Correct handling of seeds for MC on the fly trains
[u/mrichter/AliRoot.git] / PYTHIA6 / pythia_doc.update
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952cc209 1 ****************************
2 * *
3 * PYTHIA version 6.1 *
4 * *
5 ****************************
6
7 (Last updated 17 August 2000)
8
9The new PYTHIA version is a logical continuation of previous versions.
10Therefore a user should not feel lost. However, many details have been
11changed. 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
20Below follows a more extensive list of main changes, performed to move
21from Pythia 5.7 and Jetset 7.4 to Pythia 6.1. Eventually this file
22will be complemented by a completely updated manual. However, based
23on the information here and some common sense it should be possible
24to use the program already now, if you are familiar with previous
25versions.
26
27-----------------------------------------------------------------------
28
29PYTHIA/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
63DOUBLE 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
95PARTICLE 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
265RESONANCE 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
408HARD 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
1256THE 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
1315PARTON 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
1396PARTON 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
1559UNDERLYING 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
1609HADRONIZATION
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
1746BARYON 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
2014INTERFACES 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
2260HISTOGRAMS
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
2400MISCELLANEOUS
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
2467CHANGES FROM BASELINE VERSION
2468
24696.100 : 4 March 1997 - baseline.
2470
24716.101 : 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
24846.102 : 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
24956.103 : 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
25006.104 : 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
25076.110 : 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
25276.111 : 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
25316.112 : 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
25386.113 : 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
25436.114 : 19 November 1997
2544 - GOTO jump into IF...ENDIF block removed from PYSTRF.
2545 - Underscore replaced by W in some PYKFIN variable names.
2546
25476.115 : 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
25666.116 : 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
25966.117 : 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
26016.118 : 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
26356.119 : 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
26756.120 : 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
26906.121 : 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
27146.122 : 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
27606.123 : 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
27746.124 : 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
28086.125 : 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
28366.126 : 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
28596.127 : 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
29026.128 : 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
29196.129 : 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
29426.130 : 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
29686.131 : 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
29746.132 : 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
29846.133 : 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
29946.134 : 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
30026.135 : 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
30886.136 : 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
31256.137 : 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
31356.138 : 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
31606.139 : 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
32136.140 : 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
32276.143 : 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
32646.144 : 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
32826.145 : 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
32906.146 : 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
33016.147 : 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
33086.148 : 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
33196.150 : 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
33256.151 : 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
33356.152 : 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