2314 | = '+', '-', '*', '/': add, subract, multiply or divide the | |

2315 | contents in ID1 and ID2 and put the result in ID3. F1 and F2, | |

2316 | if not 1D0, give factors by which the ID1 and ID2 bin contents | |

2317 | are multiplied before the indicated operation. (Division with a | |

2318 | vanishing bin content will give 0.) | |

2319 | = 'A', 'S', 'L': for 'S' the square root of the content in ID1 | |

2320 | is taken (result 0 for negative bin contents) and for 'L' the | |

2321 | 10-logarithm is taken (a nonpositive bin content is before that | |

2322 | replaced by 0.8 times the smallest positive bin content). | |

2323 | Thereafter, in all three cases, the content is multiplied by F1 | |

2324 | and added with F2, and the result is placed in ID3. Thus ID2 | |

2325 | is dummy in these cases. | |

2326 | = 'M': intended for statistical analysis, bin-by-bin mean and | |

2327 | standard deviation of a variable, assuming that ID1 contains | |

2328 | accumulated weights, ID2 accumulated weight*variable and | |

2329 | ID3 accumulated weight*variable-squared. Afterwards ID2 will | |

2330 | contain the mean values (=ID2/ID1) and ID3 the standard | |

2331 | deviations (=sqrt(ID3/ID1-(ID2/ID1)**2)). In the end, F1 | |

2332 | multiplies ID1 (for normalization purposes), while F2 is dummy. | |

2333 | ID1, ID2, ID3 : histogram numbers, used as described above. | |

2334 | F1, F2 : factors or offsets, used as described above. | |

2335 | ||

2336 | * CALL PYHIST | |

2337 | Purpose: to print all histograms that have been filled, and | |

2338 | thereafter reset their bin contents to 0. | |

2339 | ||

2340 | * CALL PYPLOT(ID) | |

2341 | Purpose: to print out a single histogram. | |

2342 | ID : histogram to be printed. | |

2343 | ||

2344 | * CALL PYNULL(ID) | |

2345 | Purpose: to reset all bin contents, including overflow etc., to 0. | |

2346 | ID : histogram to be reset. | |

2347 | ||

2348 | * CALL PYDUMP(MDUMP,LFN,NHI,IHI) | |

2349 | Purpose: to dump the contents of existing histograms on an external | |

2350 | file, from which they could be read in to another program. | |

2351 | MDUMP : the action to be taken. | |

2352 | = 1 : dump histograms, each with the first line giving histogram | |

2353 | number and title, the second the number of x bins and lower | |

2354 | and upper limit, the third the total number of entries and | |

2355 | under-, inside- and overflow, and subsequent ones the bin | |

2356 | contents grouped five per line. If NHI=0 all existing | |

2357 | histograms are dumped and IHI is dummy, else the NHI | |

2358 | histograms with numbers IHI(1) through IHI(NHI) are dumped. | |

2359 | = 2 : read in histograms dumped with MDUMP=1 and book and | |

2360 | fill histograms according to this information. (With | |

2361 | modest modifications this option could instead be used | |

2362 | to write the info to HBOOK/HPLOT format, or whatever.) | |

2363 | NHI and IHI are dummy. | |

2364 | = 3 : dump histogram contents in column style, where the | |

2365 | first column contains the x values (average of respective | |

2366 | bin) of the first histogram, and subsequent columns the | |

2367 | histogram contents. All histograms dumped this way must | |

2368 | have the same number of x bins, but it is not checked whether | |

2369 | the x range is also the same. If NHI=0 all existing histograms | |

2370 | are dumped and IHI is dummy, else the NHI histograms with | |

2371 | numbers IHI(1) through IHI(NHI) are dumped. A file | |

2372 | written this way can be read e.g. by GNUPLOT. | |

2373 | LFN : the file number to which the contents should be written. | |

2374 | You must see to it that this file is properly opened for write | |

2375 | (since the definition of file names is machine dependent). | |

2376 | NHI : number of histograms to be dumped; if 0 then all existing | |

2377 | histograms are dumped. | |

2378 | IHI : array containing histogram numbers in the first NHI positions | |

2379 | for NHI nonzero. | |

2380 | ||

2381 | * COMMON/PYBINS/IHIST(4),INDX(1000),BIN(20000) | |

2382 | Purpose: to contain all information on histograms. | |

2383 | IHIST(1) : (D=1000) maximum allowed histogram number, i.e. dimension | |

2384 | of the INDX array. | |

2385 | IHIST(2) : (D=20000) size of histogram storage, i.e. dimension of | |

2386 | the BIN array. | |

2387 | IHIST(3) : (D=55) maximum number of lines per page assumed for | |

2388 | printing histograms. 18 lines are reserved for title, | |

2389 | bin contents and statistics, while the rest can be used for the | |

2390 | histogram proper. | |

2391 | IHIST(4) : internal counter for space usage in the BIN array. | |

2392 | INDX : gives the initial address in BIN for each histogram. | |

2393 | If this array is expanded, also IHIST(1) should be changed. | |

2394 | BIN : gives bin contents and some further histogram information for | |

2395 | the booked histograms. If this array is expanded, also IHIST(2) | |

2396 | should be changed. | |

2397 | ||

2398 | ----------------------------------------------------------------------- | |

2399 | ||

2400 | MISCELLANEOUS | |

2401 | ||

2402 | * Improved clarity of code and comments. | |

2403 | - The contents of DO loops are indented two steps. | |

2404 | - The header info given for each subroutine has been moved and modified. | |

2405 | - Title page with PYLOGO has been modified. | |

2406 | ||

2407 | * LUDBRB has been removed. The new PYROBO always requires two | |

2408 | integer arguments to give range of action, followed by the angles | |

2409 | and the boost vector. The integer arguments can be picked 0 to indicate | |

2410 | standard range (1-N). | |

2411 | ||

2412 | * MSTP(126) is now by default 50, giving the number of documentation | |

2413 | lines at the beginning of the record. | |

2414 | ||

2415 | * PYGIVE has been updated with new commonblock variables and changed | |

2416 | array dimensions. | |

2417 | ||

2418 | * The random number generator PYR now works in double precision, | |

2419 | i.e. 48 bits are set. The Marsaglia-Zaman algorithm is used, as before, | |

2420 | with a minor extension at the initialization stage. | |

2421 | ||

2422 | * PYCLUS has been expanded with new options 5 and 6, which do the | |

2423 | Durham algorithm as option 3 and 4 do the JADE one. | |

2424 | ||

2425 | * A new subroutine PYTIME has been added to give the date and time, | |

2426 | for use in PYLOGO and elsewhere. Since Fortran 77 does not contain | |

2427 | a standard way of obtaining this information, the routine is dummy, | |

2428 | to be replaced by the user. The output is given in an integer array | |

2429 | ITIME(6), with components year, month, day, hour, minute and second. | |

2430 | If there should be no such information available on a system, it is | |

2431 | acceptable to put all the numbers above to 0. | |

2432 | ||

2433 | * Extra check in PYSCAT for low remnant energies (mainly for heavy | |

2434 | quarks). | |

2435 | ||

2436 | * A new function PYMRUN to allow running (Q2-dependent) masses. | |

2437 | - PM = PYMRUN(KF,Q2) | |

2438 | Purpose: to give running masses of d, u, s, c and b quarks. For all other | |

2439 | particles, the PYMASS function is called by PYMRUN to give the normal | |

2440 | mass. | |

2441 | KF : flavour code. | |

2442 | Q2 : the scale at which the mass is evaluated. | |

2443 | Note: The nominal values, valid at a reference scale | |

2444 | Q2ref = max((PARP(37)*nominalmass)^2 , 4*Lambda^2), | |

2445 | are stored in PARF(91)-PARF(95). | |

2446 | - PARF(91) - PARF(95) : (D = 0.0099, 0.0056, 0.199, 1.35, 4.5 GeV) default | |

2447 | nominal masses, used to give the running masses. (Note change of b | |

2448 | quark mass from the 5 GeV previously used.) | |

2449 | - The result is that, for the d, u, s, c and b quarks, there are now | |

2450 | three different sets of masses in use in the program. | |

2451 | PMAS(KF,1) : the "on-shell" masses used to set up the kinematics of | |

2452 | partonic state produced in an event. | |

2453 | PARF(100+KF) : constituent masses, used in the fragmentation description, | |

2454 | recommended not to change. | |

2455 | PARF(90+KF) : the current algebra style masses, used as input for running | |

2456 | masses in Higgs physics. | |

2457 | For diquarks, only the first two exist, and for the others only the first | |

2458 | one. | |

2459 | ||

2460 | * For the HEPEVT common, NMXHEP is 4000 rather than 2000 and real variables | |

2461 | are DOUBLE PRECISION, to conform with the LEP 2 workshop agreement. | |

2462 | ||

2463 | * Some bug fixes. | |

2464 | ||

2465 | ----------------------------------------------------------------------- | |

2466 | ||

2467 | CHANGES FROM BASELINE VERSION | |

2468 | ||

2469 | 6.100 : 4 March 1997 - baseline. | |

2470 | ||

2471 | 6.101 : 17 March 1997 | |

2472 | - PYRECO: DETER(I,J,K) -> DETER(I,J,L) to avoid problems with some | |

2473 | compilers. | |

2474 | - PYDUMP: bug END=180 -> END=170. | |

2475 | - PYWIDT: calculation of beta threshold factor reorganized to avoid | |

2476 | overflow at high energies and to remove an inconsistency. | |

2477 | - PYSTAT: option 2 changed to allow listing of third decay product | |

2478 | in some channels. | |

2479 | - PYTIME: alternative timing suited for GNU LINUX libU77. | |

2480 | - PYRAND: information on where in phase space a maximum has been | |

2481 | violated has been reduced (MSTP(122)=0 : not at all; =1 : only | |

2482 | when error (i.e. not for warnings); =2 : always). | |

2483 | ||

2484 | 6.102 : 22 April 1997 | |

2485 | - PYMASS: the special options for MSTJ(93) nonzero, used especially | |

2486 | in the fragmentation process, have been corrected. This corrects | |

2487 | an error in the translation from JETSET 7.4 to PYTHIA 6.1. The | |

2488 | error has somewhat suppressed the amount of baryon production | |

2489 | relative to JETSET 7.4, but effects are not drastic. | |

2490 | - PYMULT: the comparison XT2.LE.0.01D0*VINT(149) has been changed to | |

2491 | 0.01001 to avoid possibility of infinite loop. | |

2492 | - PYSIGH: further check for process 145 that IA not equal to JA | |

2493 | (purely preventive; not known to have caused any problems). | |

2494 | ||

2495 | 6.103 : 23 May 1997 | |

2496 | - PYSIGH, PYVACU, PYHGGM: some updates/corrections of the SUSY | |

2497 | generation. | |

2498 | - PYUPIN: allow external process numbers up to 500. | |

2499 | ||

2500 | 6.104 : 30 June 1997 | |

2501 | - Three new processes for J/psi production: 106 - 108, see above, | |

2502 | in the section on `hard processes'. | |

2503 | - PYRESD: a major bug in the angular distribution of process 1, | |

2504 | caused by a missing factor of 2 in the WTMAX expression. This | |

2505 | leads to an essentially flat distribution in cos(theta). | |

2506 | ||

2507 | 6.110 : 10 October 1997 | |

2508 | - Modified code for baryon production. The default behaviour is | |

2509 | essentially unchanged, while an advanced popcorn scheme has been | |

2510 | added as a further option. Also some intermediate new options | |

2511 | are implemented. The physics aspects are described above, in | |

2512 | the section on `baryon production models'. | |

2513 | - The Breit-Wigner evaluation in process 35 corrected in the same | |

2514 | way as has already been implemented for the other Z-production | |

2515 | processes (but apparently overlooked here). | |

2516 | - Restore bug fix to process 145, erroneously not carried over to | |

2517 | version 6.104. | |

2518 | - In the fixed-alpha_s option MSTU(111)=0 the Lambda=PARU(117) is | |

2519 | set so that the first-order running alpha_s agrees with the | |

2520 | desired fixed alpha_s for the Q2 value used. Of no consequence | |

2521 | except as extra safety. | |

2522 | - Error message if PYFILL is used with an unbooked histogram number. | |

2523 | - Further line added to output/input for PYDUMP options 1 and 2, | |

2524 | giving information on the total number of entries and under-, | |

2525 | inside- and overflow. | |

2526 | ||

2527 | 6.111 : 27 October 1997 | |

2528 | - Forgotten values for XLO and XHI inserted in PYFINT routine. | |

2529 | - Change of sign convention for RMSS(16) in PYAPPS routine. | |

2530 | ||

2531 | 6.112 : 30 October 1997 | |

2532 | - PYRESD has been modified to cope with the decay t -> W + b + Z | |

2533 | (note order of decay products), by including the necessary | |

2534 | colour flow option and by setting angular weight according to | |

2535 | isotropic decay of the W and Z. The program does not calculate | |

2536 | the partial width to this potential channel. | |

2537 | ||

2538 | 6.113 : 11 November 1997 | |

2539 | - PYEIG4 has been expanded to cover a missed ambiguity in the solution | |

2540 | of a fourth-degree equation. This ambiguity could, for some parameter | |

2541 | values, give the wrong mass eigenstates in the neutralino sector. | |

2542 | ||

2543 | 6.114 : 19 November 1997 | |

2544 | - GOTO jump into IF...ENDIF block removed from PYSTRF. | |

2545 | - Underscore replaced by W in some PYKFIN variable names. | |

2546 | ||

2547 | 6.115 : 27 January 1998 | |

2548 | - In the intermediate scenario of colour reconnection, MSTP(115)=11, | |

2549 | the QCD radiation has been reduced until now by an untentional | |

2550 | application of the colour interference machinery. This is now | |

2551 | solved by having MSTJ(50)=0 during the shower call. | |

2552 | - A factor 1/SH has been missing in the width expression for | |

2553 | t -> stop + neutralino, thus giving too large partial width. | |

2554 | - Two errors in PYRESD corrected for the case the routine is called | |

2555 | from outside the standard PYINIT/PYEVNT machinery, i.e. without having | |

2556 | a subprocess number defined. The first ensures isotropic decay angles, | |

2557 | the second correct history pointers in K(I,3). | |

2558 | - D-format changed to E-format in PYDUMP(3), to be consistent with | |

2559 | GNUPLOT input conventions. | |

2560 | - Further check on allowed histogram numbers in PYFILL, PYFACT, | |

2561 | PYOPER, PYPLOT and PYNULL. | |

2562 | - Removed redundant/erroneous check on MSTU(183) in PYLOGO. | |

2563 | - MSTP(48) default changed from 2 to 0 as intended. (Should not | |

2564 | have mattered anywhere.) | |

2565 | ||

2566 | 6.116 : 8 July 1998 | |

2567 | - Initial-state radiation for a muon beam is now allowed (and also for a | |

2568 | tau beam). The radiation machinery is as for an electron, with a | |

2569 | trivial replacement of the electron mass. To distinguish the e/mu/tau | |

2570 | cases, the PYPDEL routine has KFA as a further argument and PYPDFU is | |

2571 | modified accordingly. | |

2572 | - Ten new processes, 131 - 140, for reactions involving virtual photons. | |

2573 | The (square root with appropriate sign of the) photon virtualities can | |

2574 | be set in P(1,5) and P(2,5) when PYINIT is called with the 'FIVE' option. | |

2575 | - Two new processes, 104 and 105, for chi_c production. | |

2576 | - PYPDFU and other routines are modified to allow virtual photons. A dummy | |

2577 | copy of STRUCTP (the PDFLIB routine for virtual photons) is included in | |

2578 | case PDFLIB is not linked. | |

2579 | - New variable VINT(120) coincides with VINT(3) or VINT(4), depending on | |

2580 | which side of the event is considered. Is used to bring information on the | |

2581 | user-defined virtuality of a photon beam to the parton distributions | |

2582 | of the photon. | |

2583 | - GRV 92L parton distribution is reinserted, for crosschecks with | |

2584 | Pythia 5.7. Affects PYPDPR, PYPDFU and PYINIT. | |

2585 | - The technipi partial width to quarks corrected down by factor 3 | |

2586 | (avoiding doublecounting of colour factor). | |

2587 | - A fudge factor PARP(146) has been introduced for the technipi partial | |

2588 | width to a fermion pair. | |

2589 | - Address of the Pythia webpage is updated. | |

2590 | - In PYSHOW an IF-test has been broken into two nested ones to avoid | |

2591 | testing on meaningless condition. | |

2592 | - PYMAXI has been modified to handle the case when a user-defined | |

2593 | process is implemented but switched off (calculation of XSEC(0,1)). | |

2594 | - Protection against square root of negative number in PYTHRG. | |

2595 | ||

2596 | 6.117 : 19 August 1998 | |

2597 | - New options 11 - 25 for MSTP(14) to mix alternatives for virtual photons. | |

2598 | - PYCLUS and PYCELL modified to ensure that N is unchanged and MSTU(3)=0 | |

2599 | when NJET is negative (to signal failure of the algorithm). | |

2600 | ||

2601 | 6.118 : 13 September 1998 | |

2602 | - Bottom squark production is now treated separately, as for the top | |

2603 | squark. However, there are more processes because bottom is in the | |

2604 | PDF. The new processes 281 - 296 are listed in the Hard Subprocess | |

2605 | section above. | |

2606 | - Displaced vertices are now produced for resonances. This can be | |

2607 | particularly important for delayed decays of SUSY particles to | |

2608 | gravitinos, e.g. ~chi0_2 -> ~gravitino + photon. | |

2609 | - The angular distribution in chargino pair production has been | |

2610 | reversed (i.e. that <-> uhat) for some charge combinations. | |

2611 | - The width for ~g -> ~squark + quark has been fixed. The sign of | |

2612 | a squark mixing angle was reversed. | |

2613 | - PYMSIN modified so that several (SUSY parameter) initializations can | |

2614 | be done in a single run without setting up conflicting information. | |

2615 | - Some bugs in the technicolor decay widths have been fixed, and some | |

2616 | new options are now available, see PARP(146) - PARP(151). | |

2617 | - New option IMSS(5)=1 added. | |

2618 | - New Higgs pair production processes 297-301. A few of these are | |

2619 | already available as Z' decays, where the Z' part can be killed, | |

2620 | but this provides a more direct implementation. | |

2621 | - Expanded top decays to include gravitino stop and gluino stop. | |

2622 | Added entries for virtual chargino decays of stop that might | |

2623 | be important for light stop and light staus: | |

2624 | ~t_1 -> ~nu_tauL tau+ b | |

2625 | ~t_1 -> ~tau_1+ nu_tau b | |

2626 | Also added entries for the neutralino: | |

2627 | ~chi_10 -> c dbar e- | |

2628 | -> d sbar nu_e | |

2629 | The latter two would be R-parity violating decays. | |

2630 | The status of these decays modes is -1, and they have not | |

2631 | been tested. | |

2632 | - The branching ratios are zeroed out before refilling when | |

2633 | initializing SUSY decays. | |

2634 | ||

2635 | 6.119 : 25 September 1998 | |

2636 | - Machinery introduced to allow photon inside lepton beam. | |

2637 | See further description above, section on hard processes. | |

2638 | - Extended Bose-Einstein treatment, with many new options for | |

2639 | W pair studies, see above on hadronization. Default behaviour | |

2640 | changed. | |

2641 | - PYSSPA modified so that the PARP(65) parameter for minimum gluon | |

2642 | energy emitted in spacelike showers is modifed by an extra factor | |

2643 | roughly corresponding to the 1/gamma factor for the boost to the | |

2644 | hard subprocess frame. Earlier, when a subsystem was strongly | |

2645 | boosted, i.e. at large rapidities in the cm frame of the collision, | |

2646 | the PARP(65) requirement became quite stringent on the low-energy | |

2647 | incoming side. Therefore much radiation could be cut out. | |

2648 | - PYSSPA modified so that the angular ordering requirement is based | |

2649 | on ordering p_T/p rather than p_T/p_L, i.e. replacing tan(theta) | |

2650 | by sin(theta). Earlier the starting value tan(theta)_max = 10 | |

2651 | could actually be violated by some bona fide emissions for strongly | |

2652 | boosted subsystems, where one side has small p_L. Therefore some | |

2653 | emissions were incorrectly removed when MSTP(62)=3, i.e. default. | |

2654 | - PYSSPA now sets relevant mass for QED emission to be mu or tau one | |

2655 | rather than e one for such incoming beams. (For a collider between | |

2656 | two different lepton species, the more massive one is used as | |

2657 | reference.) | |

2658 | - PYSHOW modified, so that photon emission off a lepton is governed | |

2659 | by the PARJ(90) parameter rather than PARJ(83) (see PARTON SHOWERS). | |

2660 | - PYUPDA corrected for bug in calculation of phase space available | |

2661 | in decay (generated unnecessary warnings). | |

2662 | - PYRESD modified to avoid calculation of undefined four-products | |

2663 | when called for an odd resonance (i.e. one not part of the | |

2664 | standard PYTHIA machinery, e.g. filled with PY1ENT). | |

2665 | - In pair production of heavy flavour (top) in processes 81,82, 84 | |

2666 | and 85, earlier only one of the masses was used in the matrix element, | |

2667 | under the assumption that the two were identical. Since we do not | |

2668 | have expressions involving the two separately, we now use an average | |

2669 | value constructed so that the beta kinematics factor is the same | |

2670 | for both having the average as for each having its correct value. | |

2671 | - Move technicolour parameter PARP(151) to PARP(140) to avoid clash. | |

2672 | - Effects of secondary widths included if leptoquark decays to top | |

2673 | (or fourth-generation fermions). | |

2674 | ||

2675 | 6.120 : 1 October 1998 | |

2676 | - The pTmin and pT0 cutoff parameters of the multiple interactions | |

2677 | scenario(s) are now made explicitly energy-dependent (see | |

2678 | MISCELLANEOUS). | |

2679 | - MINT(45), MINT(46) set correctly to allow photon radiation off a | |

2680 | muon beam. Also some other minor bugs corrected for muon beams. | |

2681 | Note, however, that the MSTP(12)=1 option to obtain e.g. electrons | |

2682 | inside photons inside electrons does not work for muons. | |

2683 | - PYSSPA modified so lower Q2 cutoff for QED radiation off lepton | |

2684 | is always at least twice the mass-squared, in addition to the | |

2685 | cutoff provided by PARP(68). | |

2686 | - W2 limits in CKIN(39) and CKIN(40) not checked if process 10 is | |

2687 | called for two lepton beams. | |

2688 | - Labels cleaned up. | |

2689 | ||

2690 | 6.121 : 15 October 1998 | |

2691 | - New routines PY2FRM, PY4FRM and PY6FRM added as generic interfaces | |

2692 | to two-, four- and six-fermion generators, see MISCELLANEOUS. | |

2693 | - The MSTP(14) switch has been expanded so that MSTP(14)=20 and =25 | |

2694 | works also for gamma-hadron, not only for gamma-gamma. These two | |

2695 | values would therefore be the two main alternatives for users. | |

2696 | The default has been changed to MSTP(14)=20. | |

2697 | - The MSTP(32) parameter for choice of Q2 scale has been expanded | |

2698 | with new options intended for virtual incoming photons. | |

2699 | - New function PYMRUN(KF,Q2) gives running (MSbar) mass of d, u, s, | |

2700 | c and b quarks. For all other KF, the PYMASS function is called by | |

2701 | PYMRUN to give the normal mass. PYWIDT and PYSIGH has been modified | |

2702 | for Higgs (and some technicolour) widths and production processes | |

2703 | to call PYMRUN rather than to implement the running inline. The | |

2704 | code for the running is identical, so the difference is that now | |

2705 | the PMAS(KC,1) masses can be set to the "on-shell" values expected | |

2706 | rather than the MSbar ones. The nominal b quark mass has been reduced | |

2707 | from 5 to 4.5 GeV, affecting some Higgs branching ratios. | |

2708 | The technipi rate to leptons has been somewhat changed. | |

2709 | See also MISCELLANEOUS. | |

2710 | - Correct minor bug in partial width of top to gravitino + stop. | |

2711 | - Reimplement PARP(146)-(148) in code (had been lost). | |

2712 | - Minor correction in the initialization printout. | |

2713 | ||

2714 | 6.122 : 4 January 1999 | |

2715 | - New matrix-element correction scheme for initial-state radiation, | |

2716 | especially relevant for the production of a single s-channel | |

2717 | resonance. This allows much better description e.g. of the pT | |

2718 | properties of W and Z produced in hadron colliders. See | |

2719 | MISCELLANEOUS for further details. | |

2720 | - Change in PYRESD so that, when Z' or W' decays to (one or two) | |

2721 | top quarks, these are allowed to decay isotropically. (Previously | |

2722 | the matrix element for Z' -> W+ W- -> 4 fermions was erroneously | |

2723 | called.) | |

2724 | - Minor change in PYSHOW to catch one case where K(I,1) values | |

2725 | can become incorrectly set if the routine is called for a lepton | |

2726 | pair of very low mass (roughly below 1 GeV). | |

2727 | - The lepton-inside-lepton parton distribution is changed. Previously | |

2728 | f_e^e(x) was normal for x < 1 - 10^-4, scaled up for | |

2729 | 1 - 10^-4 < x < 1 - 10^-6 and 0 for x > 1 - 10^-6, where the | |

2730 | rescaling was arranged so as to give the correct integral of | |

2731 | f_e^e(x) from 0 to 1. Now the border at 1 - 10^-4 has been moved | |

2732 | to 1 - 10^-7 and the one at 1 - 10^-6 to 1 - 10^-10. This way any | |

2733 | irregularities in the line shape has been pushed into a much narrower | |

2734 | region; of some interest e.g. for a muon collider. | |

2735 | - Angular distribution included in decay of W in process 36, | |

2736 | gamma + f -> W + f', by analogy with process 31. | |

2737 | - DATA PARU split in two statements to avoid the 19-continuation-lines | |

2738 | limit. | |

2739 | - Extra safety check in PYREMN to avoid division by zero if | |

2740 | chi = 0 or 1. | |

2741 | - Matrix-element code MDME(IDC,2)=32 restored for h0, H0, A0, H+- -> | |

2742 | q qbar (set 0 in recent versions). This code is irrelevant when | |

2743 | resonance decays is performed in PYRESD, as is almost always the | |

2744 | case. | |

2745 | - PYMSIN modified so that MWID(KC) and MDCY(KC,1) values are saved | |

2746 | and restored for the lightest supersymmetric particle. Is relevent | |

2747 | where a single run contains several PYINIT calls for different | |

2748 | SUSY parameter sets, and hence different LSP's: it switches back on | |

2749 | the decays of a particle that was LSP but no longer is it. | |

2750 | - PYLOGO, PYTIME and PYHIST slightly modified for year | |

2751 | 2000-compatible output. | |

2752 | - New option MSTP(39)=5, where the Q2 scale of the gg, qqbar -> QQbarH | |

2753 | processes is set equal to the squared nominal Higgs mass | |

2754 | (cf. MSTP(39)=3 is the actual Higgs mass, i.e. fluctuating between | |

2755 | events). | |

2756 | - Introduce a line | |

2757 | IMPLICIT INTEGER(I-N) | |

2758 | in routines. This helps avoid a bug in the SGI Fortran compiler. | |

2759 | ||

2760 | 6.123 : 2 February 1999 | |

2761 | - New process machinery for doubly charged Higgs production in a | |

2762 | left-right-symmetric scenario. Includes new particles and new hard | |

2763 | subprocesses; see these subsections. | |

2764 | - Introduce missing shat factor in cross section for process 140. | |

2765 | - Correct logic of photon virtuality choice in processes 131 - 136, | |

2766 | which gave erroneous results for the direct*resolved cases of | |

2767 | gamma*gamma* events. | |

2768 | - Explicit DOUBLE PRECISION declaration for EXTERNAL functions and | |

2769 | some DATA statements moved after all declarations to avoid problems | |

2770 | on some compilers. | |

2771 | - The pT^2 fluctuation margin allowed for independent fragmentation | |

2772 | in PYTEST increased. | |

2773 | ||

2774 | 6.124 : 7 February 1999 | |

2775 | - The effects of longitudinal resolved photons can be approximated | |

2776 | by a multiplicative factor to the transverse resolved cross sections, | |

2777 | see PARP(165) in the hard processes section. | |

2778 | - Possibility to choose between e -> gamma splitting variable | |

2779 | being energy fraction x or lightcome fraction y, see MSTP(16). | |

2780 | - Cross sections for direct photon processes 137-140 corrected by a | |

2781 | factor shat/lambda, usually very close to unity, to better describe | |

2782 | phase space relations. | |

2783 | - A few bug corrections in the new popcorn scenario (see section | |

2784 | above on baryon production models). Especially, one bug also came | |

2785 | to affect the default baryon production scenario, and could in | |

2786 | some cases result in charge and baryon number nonconservation in | |

2787 | the beam remnant fragmentation process (PYREMN). | |

2788 | - PYKFIN extensively rewritten. Mostly cosmetics, but also | |

2789 | 1) For MSTJ(12)=5, a factor 2 was misplaced for ud_1 and uu_1 | |

2790 | diquark production in the process (rank 0 qq) -> ... M + B + ... | |

2791 | 2) In the old algorithm the diquark SU(6) survival factor was not | |

2792 | considered when generating the final diquark of a popcorn | |

2793 | system. In Pythia 6.110, this factor was introduced for the new | |

2794 | options MSTJ(12)>2, but unintentionally also for MSTJ(12)=2. For | |

2795 | backward compatibility, the diquark SU(6) survival factors are now | |

2796 | set to 1 if MSTJ(12)<3. | |

2797 | - IN PYRAND the VINT(25) = x_T^2 calculation was incorrect for a | |

2798 | user-defined process; normalization now changed from VINT(1) to | |

2799 | VINT(2). Will have given too high a starting x_T for multiple | |

2800 | interactions. | |

2801 | - The well-known but harmless rho0 -> eta gamma and a_2 -> eta' pi | |

2802 | possibilities of looping in PYDECY no longer cause a warning | |

2803 | message. | |

2804 | - In process 23 the cross section in PYSIGH is explicitly ensured to | |

2805 | be non-negative. This is likely a problem of the far-out wings of | |

2806 | the Breit-Wigners, which the cross section is not set up to handle. | |

2807 | ||

2808 | 6.125 : 21 February 1999 | |

2809 | - PYSTRF corrected for a bug in the choice of the string region | |

2810 | which defines the longitudinal directions of the final two hadrons. | |

2811 | In principle the bug is severe, but in practice its consequences | |

2812 | are limited, since in many events the final string region is | |

2813 | uniquely defined so that the choice is irrelevent, and since, | |

2814 | even when there is a choice, the procedure would work whichever | |

2815 | of the two possible regions is selected. | |

2816 | - PYSSPA treatment of QED showers corrected, in three respect. First, | |

2817 | lower x cutoff (XEE) changed from 1D-7 to 1D-10 to match changes | |

2818 | in lepton-in-lepton distributions of 6.122. Second, the | |

2819 | matrix-element matching can made also for QED processes. | |

2820 | Third, a scattered lepton does not (occasionally) get K(I,1)=3. | |

2821 | - New (default) option for lower parton-shower cutoff (and `primordial | |

2822 | kT') in resolved photons, see MSTP(66) above. | |

2823 | - New parameter and default behaviour for multiple interactions in the | |

2824 | VMD component of virtual photons, see MSTP(84) above. | |

2825 | - For non-QCD processes, a photon is now assumed unresolved when | |

2826 | MSTP(14)=10, 20 or 25. (In principle, both the resolved and direct | |

2827 | possibilities ought to be explored, but this mixing is not currently | |

2828 | implemented, so picking direct at least will explore one of the two | |

2829 | main alternatives rather than none.) Also a minor change in PYMAXI | |

2830 | to correct the calculation of number of points in y* for a photon | |

2831 | beam. | |

2832 | - New option MSTP(32)=10 : Q2 scale is equal to CM energy. No special | |

2833 | reason except as extreme contrast (or not so extreme, for many e+e- | |

2834 | processes). | |

2835 | ||

2836 | 6.126 : 26 March 1999 | |

2837 | - The simulation of the production and decays of technicolor particles | |

2838 | has been substantially upgraded. The processes 149, 191, 192, and 193 | |

2839 | are to be considered obsolete, and are temporarily retained to allow | |

2840 | cross checking with the new processes. Process 194 has been changed | |

2841 | to more accurately represent the mixing between the photon, Z, | |

2842 | techni_rho0, and techni_omega particles in the Drell-Yan process. | |

2843 | Process 195 is the analogous process including W and techni_rho+/- | |

2844 | mixing. Processes 361 - 377 are completely new. For a listing of | |

2845 | processes and parameters, see the description in the Hard Processes | |

2846 | section. | |

2847 | - The possibility of flavor--dependent Z0' couplings has been considered. | |

2848 | The previous set of parameters PARU(121)--PARU(128) now affect only | |

2849 | the first generation of fermions. As before, these parameters represent | |

2850 | the V and A couplings for the d-quark, u-quark, electron, and nu_e, | |

2851 | respectively. The parameters PARJ(180)--PARJ(187) and | |

2852 | PARJ(188)--PARJ(195) represent the V and A couplings of the s-quark, | |

2853 | c-quark, muon, nu_mu and b-quark, t-quark, tau, and nu_tau, | |

2854 | respectively. The default value for all parameters are the standard | |

2855 | model values. | |

2856 | - PYMSIN : improve zeroing of branching ratios when several parameter | |

2857 | sets are considered in the same run. | |

2858 | ||

2859 | 6.127 : 18 May 1999 | |

2860 | - In process 226, for chargino pair production, the sign in the | |

2861 | u quark inteference term in the cross section is changed. | |

2862 | - In PYRESD, the HGZ array of relative Z/gamma weights in processes | |

2863 | 15, 19, 30 and 35 was not always stored and read out with the same | |

2864 | index, leading to a potentially incorrect angular distribution in | |

2865 | the Z decay, specifically concerning forward-backward asymmetries. | |

2866 | - In process 25, W pair production, the contribution from Z exchange | |

2867 | to the cross section is now evaluated with a fixed width for the Z | |

2868 | in the propagator, in PYSIGH. That is, the GMMZ = mass * width | |

2869 | in the denominator of the progagator is used with the nominal mass | |

2870 | and width of the Z. Previously the actual mass and the running width | |

2871 | were used, which gave rise to divering cross sections, by imperfect | |

2872 | gauge cancellation, at large energies. | |

2873 | - In process 140, for longitudinal photon interactions, the cross section | |

2874 | corrected for an erroneous factor shat too much. | |

2875 | - In photon physics, the setting of MINT(15), MINT(16), VINT(307) and | |

2876 | VINT(308) have been corrected for some cases, affecting PYRAND, | |

2877 | PYGAGA, PYMAXI and PYINPR. | |

2878 | - The normalizing cross section used for multiple interactions in | |

2879 | photon collisions is scaled by a factor m^4/(m^2+Q^2)^2 for virtual | |

2880 | photons, rather than only the square root of that. | |

2881 | - New variable MSTP(69), replacing some of the functionality previously | |

2882 | provided by MSTP(68) but removed with the change in PYSSPA with | |

2883 | Pythia 6.119: | |

2884 | MSTP(69) : (D=0) possibility to change Q2 scale for parton distributions | |

2885 | from the MSTP(32) choice, especially for e+e-. | |

2886 | = 0 : use MSTP(32) scale. | |

2887 | = 1 : in lepton-lepton collisions, the QED lepton-inside-lepton | |

2888 | parton distributions are evaluated with s, the full squared CM | |

2889 | energy, as scale. | |

2890 | = 2 : s is used as parton distribution scale also in other | |

2891 | processes. | |

2892 | - Insert WID2=1 in a few more places in PYWIDT, to avoid it being | |

2893 | undefined. | |

2894 | - THE, PHI, CHI -> THEZ, PHIZ, CHIZ in the special e+e- -> Z option | |

2895 | of PYRESD, to avoid a name clash. | |

2896 | - Insert extra demand when storing THE2T in PYSSPA, for consistency | |

2897 | (to avoid storing an undefined variable). | |

2898 | - Replace SQMW*PMAS(24,2)**2 by GMMW**2 in PYSIGH. | |

2899 | - Remove unused SR2 in PYSIGH. | |

2900 | - Further examples of PYTIME solutions on some machines have been added. | |

2901 | ||

2902 | 6.128 : 3 June 1999 | |

2903 | - Introduce new options for MINT(47): | |

2904 | = 6 : parton distribution is peaked at x=1 for target and no | |

2905 | distribution at all for beam. | |

2906 | = 7 : parton distribution is peaked at x=1 for beam and no | |

2907 | distribution at all for target. | |

2908 | This prompts modifications in several routines, especially | |

2909 | PYSIGH and PYKMAP, with modified checks and phase space factors, | |

2910 | and also e.g. the possibility of having a 1/(1-tau) term in | |

2911 | the selecion procedure of e-gamma collisions. | |

2912 | - Put MINT(15 or 16) = 22 in PYGAGA for a photon whenever MSTP(14)=0. | |

2913 | - Modify the cross section for process 35 to depend on the | |

2914 | virtuality of the incoming photon in e gamma* -> e Z0. | |

2915 | (Exact form ambiguous, but hopefully sensible choice.) | |

2916 | - In PYOFSH the modification of the allowed resonace mass range by | |

2917 | CKIN values is modified when the other particle is not a resonance. | |

2918 | ||

2919 | 6.129 : 9 July 1999 | |

2920 | - Correct severe bug in colour reconnection with PYRECO, whereby the | |

2921 | W+ and W- decay vertices were swapped if the resonaces were given | |

2922 | in the order W- W+. This is the case when PYINIT is called with the | |

2923 | beam arguments 'e-','e+' rather than 'e+','e-'. In scenarios I, II | |

2924 | and II', the rearrangement rate is then overestimated. | |

2925 | - Allow to switch on colour reconnection also for subprocess 22, Z0Z0 | |

2926 | pair production, analogously with rearrangement in W+W- events. | |

2927 | Note, however, that the Z0 decay vertex position is calculated | |

2928 | without any regard to the gamma* component of the cross section. | |

2929 | Thus, the description in scenarios I, II and II' would not be | |

2930 | sensible e.g. for a light-mass gamma*gamma* pair. | |

2931 | - The width of the A0 higgs particle to a fermion pair is corrected | |

2932 | to be like beta (=velocity), rather than like beta**3. | |

2933 | - Default decay status for Higgs modes to supersymmetric particles | |

2934 | changed from -1 to 1 in MDME(IDC,1) in PYDATA. | |

2935 | - New options to take into account the effects of resolved longitudinal | |

2936 | photons, see above MSTP(17), PARP(165), PARP(167) and PARP(168) | |

2937 | (PYSIGH routine). | |

2938 | - When calling virtual-photon PDF's via PDFLIB, ensure that P^2 < Q^2. | |

2939 | For GRS, also ensure the specific cuts in that parameterization, | |

2940 | specifically P^2 < Q^2/5. | |

2941 | ||

2942 | 6.130 : 6 September 1999 | |

2943 | - pi0 decay was unintentionally switched off by default from version | |

2944 | 6.126 onwards, but is now again allowed to decay. | |

2945 | - A major bug has been corrected in the PYBOEI routine for Bose-Einstein | |

2946 | corrections. It does not affect the BE_0, BE_3 and BE_32 (default) | |

2947 | options, but only the BE_m and BE_lambda alternatives, MSTJ(54)=-1 | |

2948 | and -2 (when MSTJ(57)=1, which is default). The weight used to | |

2949 | define the most likely particles to carry the energy/momentum | |

2950 | compensation of BE pairs contained a sign error in an exponential, | |

2951 | which meant that not always the intended particles were selected. | |

2952 | This affects several of the distributions obtained with these | |

2953 | algorithms in the past. The predicted average W mass shift is among | |

2954 | the quantities changed, but stays of the same order. | |

2955 | - PYSHOW has been modified and expanded with new options, see the | |

2956 | PARTON SHOWERS section above for further details. | |

2957 | First, the emission of gluons off primary quarks in gamma*/Z0 decays | |

2958 | has been modified. This increases the amount of gluon radiation off | |

2959 | heavier quarks like b's (by about 5% at LEP1), while light quarks | |

2960 | are not affected. | |

2961 | Second, the description of g -> q qbar branchings has been expanded | |

2962 | with several new options, MSTJ(42)=3 and 4 and MSTJ(44)=3, in order | |

2963 | to explore a larger range of uncertainty in predictions. | |

2964 | - PY6FRM has been improved, so that if an event is classified as a | |

2965 | ttbar one, the t pair is allowed to radiate gluons before the top | |

2966 | decay. Radiation off the b's and in the W decays is there as earlier. | |

2967 | ||

2968 | 6.131 : 13 September 1999 | |

2969 | - New routine PY4JET introduced (with auxiliary routines PY4JTW and | |

2970 | PY4JTS, and significant additions to PYSHOW) to provide interface | |

2971 | from a four-jet QCD generator to a parton-shower evolution. See | |

2972 | further description in section on INTERFACES TO OTHER GENERATORS. | |

2973 | ||

2974 | 6.132 : 23 September 1999 | |

2975 | - Default (pseudo)rapidity limits in CKIN(9)-CKIN(16) changed from | |

2976 | +-10 to +-40, since former still can imply an unwanted pTmin cut. | |

2977 | - Also PYKLIM modified to avoid erroneous rejections at extreme | |

2978 | (pseudo)rapidities. | |

2979 | - When MINT(15)=1 is set before a PYWIDT call, the original value | |

2980 | is restored afterwards. | |

2981 | - Incoming/outgoing lepton mass included in cross section for | |

2982 | process 35. | |

2983 | ||

2984 | 6.133 : 29 September 1999 | |

2985 | - Correct the calculation of that and uhat for process 35 when the | |

2986 | incoming photon is virtual, so that masses are assigned assuming | |

2987 | that incoming parton number 2 is the photon (for the internal | |

2988 | numbering). | |

2989 | - Introduce a missing factor of 1/2 in the cross section for processes | |

2990 | 351 and 352, H++/H-- production, and change the rules for when | |

2991 | t^ and u^ contributions should be symmetrized so it is only done | |

2992 | for identical leptons. | |

2993 | ||

2994 | 6.134 : 10 October 1999 | |

2995 | - New internally available parton distribution parameterizations for | |

2996 | the proton, CTEQ 5L and CTEQ 5M1. These are obtained with | |

2997 | MSTP(51)=7 and =8, respectively. Internally: changes in PYPDPR and | |

2998 | PYINIT, and additions of new routines PYCT5L and PYCT5M. All is based | |

2999 | on code written by Jon Pumplin, with minor modifications to fit the | |

3000 | PYTHIA framework. | |

3001 | ||

3002 | 6.135 : 3 November 1999 | |

3003 | - Major modifications in routine PYPREP, intended e.g. to improve | |

3004 | modelling of charm/bottom production from small-mass parton systems, | |

3005 | "clusters". It is now possible to select between a few alternative | |

3006 | descriptions of how energy/momentum is shuffled when a cluster | |

3007 | collapses to a single particle, and to have anisotropic decay when | |

3008 | a cluster gives two particles. See E. Norrbin and T. Sjostrand, | |

3009 | Phys. Lett. B442 (1998) 407 and in preparation. | |

3010 | MSTJ(16) : (D=2) mode of cluster treatment. | |

3011 | = 0 : old scheme. Cluster decays (to two hadrons) are isotropic. | |

3012 | In cluster collapses (to one hadron), energy-momentum | |

3013 | compensation is to/from the parton or hadron furthest away | |

3014 | in mass. | |

3015 | = 1 : intermediate scheme. Cluster decays are anisotropic in a | |

3016 | way that is intended to mimic the Gaussian pT suppression and | |

3017 | string 'area law' of suppressed rapidity orderings of ordinary | |

3018 | string fragmentation. In cluster collapses, energy-momentum | |

3019 | compensation is to/from the string piece most closely moving | |

3020 | in the same direction as the cluster. Excess energy is put | |

3021 | as an extra gluon on this string piece, while a deficit | |

3022 | is taken from both endpoints of this string piece as a common | |

3023 | fraction of their original momentum. | |

3024 | = 2 : new default scheme. Essentially as above, except that a | |

3025 | energy deficit is preferentially taken from the endpoint of | |

3026 | the string piece that is moving closest in direction to the | |

3027 | cluster. | |

3028 | MSTJ(17) : (D=2) number of attempts made to find two hadrons that | |

3029 | have a combined mass below the cluster mass, and thus allow a | |

3030 | cluster to decay to two hadrons rather than collapse to one. | |

3031 | Thus the larger MSTJ(17), the smaller the fraction of collapses. | |

3032 | At least one attempt is always made, and this was the old default | |

3033 | behaviour. | |

3034 | - In order to better match the data on charm production asymmetries, | |

3035 | the quark masses in PMAS(I,1) have been changed to be in line with | |

3036 | constituent quark masses. These are the masses that are used for | |

3037 | kinematics construction, and also influence production cross sections. | |

3038 | After the introduction of the PYMRUN routine for running quark masses | |

3039 | of relevance e.g. as Higgs couplings, there is no longer the previous | |

3040 | need to store current algebra masses in PMAS. (Actually, this is | |

3041 | thereby a return to the practice in very old versions of the program, | |

3042 | before the Higgs considerations lead to a change.) | |

3043 | PMAS(1,1) - PMAS(5,1) : (D= 0.33, 0.33, 0.5, 1.5, 4.8 GeV) | |

3044 | - The default primordial kT value has been raised by about a factor of | |

3045 | two to better account for a number of production characteristics, | |

3046 | such as charm azimuthal correlations. The new default is very | |

3047 | difficult to consider as a purely nonperturbative number, but | |

3048 | could be viewed as also resumming some soft perturbative gluon | |

3049 | emissions. | |

3050 | PARP(91) : (D=1 GeV) Gaussian width. | |

3051 | PARP(92) : (D=0.4 GeV) equivalent exponential width. | |

3052 | PARP(93) : (D=5 GeV) upper cut on primordial kT spectrum. | |

3053 | PARP(99) : (D=1 GeV) Gaussian width for photon remnant. | |

3054 | PARP(100) : (D=5 GeV) upper cut on primordial kT spectrum for photon | |

3055 | remnant. | |

3056 | - The default MSTP(92) value has been changed to 3. This provides a | |

3057 | somewhat more even energy sharing between two coloured beam remnants, | |

3058 | and again helps improve charm production phenomenology. | |

3059 | - Changed parameterization of the probability for reverse rapidity | |

3060 | ordering in the joining of the final two hadrons in string | |

3061 | fragmentation, and now also for a cluster decaying to two hadrons. | |

3062 | P_rev = 1/(1 + exp(b Delta)) with | |

3063 | Delta = Gamma_2 - Gamma_1 | |

3064 | = sqrt((mT0**2 - mT1**2 - mT2)**2)**2 - 4 mT1**2 mT2**2). | |

3065 | Here Gamma_1 and Gamma_2 are the string squared invariant times of | |

3066 | the two possible breaks, of a subsystem with transverse masses | |

3067 | mT0 -> mT1 + mT2. For Lund fragmentation functions, b = PARJ(42), | |

3068 | and thus PARJ(38) is no longer used, while for other functions | |

3069 | b = PARJ(39) has been refitted. Note that this does not represent | |

3070 | any noticeable change of the physics output. | |

3071 | PARJ(39) (D=0.08 GeV^-2) related to probability for reverse | |

3072 | rapidity ordering for Field-Feynman type fragmentation functions, | |

3073 | as above. | |

3074 | - PYDIFF: remove a check on minimum invariant masses that, for the new | |

3075 | default quark masses, could lead to an infinite loop. | |

3076 | - PYSSPA, PYREMN: remove a check on and rescaling of (boost) beta | |

3077 | values close to 1, that were leftovers from the single-precision | |

3078 | version. In some rare events at very high energies, this could give | |

3079 | significant energy-momentum nonconservation. | |

3080 | - PYDATA: adjust the length of some PROC character constants that had | |

3081 | wrong number of trailing blanks. | |

3082 | - PYDECY: change a DO 310 I=1,4 to I=1,NQ to avoid that the routine | |

3083 | may copy unnitialized values, which gives problems on some compilers. | |

3084 | - PYSHOW: change dimension of ISSET from 2 to 3. The too small size | |

3085 | may have given problems for showers in Upsilon decays, but not in | |

3086 | normal showers from two partons. | |

3087 | ||

3088 | 6.136 : 30 November 1999 | |

3089 | - PYSSPA: two changes, for initial-state showers related to flavour | |

3090 | excitation, where a c (or b) quark enters the hard scattering and | |

3091 | should be reconstructed by the shower as coming from a g -> c cbar | |

3092 | (or g -> b bbar) branching. | |

3093 | First, an x value for the incoming c above Q_max^2/(Q_max^2 + m_c^2) | |

3094 | does not allow a kinematical reconstruction of the gluon branching | |

3095 | with an x_g < 1, and is thus outside the allowed phase space. Such | |

3096 | events (with some safety margin) are rejected. Currently they will | |

3097 | appear in PYSTAT(1) listings in the 'Fraction of events that fail | |

3098 | fragmentation cuts', which is partly misleading, but has the correct | |

3099 | consequence of suppressing the physical cross section. | |

3100 | Second, the Q^2 value of the backwards evolution of a c quark is | |

3101 | by force kept above m_c^2, so as to ensure that the branching | |

3102 | g -> c cbar is not 'forgotten' by evolving Q^2 below Q_0^2. Thereby | |

3103 | the possibility of having a c in the beam remnant proper is eliminated. | |

3104 | Warning: as a consequence of the changes above, flavour excitation | |

3105 | is not at all possible too close to threshold. If the KFIN array | |

3106 | in PYSUBS is set so as to require a c (or b) on either side, and | |

3107 | the phase space is closed for such a c to come from a g -> c cbar | |

3108 | branching, the program will enter an infinite loop. | |

3109 | - The older EHLQ1, EHLQ2, DO1 and DO2 parton distributions of the | |

3110 | proton have been ported from Pythia 5 and inserted as | |

3111 | MSTP(51) = 12 - 15. Not intended for current studies, but good | |

3112 | for checks of backwards compatibility. In this connection, the | |

3113 | default of MSTP(58) is changed from 6 to 5, since the EHLQ | |

3114 | distributions also contain top, that one nowadays probably would | |

3115 | not want to see included by default. | |

3116 | - PYREMN is modified, so that when a hadronic remnant is split in two, | |

3117 | the primordial kT recoil is shared evenly between them (with a | |

3118 | relative pT kick added). | |

3119 | - The default for MSTP(94) is changed from 2 to 3, meaning that the | |

3120 | standard Lund symmetric fragmentation function is used for the | |

3121 | lightcome momentum fraction of the hadron produced from a multiquark | |

3122 | remnant. | |

3123 | - PYTHRG: protect against negative square root (by roundoff) in RT(1,2). | |

3124 | ||

3125 | 6.137 : 2 February 2000 | |

3126 | - Introduce new process 146, e + gamma -> e*. | |

3127 | - Process 161 Breit-Wigner corrected to suppress low-mass tail. | |

3128 | - PYSHOW corrected for possibility of populating unallowed region of | |

3129 | phase space (and thereby breaking energy-momentum conservation) | |

3130 | in the option where a given four-parton configuration is used to | |

3131 | start the shower, e.g. from PY4JET. | |

3132 | - The default value of PARP(67) changed from 4 to 1; relates to scale | |

3133 | matching between initial-state parton shower and hard scattering. | |

3134 | ||

3135 | 6.138 : 2 March 2000 | |

3136 | - Introduce new process 169, q + qbar -> e + e*. | |

3137 | - For QCD processes in the multiple interactions description, also | |

3138 | c and b quarks are allowed as incoming partons. Thus also charm | |

3139 | and bottom production by flavour excitation is included in this | |

3140 | framework. (Only for the hardest interaction, however, related to | |

3141 | limitations in the beam-remnant treatment.) | |

3142 | - Exclude by default the possibility of top-antitop pair production | |

3143 | for processes where a new flavour pair is produced at a gluon or | |

3144 | photon vertex, i.e. processes 12, 53, 54, 58, 96 and 135-140. | |

3145 | (This is achieved by changing the g, gamma -> t + tbar decay | |

3146 | channels to MDME(IDC,1)=0.) | |

3147 | - Correct severe errors in the width calculation in PYWIDT for Z'/Z. | |

3148 | Affects process 141. | |

3149 | - Insert missing SQMW and SQMZ definitions for techni-rho width | |

3150 | calculations. | |

3151 | - Redistribute colour interference term of cross section in process | |

3152 | 11 (and corresponding part of process 96) to avoid some part of | |

3153 | the cross section from becoming negative. | |

3154 | - Avoid rare division by zero in boost in PYSTRF. | |

3155 | - Minor further improvement of the PYSHOW modification of the 6.137 | |

3156 | version. | |

3157 | - Minor modification to PYSSPA to allow Q2 scale to be raised | |

3158 | slightly if g -> Q + Qbar branching is kinematically problematical. | |

3159 | ||

3160 | 6.139 : 23 March 2000 | |

3161 | - A severe bug has been found for the multiple interactions scenario | |

3162 | when MSTP(82) >= 3, i.e. when using variable impact parameters. | |

3163 | It is only important when the main, "hard" process (the one(s) | |

3164 | selected with the MSEL oand MSUB switches) can become rather soft, | |

3165 | like e.g. in gamma*/Z production at small masses. Here follows | |

3166 | more details. | |

3167 | The traditional multiple interactions procedure is to let the main | |

3168 | interaction set the upper pT scale for subsequent multiple | |

3169 | interactions. For QCD, this is a matter of avoiding doublecounting. | |

3170 | Other processes normally are hard, so the procedure is then also | |

3171 | sensible. However, for a soft main interaction, further softer | |

3172 | interactions are hardly possible, i.e. multiple interactions are | |

3173 | more or less killed. | |

3174 | For MSTP(82) >= 3 it is even worse, since also the events themselves | |

3175 | are likely to be rejected in the impact-parameter selection stage. | |

3176 | Thus the spectrum of main events that survive is biased, with the | |

3177 | soft tail suppressed. Such a behaviour could be motivated by the | |

3178 | rejected events instead appearing as part of the interactions | |

3179 | underneath a normal QCD hard interaction, but in practice the latter | |

3180 | mechanism is not implemented. (And would have been very inefficient | |

3181 | to work with, had it been.) Furthermore, even when events are | |

3182 | rejected by the impact parameter procedure, this is not reflected | |

3183 | in the cross section for the process, as it should have been. | |

3184 | Therefore the default behaviour has been modified, so that only | |

3185 | for QCD processes is the main process enforcing a limit on the | |

3186 | subsequent interactions. Note that this also allows more underlying | |

3187 | event activity in the default options MSTP(82)<=2. | |

3188 | MSTP(86) : (D=2) requirements on multiple interactions based on | |

3189 | the hardness scale of the main process. | |

3190 | = 1 : the main collision is harder than all the subsequent | |

3191 | ones (old behaviour, for backwards compatibility, with | |

3192 | dangers and errors as noted above). | |

3193 | = 2 : when the main process is of the QCD jets type (the same | |

3194 | as those in multiple interactions) subsequent jets are | |

3195 | requested to be softer, but for other processes no such | |

3196 | requirement exists. | |

3197 | = 3 : no requirements at all that multiple interactions have | |

3198 | to be softer than the main interactions (of dubious use for | |

3199 | QCD processes but intended for crosschecks). | |

3200 | Note : process cross sections are unreliable whenever the | |

3201 | main process does restrict subsequent interactions, and the | |

3202 | main process can become soft. For QCD jet studies in this | |

3203 | region it is then better to put CKIN(3) < PARP(81) or | |

3204 | PARP(82) and get the "correct" total cross section. | |

3205 | - The default primordial kT value has been raised by about a factor of | |

3206 | two for protons in version 6.136; now also the photons are changed | |

3207 | the same way. | |

3208 | PARP(99) : (D=1 GeV) Gaussian width for photon remnant. | |

3209 | PARP(100) : (D=5 GeV) upper cut on primordial kT spectrum for photon | |

3210 | remnant. | |

3211 | - Correct minor bug in PYBOEI, causing division by zero in rare cases. | |

3212 | ||

3213 | 6.140 : 2 May 2000 | |

3214 | - Correct bug in PYSCAT for processes 203, 206 and 209, giving wrong | |

3215 | colour flow. | |

3216 | - Correct final mass selection machinery in PYSCAT, for cases when | |

3217 | a generic quark happens to become a top (or another heavy one) and | |

3218 | thus have to be assigned a large and variable mass. | |

3219 | - Change PYSSPA for a photon beam so that a c (or b) heavy quark is | |

3220 | not necessarily to be reconstructed as coming from a branching | |

3221 | g -> c + cbar. (Since a photon has a c/b valence quark content, | |

3222 | unlike normal hadronic beam particles.) | |

3223 | - Introduce new loop counter to PYSSPA, to interrupt event in case | |

3224 | it seems to be impossible to find a consistent kinematics for | |

3225 | shower branchings. (Rare, but can happen for heavy flavours.) | |

3226 | ||

3227 | 6.143 : 15 May 2000 | |

3228 | - New machinery for treatment of minimum bias processes in | |

3229 | gamma*-p and gamma* gamma* processes, not yet quite complete | |

3230 | but released to give some first feedback. Extensive changes in | |

3231 | PYXTOT, PYGAGA, PYRAND, PYSCAT, PYINPR, PYSIGH, PYMULT, PYSSPA | |

3232 | and PYKLIM. Default behaviour changed, both by changes of the code | |

3233 | and by changes of some default values. While it should be possible | |

3234 | to recover most of the old behaviour by suitable changes of switches | |

3235 | and parameters, complete backwards compatibility is not assured. | |

3236 | Therefore it is better to think of this version as a clean break | |

3237 | in the area of minimum bias physics for virtual photons. The | |

3238 | machinery can be used either for photons of fixed virtuality, | |

3239 | by using the 'FIVE' option of the PYINIT call, or for a spectrum of | |

3240 | photon virtualities by using the GAMMA/E beam particle option. | |

3241 | For the latter option, photon kinematics can be constrained | |

3242 | with CKIN variables. In either case, the CKIN(3) variable is | |

3243 | used to switch between a minimum-bias and a jet description, | |

3244 | just like for hadronic collisions. MSEL=2 also gives diffractive | |

3245 | and 'elastic' events. What is still missing is mainly the admixing | |

3246 | of DIS-type events; work is underway. Further details can be found | |

3247 | in the HARD PROCESSES section above. | |

3248 | - Bug found and corrected for process 137-140, where before the | |

3249 | flavour selection machinery did not allow the production of | |

3250 | gamma * gamma* -> lepton+ lepton-, even when this kind of | |

3251 | processes were switched on and included in the cross section. | |

3252 | - Checks on the x values allowed for colliding beams have been | |

3253 | extended. For a hadron beam, x is not allowed to be above | |

3254 | 1 - 2 * PARP(111)/E_CM. This ensures that the hadronic beam remnant | |

3255 | has an energy of at least PARP(111) in the rest frame of the event, | |

3256 | as is required (with some safety margin) in order to construct a | |

3257 | realistic beam renmnant. The need emerged out of studies with | |

3258 | anomalous photons, where the parton distributiosn are large close | |

3259 | to x = 1, but the correction is applied to all kinds of hadronic | |

3260 | events. | |

3261 | - Break out of loop in PYPOLE routine if no convergence after 100 | |

3262 | iterations. | |

3263 | ||

3264 | 6.144 : 25 May 2000 | |

3265 | - New process 99, for DIS scattering gamma* + q -> q, where it is | |

3266 | assumed that the photon flux is provided separately. New code | |

3267 | ISET(ISUB)=8 represents this kinematics. New routine PYDISG to | |

3268 | handle the kinematics of this process. Thus the gamma*-p | |

3269 | and gamma* gamma* machineries are extended also to include | |

3270 | automatic mixing with DIS processes, see comment for version 6.143. | |

3271 | Many changes in code. New options for MSTP(14), MSTP(18), MSTP(19), | |

3272 | and several MINT and VINT variables. New default value MSTP(14)=30 | |

3273 | gives automatic mix with DIS processes. | |

3274 | - Enhancement from longitudinal photons (see MSTP(17)) did not work | |

3275 | in 6.143 and has now been corrected. | |

3276 | - Default values for x_min and y_min of emitted photons (CKIN(61), | |

3277 | CKIN(63), CKIN(73), CKIN(75) changed from 0.01 to 0.0001). | |

3278 | - PYTECM declarations changed to Pythia standard ones. | |

3279 | - PYK: minor change to nest IF requirements to avoid problems with | |

3280 | some compilers. | |

3281 | ||

3282 | 6.145 : 29 May 2000 | |

3283 | - Insert forgotten conversion factor in process 99 cross section. | |

3284 | - Exclude leptons from direct*direct process for MSEL=1 or 2. | |

3285 | - Check against infinite loop for small systems with diffraction. | |

3286 | - Correct error in mother pointer for cluster collapse. | |

3287 | - Correct mixup of process types in PYSTAT(1) listing. | |

3288 | - Modify initialization scale for DIS processes. | |

3289 | ||

3290 | 6.146 : 8 June 2000 | |

3291 | - Updated PYDISG routine now handles beam remnant in DIS processes | |

3292 | like in PYREMN and includes final-state radiation of scattered | |

3293 | quark. (Initial-state radiation still missing.) | |

3294 | - Correct bug in PYRAND that reset pTmin incorrectly when asking for | |

3295 | high-pT events only in gamma*-p or gamma*-gamma*. | |

3296 | - Document DIS process with pT = PARI(17) = 0. | |

3297 | - Increase initialization/maximum search scale for DIS * anomalous. | |

3298 | - Include kinematical factor 1/(1-x) in the conversion formula from | |

3299 | F2 to photon cross section. | |

3300 | ||

3301 | 6.147 : 19 June 2000 | |

3302 | - PYSIGH updated in a few places to avoid division by zero. The | |

3303 | error occured in calculations that are ultimately not used, so | |

3304 | therefore do not affect any output. | |

3305 | - Avoid a division by zero in PYSHOW, appearing in the showering | |

3306 | of the new DIS process 99. | |

3307 | ||

3308 | 6.148 : 27 June 2000 | |

3309 | - New treatment of elastic/diffractive processes of the GVMD | |

3310 | component. VINT(69) and VINT(70) denote the masses of the GVMD | |

3311 | states. See above, section on hard processes. | |

3312 | - New and upgraded treatment of primordial kT of anomalous photon, | |

3313 | which also before contained a bug. Also affects e.g. DIS scattering | |

3314 | off an anomalous photon. New options MSTP(66)=4 and =5, with the | |

3315 | latter new default. See above, section on hard processes. | |

3316 | - Switch off DIS process 99 if vanishing maximum at initialization. | |

3317 | Stop run if no process has nonvanishing maximum. | |

3318 | ||

3319 | 6.150 : 30 June 2000 | |

3320 | - Include virtuality dependence for a photon target in the form factor | |

3321 | of the DIS process 99 in PYSIGH (this factor accounts for overlap | |

3322 | with the direct*direct process). | |

3323 | - Some insignificant changes for better Fortran 77 standard conformance. | |

3324 | ||

3325 | 6.151 : 7 August 2000 | |

3326 | - Increase the maximum scale of final-state shower evolution for DIS | |

3327 | events in the PYDISG routine by a factor of 2, to obtain a smoother | |

3328 | matching to the activity in the direct process group. | |

3329 | - For elastic and diffractive scattering, store m**2/4 (approximately | |

3330 | pT**2) in VINT(283) or VINT(284), respectively. Here m is the mass | |

3331 | of the state being diffracted, which may be of interest when | |

3332 | analyzing GVMD diffractive scattering. | |

3333 | - Join two COMPLEX*16 declarations in PYSIGH (cosmetics). | |

3334 | ||

3335 | 6.152 : 17 August 2000 | |

3336 | - Include factor in PYSIGH cross section to take into account the | |

3337 | effects of longitudinal resolved photons probed in the | |

3338 | DIS process (99), by mistake missing so far. | |

3339 | - PYRECO colour reconnection for scenario I: check that selected | |

3340 | space-time point is in the forward light cone before studying it | |

3341 | further (thereby saving some time). | |

3342 | ||

3343 | ----------------------------------------------------------------------- | |

3344 | ||

3345 |