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d909f169 1
2 A new version of the Monte Carlo program HERWIG (version 6.1) is now
3 available, and can be obtained from the following web site:
4
5 http://hepwww.rl.ac.uk/theory/seymour/herwig/
6
7 This will temporarily be mirrored at CERN for the next few weeks:
8
9 http://home.cern.ch/~seymour/herwig/
10
11 More complete information on HERWIG can be found in the publication
12 G. Marchesini, B.R. Webber, G. Abbiendi, I.G. Knowles, M.H. Seymour
13 and L. Stanco, Computer Phys. Commun. 67 (1992) 465 and also in the
14 documentation for the previous version (5.9), which are available at
15 the same site, together with other useful files and information.
16 Here we merely give the new features relative to 5.9.
17
18 If you use HERWIG, please refer to it something along the lines of:
19
20 HERWIG 6.1, hep-ph/9912396; G. Marchesini, B.R. Webber, G. Abbiendi,
21 I.G. Knowles, M.H. Seymour and L. Stanco,
22 Computer Phys. Commun. 67 (1992) 465.
23
24
25 *** NEW FEATURES OF THIS VERSION ***
26
27 *---------------------------------------------------------------*
28 | The main new features are: supersymmetric processes (both |
29 | R-parity conserving & violating) in hadron-hadron collisions; |
30 | new e+e- to four jets process; matrix element corrections to |
31 | top decay and Drell-Yan processes; new soft underlying event |
32 | options; updates to default particle data tables; new LaTeX & |
33 | html printout options. |
34 *---------------------------------------------------------------*
35
36 * [N.B. Default values for input variables shown in square brackets.]
37
38 * All R-parity conserving SUSY two-to-two processes in hadron-hadron
39 collisions have been added. Their process numbers are:
40
41 +-------+----------------------------------------------------------+
42 | IPROC | Process |
43 +-------+----------------------------------------------------------+
44 | 3000 | 2 parton to 2 sparticles: the sum of 3010,3020 and 3030 |
45 | 3010 | 2 parton to 2 spartons |
46 | 3020 | 2 parton to 2 gauginos |
47 | 3030 | 2 parton to 2 sleptons |
48 +-------+----------------------------------------------------------+
49
50 Further details of the inclusion of superpartners and their decays
51 are given below.
52
53 Additional processes for the SUSY two Higgs doublet model are
54 currently under test and will be released shortly.
55
56 * All R-parity violating SUSY two-to-two processes via resonant
57 sleptons and squarks in hadron collisions have been added. Their
58 process numbers are:
59
60 +-------+----------------------------------------------------------+
61 | IPROC | Processes derived from the LQD term in the superpotential|
62 +-------+----------------------------------------------------------+
63 | 4000 | The sum of 4010,4020,4040 and 4050 |
64 | 4010 | Neutralino lepton production (all neutralinos) |
65 | 401i | As 4010 but only the ith neutralino |
66 | 4020 | Chargino lepton production (all charginos) |
67 | 402i | As 4020 but only the ith chargino |
68 | 4040 | Slepton W/Z production |
69 | 4050 | Slepton Higgs production |
70 +-------+----------------------------------------------------------+
71 | 4060 | Sum of 4070 and 4080 |
72 | 4070 | quark-antiquark production via LQD |
73 | 4080 | lepton production via LLE and LQD |
74 +=======+==========================================================+
75 | IPROC | Processes derived from the UDD term in the superpotential|
76 +-------+----------------------------------------------------------+
77 | 4100 | The sum of 4110, 4120, 4130, 4140 and 4150 |
78 | 4110 | Neutralino quark production (all neutralinos) |
79 | 411i | As 4110 but only the ith neutralino |
80 | 4120 | Chargino quark production (all charginos) |
81 | 412i | As 4120 but only the ith chargino |
82 | 4130 | Gluino quark production |
83 | 4140 | Squark W/Z production |
84 | 4150 | Squark Higgs production |
85 +-------+----------------------------------------------------------+
86 | 4160 | quark-quark production |
87 +-------+----------------------------------------------------------+
88
89 In addition the R-parity violating decays of all superpartners is
90 included.
91
92 * A new process describing electron-positron annihilation to four jets
93 has been added. This has IPROC=600+IQ, where a non-zero value for IQ
94 guarantees production of quark flavour IQ whilst IQ=0 corresponds to
95 the natural flavour mix. IPROC=650+IQ is as above but without those
96 terms in the matrix element which orient the event w.r.t. the lepton
97 beam direction. The matrix elements are based on those of Ellis Ross
98 & Terrano with orientation terms from Catani & Seymour. The soft and
99 collinear divergences are avoided by imposing a minimum y-cut, Y4JT
100 [.01], on the initial 4 partons. The interjet distance is calculated
101 using either the Durham or JADE metrics. This choice is governed by
102 the logical variable DURHAM [.TRUE.]. Note that parameterizations of
103 the volume of four-body phase space are used: these are accurate up
104 to a few percent for y-cut values less than 0.14. Note, also that
105 the phase space is for massless partons, as are the matrix elements,
106 though a mass threshold cut is applied. Finally, the matrix elements
107 for the q-qbar-g-g & q-qbar-q-qbar (same flavour quark) final states
108 receive contributions from 2 colour flows each, the treatment of the
109 interference terms being controlled by the array IOP4JT:
110
111 q-qbar-g-g case:
112 IOP4JT(1)=0 neglect, =1 extreme 2341; =2 extreme 3421 [0]
113
114 q-qbar-q-qbar (identical quark flavour) case:
115 IOP4JT(2)=0 neglect, =1 extreme 4123; =2 extreme 2143 [0]
116
117 The scale EMSCA for the parton showers is set equal to SQRT(s*ymin)
118 where ymin is the least distance, according to the selected metric,
119 between any two partons.
120
121 * Matrix element corrections to the simulation of top quark decays and
122 Drell-Yan processes are now available using the same general method
123 as already implemented for e+e- annihilation and DIS. If HARDME
124 [.TRUE.] then fill the missing phase-space (`dead zone') using the
125 exact 1st-order M.E. result (`hard corrections'). If SOFTME
126 [.TRUE.] then correct emissions in the already-populated region of
127 phase space using the exact amplitude for every emission that is
128 capable of being the hardest so far (`soft corrections').
129
130 - For t -> bW decays the routine HWBTOP implements hard corrections.
131 HWBRAN has been modified to implement the soft corrections to top
132 decays. Since the dead zone includes part of the soft singularity
133 a cutoff is required: only gluons with energy above GCUTME [2 GeV]
134 (in the top rest frame) are corrected. Physical quantities are not
135 strongly dependent on GCUTME in the range 1 to 5 GeV. For details
136 see:
137
138 G. Corcella and M.H. Seymour, Phys. Lett. B442 (1998) 417.
139
140 - For the Drell-Yan process the routine HWBDYP implements the hard
141 corrections whilst HWSBRN has been modified to implement the soft
142 corrections to the initial state radiation. For details see:
143
144 G. Corcella and M.H. Seymour, hep-ph/9908338.
145
146 * The parameters of the model used for soft interactions are now
147 available to the user for modification. The model is based on the
148 minimum-bias event generator of the UA5 Collaboration, which starts
149 from a parametrization of the pbar p inelastic charged multiplicity
150 distribution as a negative binomial. The parameters are as follows
151 (default parameter values are the UA5 ones used in previous
152 versions):
153
154 +-------+---------------------------+---------+
155 | Name | Description | Default |
156 +-------+---------------------------+---------+
157 | PMBN1 | a in <n> = a*S^b+c | 9.11 |
158 | PMBN2 | b in <n> = a*S^b+c | 0.115 |
159 | PMBN3 | c in <n> = a*S^b+c | -9.50 |
160 | | | |
161 | PMBK1 | a in 1/k = a*log_e(S)+b | 0.029 |
162 | PMBK2 | b in 1/k = a*log_e(S)+b | -0.104 |
163 | | | |
164 | PMBM1 | a in (M-m_1-m_2-a)e^{-bM} | 0.4 |
165 | PMBM2 | b in (M-m_1-m_2-a)e^{-bM} | 2.0 |
166 | | | |
167 | PMBP1 | p_t slope for d,u | 5.2 |
168 | PMBP2 | p_t slope for s,c | 3.0 |
169 | PMBP3 | p_t slope for qq | 5.2 |
170 +-------+---------------------------+---------+
171
172 The first three parametrize the mean charged multiplicity at
173 c.m. energy \sqrt{s} as indicated. The next two specify the
174 parameter k in the negative binomial charged multiplicity
175 distribution. The parameters PMBM1 and PMBM2 describe the
176 distribution of cluster masses M in the soft collision. These soft
177 clusters are generated with a flat rapidity distribution with
178 gaussian shoulders. The transverse momentum distribution of soft
179 clusters has the form
180
181 P(p_t)\propto p_t\exp(-b\sqrt{p_t^2+M^2})
182
183 where the slope parameter b depends as indicated on the flavour of
184 the quark or diquark pair created when the cluster was produced.
185
186 As an option, for underlying events the value of \sqrt{s} used to
187 choose the multiplicity n may be enhanced by a parameter ENSOF to
188 allow for an enhanced underlying activity in hard events. The actual
189 charged multiplicity is then taken to be n plus the sum of the
190 moduli of the charges of the colliding hadrons or clusters.
191
192 * There have been a number of additions/changes to the default hadrons
193 included via HWUDAT. Here the identification of hadrons follows the
194 PDG ('98 edition) table 13.2 with numbering according to section 31.
195
196 New isoscalars states have been added to try to complete the 1^3D_3,
197 1^1D_2 and 1^3D_1 multiplets:
198
199 IDHW RNAME IDPDG IDHW RNAME IDPDG
200 ---- ----- ----- ---- ----- -----
201 395 OMEGA_3 227 396 PHI_3 337
202 397 ETA_2(L) 10225 398 ETA_2(H) 10335
203 399 OMEGA(H) 30223
204
205 Also the following states have been re-identified/replaced:
206
207 IDHW RNAME IDPDG IDHW RNAME IDPDG
208 ---- ----- ----- ---- ----- -----
209 57 FH_1 20333
210 293 F0P0 9010221 294 FH_00 10221
211 62 A_0(H)0 10111 290 A_00 9000111
212 63 A_0(H)+ 10211 291 A_0+ 9000211
213 64 A_0(H)- -10211 292 A_0- -9000211
214
215 The f_1(1420) state completely replaces the f_1(1520) in the 1^3P_0
216 multiplet, taking over 57. The f_0(1370) (294) replaces the f_0(980)
217 (293) in the 1^3P_0 multiplet; the latter is retained as it appears
218 in the decays of several other states. The new a_0(1450) states (62
219 -64) replace the three old a_0(980) states (290 - 292) in the 1^3P_0
220 multiplet; the latter are kept, as they appear in f_1(1285) decays.
221
222 By default production of the f_0(980) and a_0(980) states in cluster
223 decays is vetoed.
224
225 Also, the PDG numbers for the remnant particles have been changed to
226 98 for REMG and 99 for REMN.
227
228 * Since version 6.1 contains a large number of supersymmetry processes
229 several new particles have been added.
230
231 Extra scalar bosons for the two Higgs Doublet (SUSY) scenario:
232
233 IDHW RNAME IDPDG IDHW RNAME IDPDG
234 ---- ----- ----- ---- ----- -----
235 203 HIGGSL0 26 206 HIGGS+ 37
236 204 HIGGSH0 35 207 HIGGS- -37
237 205 HIGGSA0 36
238
239 Note that the lighter neutral scalar (203) is given the non-standard
240 PDG number 26, in order to distinguish it from the minimal SM Higgs,
241 PDG number 25.
242
243 Extra sfermions and gauginos for SUSY scenarios:
244
245 IDHW RNAME IDPDG IDHW RNAME IDPDG
246 ---- ----- ----- ---- ----- -----
247 401 SSDL 1000001 413 SSDR 2000001
248 | | | | | |
249 406 SST1 1000006 418 SST2 2000006
250 407 SSDLBR -1000001 419 SSDRBR -2000001
251 | | | | | |
252 412 SST1BR -1000006 424 SST2BR -2000006
253
254 425 SSEL- 1000011 437 SSER- 2000011
255 | | | | | |
256 430 SSNUTL 1000016 442 SSNUTR 2000016
257 431 SSEL+ -1000011 443 SSER+ -2000011
258 | | | | | |
259 436 SSNUTLBR -1000016 448 SSNUTRBR -2000016
260
261 449 GLUINO 1000021 454 CHGINO1+ 1000024
262 450 NTLINO1 1000022 455 CHGINO2+ 1000037
263 451 NTLINO2 1000023 456 CHGINO1 -1000024
264 452 NTLINO3 1000025 457 CHGINO2 -1000037
265 453 NTLINO4 1000035 458 GRAVTINO 1000039
266
267 The implementation of SUSY is discussed more fully below. Note that
268 the default masses of the SUSY particles are zero and that they have
269 no decay modes. Before a SUSY process can be simulated you must load
270 the appropriate masses and decay modes generated using ISAWIG (see
271 below) or an equivalent program.
272
273 These new states don't interfere with the user's ability to add new
274 particles as previously described.
275
276 * It is now possible to create particle property and event listings in
277 any combination of 3 formats - standard ASCII, LaTeX or html. These
278 options are controlled by the new, logical variables PRNDEF [.TRUE.]
279 PRNTEX [.FALSE.] and PRNWEB [.FALSE.]. The ASCII output is directed
280 to stout (screen / log file) as in previous versions. When a listing
281 of particle properties is requested (IPRINT.GE.2 or HWUDPR is called
282 explicitly) then the following files are produced:
283
284 If (PRNTEX): HW_decays.tex
285 If (PRNWEB): HW_decays/index.html
286 /PART0000001.html etc.
287
288 The HW_decays.tex file is written to the working directory whilst
289 the many **.html files appear in the sub-directory HW_decays/ which
290 must have been created previously. Paper sizes and offsets for the
291 LaTeX output are stored at the top of the block data file HWUDAT:
292 they may need modifying to suit a particular printer. When event
293 listings are requested (NEVHEP.LE.MAXPR.NE.0 or HWUEPR is called
294 explicitly) the following files are created in the current working
295 directory:
296
297 If (PRNTEX): HWEV_*******.tex where *******=0000001 etc.
298 If (PRNWEB): HWEV_*******.html is the event number
299
300 Note the .html file automatically makes links to the index.html file
301 of particle properties assumed to be in the HW_decays sub-directory.
302
303 A new integer variable NPRFMT [1] has been introduced to control how
304 many significant figures are shown in each of the 3 event outputs.
305 Basically NPRFMT=1 gives short compact outputs whilst NPRFMT=2 gives
306 long formats.
307
308 Note that all the LaTeX files use the package longtable.sty to
309 format the tables. Also if NPRFMT=2 or PRVTX=.TRUE. then the LaTeX
310 files are designed to be printed in landscape mode.
311
312 * There were previously some inconsistencies and ambiguities in our
313 conventions for the mixing of flavour `octet' and `singlet' mesons.
314 They are now:
315
316 Multiplet Octet Singlet Mixing Angle
317 --------- ----- ------- ------------
318 1^1S_0 eta eta' ETAMIX=-23.
319 1^3S_1 phi omega PHIMIX=+36.
320 1^1P_1 h_1(1380) h_1(1170) H1MIX =ANGLE
321 1^3P_0 MISSING f_0(1370) F0MIX =ANGLE
322 1^3P_1 f_1(1420) f_1(1285) F1MIX =ANGLE
323 1^3P_2 f'_2 f_2 F2MIX =+26.
324 1^1D_2 eta_2(1645) eta_2(1870) ET2MIX=ANGLE
325 1^3D_1 MISSING omega(1600) OMHMIX=ANGLE
326 1^3D_3 phi_3 omega_3 PH3MIX=+28.
327
328 After mixing the quark content of the physical states is given, in
329 terms of the mixing angle, theta, by:
330
331 (ddbar+uubar)/sqrt(2) ssbar
332 --------------------- -----
333 Octet: cos(theta+theta_0) -sin(theta+theta_0)
334 Singlet: sin(theta+theta_0) cos(theta+theta_0)
335
336 where theta_0=ATAN(SQRT(2)). Hence, using the default value of
337 ANGLE=ATAN(1/SQRT(2))*180/ACOS(-ONE) for theta gives ideal mixing,
338 that is, the `octet' state = ssbar and the `singlet' =
339 (ddbar+uubar)/sqrt(2). This choice is important to avoid large
340 isospin violations in the 1^3P_0 and 1^3D_1 multiplets in which the
341 octet member is unknown.
342
343 * A new treatment of the colour interference terms in matrix elements
344 has been introduced in this version. A non-planar, interference term
345 is now shared between the planar terms corresponding to well defined
346 colour flows in proportion to the size of the planar terms. Existing
347 two-to-two QCD processes which have been affected are:
348
349 Light Quarks Heavy Quarks
350 ============ ============
351 Process IHPRO Process IHPRO
352 ------- ----- ------- -----
353 q +q --> q +q 1,2 Q +g --> Q +g 10,11
354 q +qbar --> q +qbar 5,6 Qbar+g --> Qbar+g 21,22
355 qbar+q --> qbar+q 13,14 g +Q --> g +Q 23,24
356 qbar+qbar --> qbar+qbar 18,19 g +Qbar --> g +Qbar 25,26
357 g +g --> Q +Qbar 27,28
358
359 The present and previous treatments of the interference term are the
360 same for the other two-to-two QCD processes which remain unaffected.
361
362 This new procedure has been adopted for all the SUSY QCD processes.
363
364 For details see: K. Odagiri, JHEP 10 (1998) 006
365
366 * A new process, direct gamma-gamma to charged particle pairs has been
367 added. This has IPROC=16000+IQ: if IQ=1-6 then only quark flavour IQ
368 is produced, if IQ=7,8 or 9 then only lepton flavour e, mu or tau is
369 produced and if IQ=10 then only W pairs are produced: in these cases
370 particle masses effects are included. Whilst if IQ=0 the natural mix
371 of quark pairs are produced using massless MEs but including a mass
372 threshold cut. The range of allowed transverse momenta is controlled
373 by PTMIN & PTMAX as usual.
374
375 * A new package ISAWIG has been created to work with ISAJET to produce
376 a file of the SUSY particle masses, lifetimes and decay modes which
377 can be read in by HERWIG.
378
379 This package takes the outputs of the ISAJET SUGRA or general MSSM
380 programs and produces a data file in a format that can be read in by
381 the HWISSP subroutine described below.
382
383 In addition to the decay modes included in the ISAJET package ISAWIG
384 allows for the possibility of violating R-parity and includes the
385 calculation of all 2-body squark and slepton, and 3-body gaugino and
386 gluino R-parity violating decay modes.
387
388 * A new subroutine HWISSP has been added to read the file of particle
389 properties produced by the ISAWIG program. In principle the user can
390 produce a similar file provided that the correct format is used. The
391 format should be as follows.
392
393 First the SUSY particle and top quark masses and lifetimes are given
394 as, for example:
395
396 65
397 401 927.3980 0.74510E-13
398 402 925.3307 0.74009E-13
399 ....etc.
400
401 That is,
402
403 NSUSY=Number of SUSY+top particles
404 IDHW, RMASS(IDHW) & RLTIM(IDHW)
405 repeated NSUSY times.
406
407 Next each particle's decay modes together with their branching
408 ratios and matrix element codes are given as, for example:
409
410 6
411 401 0.18842796E-01 0 450 1 0 0 0
412 | | | | | | | |
413 401 0.32755006E-02 0 457 2 0 0 0
414 6
415 402 0.94147678E-02 0 450 2 0 0 0
416 ....etc.
417
418 That is,
419
420 Number of decay modes for a given particle (IDK)
421 IDK(*), BRFRAC(*), NME(*) & IDKPRD(1-5,*)
422 repeated for each mode.
423
424 Repeated for each particle (NSUSY times).
425
426 The order in which the decay products appear is significant: this is
427 important inorder to obtain appropriate showering and hadronization.
428 The correct ordering for each decay mode is indicated below.
429
430 +----------+------------------------+------------------------------+
431 | Decaying | Type of Mode | Order of Decay Products: |
432 | Particle | | 1st | 2nd | 3rd |
433 +----------+------------------------+---------+---------+----------+
434 | Top | 2 body to Higgs | Higgs | Bottom | |
435 | +------------------------+---------+---------+----------+
436 | | 3 body via Higgs/W | quarks or leptons | Bottom |
437 | | | from W/Higgs | |
438 +----------+------------------------+---------+---------+----------+
439 | Gluino | 2 body modes: | | | |
440 | | without gluon | any order | |
441 | | with gluon | gluon | colour | |
442 | | | | neutral | |
443 | +------------------------+---------+---------+----------+
444 | | 3 body modes: | colour | q or qbar |
445 | | R-parity conserved | neutral | |
446 +----------+------------------------+---------+---------+----------+
447 | Squark/ | 2 body modes: | | | |
448 | Slepton | Gaugino/Gluino | Gaugino | quark | |
449 | | Quark/Lepton | Gluino | lepton | |
450 | +------------------------+---------+---------+----------+
451 | | 3 body modes: |sparticle| particles from |
452 | | Weak | | W decay |
453 +----------+------------------------+---------+---------+----------+
454 | Squark | 2 body modes: | | | |
455 | | Lepton Number Violated | quark | lepton | |
456 | | Baryon Number Violated | quark | quark | |
457 +----------+------------------------+---------+---------+----------+
458 | Slepton | 2 body modes: | q or qbar | |
459 | | Lepton Number Violated | | | |
460 +----------+------------------------+---------+---------+----------+
461 | Higgs | 2 body modes: | | | |
462 | | (s)quark-(s)qbar | (s)q or (s)qbar | |
463 | | (s)lepton-(s)lepton | (s)l or (s)lbar | |
464 | +------------------------+---------+---------+----------+
465 | | 3 body modes: | colour | q or qbar |
466 | | | neutral | l or lbar |
467 +----------+------------------------+---------+---------+----------+
468 | Gaugino | 2 body modes: | | | |
469 | | squark-quark | q or sq | |
470 | | slepton-lepton | l or sl | |
471 | +------------------------+---------+---------+----------+
472 | | 3 body modes: | colour | q or qbar |
473 | | R-parity conserved | neutral | l or lbar |
474 +----------+------------------------+---------+---------+----------+
475 | Gaugino/ | 3 body modes: | particles in the order i,j,k |
476 | Gluino | R-parity violating | |
477 +----------+------------------------+---------+---------+----------+
478
479 A new matrix element code has been added for these decays:
480
481 NME = 300 3 body R-parity violating gaugino and gluino decays
482
483 in addition, an extra matrix element code has been reserved for use
484 in a forthcoming version:
485
486 NME = 200 3 body top quark via charged Higgs
487
488 The indices i,j,k in R-parity violating gaugino/gluino decays refer
489 to the ordering of the indices in the R-parity violating couplings
490 in the superpotential. The convention is as in:
491
492 H.Dreiner, P.Richardson and M.H.Seymour, hep-ph/9912407.
493
494 Next a number of parameters derived from the SUSY Lagrangian must be
495 given. These are: the ratio of Higgs VEVs, tan(beta), and the scalar
496 Higgs mixing angle, alpha; the mixing parameters for the Higgses,
497 gauginos and the sleptons; the trilinear couplings; and the Higgsino
498 mass parameter mu.
499
500 Finally the logical variable RPARTY should be set: if FALSE then
501 R-parity is violated, and the R-parity violating couplings must also
502 be supplied, otherwise not.
503
504 Details of the FORMAT statements employed can be found by examining
505 the subroutine HWISSP.
506
507 The integer argument in the call to HWISSP(N) gives the unit number
508 to be read from. If the data is stored in a `fort.N' file no further
509 action is required but if the data is to be read from a file named
510 `fname.dat' then appropriate OPEN and CLOSE statements must be added
511 by hand:
512
513 OPEN(UNIT=N,FORM='FORMATTED',STATUS='UNKNOWN',FILE='fname.dat')
514 CALL HWISSP(N)
515 CLOSE(UNIT=N)
516
517 A number of sets of SUSY parameter files, produced using ISAWIG, for
518 the standard LHC SUGRA and GMSB points are available from the HERWIG
519 home page: http://hepwww.rl.ac.uk/theory/seymour/herwig/
520
521 * A large number of changes have been made to enable SUSY processes to
522 be included in hadron-hadron collisions. The main changes are:
523
524 - The subroutine HWDHQK has been replaced by HWDHOB which does both
525 heavy quark and SUSY particle decays.
526
527 - The subroutines HWBCON HWCGSP & HWCFOR have been adapted to handle
528 the colour connections found in normal SUSY decays.
529
530 - The subroutine HWBRCN has been included to deal with the inter-jet
531 colour connections arising in R-parity violating SUSY. Also HWCBVI
532 HWCBVT and HWCBCT have been added to handle the hadronization of
533 baryon number violating SUSY decays and processes. If the variable
534 RPARTY=.TRUE. [default] then the old HWBCON colour connection code
535 is used else the new HWBRCN
536
537 * The option of separate treatments for `light' and b-quark containing
538 clusters are now available. The 3 variables, PSPLT (which controls
539 the mass spectrum of the fragments in heavy cluster splitting) CLDIR
540 (which controls whether perturbatively produced (anti-)quarks retain
541 some knowledge of their direction in cluster decays to hadrons) and
542 CLSMR (which defines to what extent the hadron and constituent quark
543 directions are aligned), have been made two dimensional.
544
545 ARRAY(1) controls clusters that do NOT contain a b quark
546 ARRAY(2) controls clusters that do contain a b quark
547
548 [ Default ARRAY(1)=ARRAY(2) equivalent to earlier versions. ]
549
550 * A new variable EFFMIN [1E-3] has been introduced, it allows the user
551 to set the minimum acceptable efficiency for event generation.
552
553 * All hadron & lepton masses are now given to five significant figures
554 whenever possible.
555
556 * The treatment of the perturbative g --> qqbar vertex in the partonic
557 showers has been improved. The total rate is unchanged, but the
558 angular distribution now covers the full range, rather than being
559 confined to the angular-ordered region as before.
560
561 * The treatment of the intrinsic transverse momentum of partons in an
562 incoming hadron has been improved. It is now chosen before the
563 initial state cascade is performed, and is held fixed even if the
564 generated cascade is rejected. This removes a correlation between
565 the amount of perturbative and non-perturbative transverse momentum
566 generated that existed before.
567
568 * Space-time positioning of clusters is now smeared according to a
569 Gaussian distribution of width 1/(cluster mass).
570
571 * For e+e- processes with ISR a check was added requiring TMNISR to be
572 greater than the light quark threshold.
573
574 * The treatment of the W resonance in top decays has been improved.
575
576 * The common block file HERWIG61.INC has had many new variables added,
577 these are listed at the top of the file.
578
579 * Corrections for bugs have been made affecting the following:
580
581 - eta-eta' mixing: the parameterization was nonstandard (see above).
582
583 - 4/5 body phase space generation: was not flat - affected resonance
584 decays only.
585
586 - Drell-Yan: the overall normalization was too small by a factor 2/3
587 also the t-channel contribution to q-qbar-> q-qbar was incorrectly
588 normalized.
589
590 - HWHV1J: the normalization of Z+jet production rate was a factor 4
591 too small; there was an incorrect correlation between the (signed)
592 W and jet rapidities; the treatment of the W/Z Breit-Wigner lead a
593 normalization error by a factor 3/pi.
594
595 - HWHWPR: there was an overall normalization error of (M_ff'/M_w)^2,
596 this only affected the line shape and normalization for the t-bbar
597 final state for which M_ff' is large.
598
599 - B_d/_s mixing: an incorrect formula was used.
600
601 - VMIN2: the effective cut-off on the space-time distances travelled
602 by light partons in a shower was incorrectly implemented. Also its
603 default value has been increased to [0.1], which affects the
604 colour reconnection probability.
605
606 - A number of fixes to improve safety against overflowing the HEPEVT
607 common block.
608
609 - Fix to the underlying event to prevent errors with heavy quarks.
610
611 - HWMODK/HWIODK: a number of corrections were made and the code made
612 more robust.
613
614 - HWURES: the minimum threshold for the decay of diquark-antidiquark
615 clusters was incorrectly set.
616
617 - The calculation of the top lifetime has been corrected and the QCD
618 corrections included - this only affects the treatment of colour
619 reconnection.
620
621 - The space-time positioning of clusters sometimes led to them being
622 produced outside the forward lightcone. This has been rectified.
623
624 As usual, if you wish to be removed from the HERWIG mailing list, or
625 if you know someone who wants to be added, please let one of us
626 know.
627
628 Mike Seymour, Bryan Webber, Ian Knowles, Peter Richardson, Kosuke
629 Odagiri, Stefano Moretti, Gennaro Corcella, Pino Marchesini
630
631 CERN, Edinburgh, Oxford, RAL, Rochester, Milano, etc,
632 16th December 1999.