A new version of the Monte Carlo program HERWIG (version 6.1) is now available, and can be obtained from the following web site: http://hepwww.rl.ac.uk/theory/seymour/herwig/ This will temporarily be mirrored at CERN for the next few weeks: http://home.cern.ch/~seymour/herwig/ More complete information on HERWIG can be found in the publication G. Marchesini, B.R. Webber, G. Abbiendi, I.G. Knowles, M.H. Seymour and L. Stanco, Computer Phys. Commun. 67 (1992) 465 and also in the documentation for the previous version (5.9), which are available at the same site, together with other useful files and information. Here we merely give the new features relative to 5.9. If you use HERWIG, please refer to it something along the lines of: HERWIG 6.1, hep-ph/9912396; G. Marchesini, B.R. Webber, G. Abbiendi, I.G. Knowles, M.H. Seymour and L. Stanco, Computer Phys. Commun. 67 (1992) 465. *** NEW FEATURES OF THIS VERSION *** *---------------------------------------------------------------* | The main new features are: supersymmetric processes (both | | R-parity conserving & violating) in hadron-hadron collisions; | | new e+e- to four jets process; matrix element corrections to | | top decay and Drell-Yan processes; new soft underlying event | | options; updates to default particle data tables; new LaTeX & | | html printout options. | *---------------------------------------------------------------* * [N.B. Default values for input variables shown in square brackets.] * All R-parity conserving SUSY two-to-two processes in hadron-hadron collisions have been added. Their process numbers are: +-------+----------------------------------------------------------+ | IPROC | Process | +-------+----------------------------------------------------------+ | 3000 | 2 parton to 2 sparticles: the sum of 3010,3020 and 3030 | | 3010 | 2 parton to 2 spartons | | 3020 | 2 parton to 2 gauginos | | 3030 | 2 parton to 2 sleptons | +-------+----------------------------------------------------------+ Further details of the inclusion of superpartners and their decays are given below. Additional processes for the SUSY two Higgs doublet model are currently under test and will be released shortly. * All R-parity violating SUSY two-to-two processes via resonant sleptons and squarks in hadron collisions have been added. Their process numbers are: +-------+----------------------------------------------------------+ | IPROC | Processes derived from the LQD term in the superpotential| +-------+----------------------------------------------------------+ | 4000 | The sum of 4010,4020,4040 and 4050 | | 4010 | Neutralino lepton production (all neutralinos) | | 401i | As 4010 but only the ith neutralino | | 4020 | Chargino lepton production (all charginos) | | 402i | As 4020 but only the ith chargino | | 4040 | Slepton W/Z production | | 4050 | Slepton Higgs production | +-------+----------------------------------------------------------+ | 4060 | Sum of 4070 and 4080 | | 4070 | quark-antiquark production via LQD | | 4080 | lepton production via LLE and LQD | +=======+==========================================================+ | IPROC | Processes derived from the UDD term in the superpotential| +-------+----------------------------------------------------------+ | 4100 | The sum of 4110, 4120, 4130, 4140 and 4150 | | 4110 | Neutralino quark production (all neutralinos) | | 411i | As 4110 but only the ith neutralino | | 4120 | Chargino quark production (all charginos) | | 412i | As 4120 but only the ith chargino | | 4130 | Gluino quark production | | 4140 | Squark W/Z production | | 4150 | Squark Higgs production | +-------+----------------------------------------------------------+ | 4160 | quark-quark production | +-------+----------------------------------------------------------+ In addition the R-parity violating decays of all superpartners is included. * A new process describing electron-positron annihilation to four jets has been added. This has IPROC=600+IQ, where a non-zero value for IQ guarantees production of quark flavour IQ whilst IQ=0 corresponds to the natural flavour mix. IPROC=650+IQ is as above but without those terms in the matrix element which orient the event w.r.t. the lepton beam direction. The matrix elements are based on those of Ellis Ross & Terrano with orientation terms from Catani & Seymour. The soft and collinear divergences are avoided by imposing a minimum y-cut, Y4JT [.01], on the initial 4 partons. The interjet distance is calculated using either the Durham or JADE metrics. This choice is governed by the logical variable DURHAM [.TRUE.]. Note that parameterizations of the volume of four-body phase space are used: these are accurate up to a few percent for y-cut values less than 0.14. Note, also that the phase space is for massless partons, as are the matrix elements, though a mass threshold cut is applied. Finally, the matrix elements for the q-qbar-g-g & q-qbar-q-qbar (same flavour quark) final states receive contributions from 2 colour flows each, the treatment of the interference terms being controlled by the array IOP4JT: q-qbar-g-g case: IOP4JT(1)=0 neglect, =1 extreme 2341; =2 extreme 3421 [0] q-qbar-q-qbar (identical quark flavour) case: IOP4JT(2)=0 neglect, =1 extreme 4123; =2 extreme 2143 [0] The scale EMSCA for the parton showers is set equal to SQRT(s*ymin) where ymin is the least distance, according to the selected metric, between any two partons. * Matrix element corrections to the simulation of top quark decays and Drell-Yan processes are now available using the same general method as already implemented for e+e- annihilation and DIS. If HARDME [.TRUE.] then fill the missing phase-space (`dead zone') using the exact 1st-order M.E. result (`hard corrections'). If SOFTME [.TRUE.] then correct emissions in the already-populated region of phase space using the exact amplitude for every emission that is capable of being the hardest so far (`soft corrections'). - For t -> bW decays the routine HWBTOP implements hard corrections. HWBRAN has been modified to implement the soft corrections to top decays. Since the dead zone includes part of the soft singularity a cutoff is required: only gluons with energy above GCUTME [2 GeV] (in the top rest frame) are corrected. Physical quantities are not strongly dependent on GCUTME in the range 1 to 5 GeV. For details see: G. Corcella and M.H. Seymour, Phys. Lett. B442 (1998) 417. - For the Drell-Yan process the routine HWBDYP implements the hard corrections whilst HWSBRN has been modified to implement the soft corrections to the initial state radiation. For details see: G. Corcella and M.H. Seymour, hep-ph/9908338. * The parameters of the model used for soft interactions are now available to the user for modification. The model is based on the minimum-bias event generator of the UA5 Collaboration, which starts from a parametrization of the pbar p inelastic charged multiplicity distribution as a negative binomial. The parameters are as follows (default parameter values are the UA5 ones used in previous versions): +-------+---------------------------+---------+ | Name | Description | Default | +-------+---------------------------+---------+ | PMBN1 | a in = a*S^b+c | 9.11 | | PMBN2 | b in = a*S^b+c | 0.115 | | PMBN3 | c in = a*S^b+c | -9.50 | | | | | | PMBK1 | a in 1/k = a*log_e(S)+b | 0.029 | | PMBK2 | b in 1/k = a*log_e(S)+b | -0.104 | | | | | | PMBM1 | a in (M-m_1-m_2-a)e^{-bM} | 0.4 | | PMBM2 | b in (M-m_1-m_2-a)e^{-bM} | 2.0 | | | | | | PMBP1 | p_t slope for d,u | 5.2 | | PMBP2 | p_t slope for s,c | 3.0 | | PMBP3 | p_t slope for qq | 5.2 | +-------+---------------------------+---------+ The first three parametrize the mean charged multiplicity at c.m. energy \sqrt{s} as indicated. The next two specify the parameter k in the negative binomial charged multiplicity distribution. The parameters PMBM1 and PMBM2 describe the distribution of cluster masses M in the soft collision. These soft clusters are generated with a flat rapidity distribution with gaussian shoulders. The transverse momentum distribution of soft clusters has the form P(p_t)\propto p_t\exp(-b\sqrt{p_t^2+M^2}) where the slope parameter b depends as indicated on the flavour of the quark or diquark pair created when the cluster was produced. As an option, for underlying events the value of \sqrt{s} used to choose the multiplicity n may be enhanced by a parameter ENSOF to allow for an enhanced underlying activity in hard events. The actual charged multiplicity is then taken to be n plus the sum of the moduli of the charges of the colliding hadrons or clusters. * There have been a number of additions/changes to the default hadrons included via HWUDAT. Here the identification of hadrons follows the PDG ('98 edition) table 13.2 with numbering according to section 31. New isoscalars states have been added to try to complete the 1^3D_3, 1^1D_2 and 1^3D_1 multiplets: IDHW RNAME IDPDG IDHW RNAME IDPDG ---- ----- ----- ---- ----- ----- 395 OMEGA_3 227 396 PHI_3 337 397 ETA_2(L) 10225 398 ETA_2(H) 10335 399 OMEGA(H) 30223 Also the following states have been re-identified/replaced: IDHW RNAME IDPDG IDHW RNAME IDPDG ---- ----- ----- ---- ----- ----- 57 FH_1 20333 293 F0P0 9010221 294 FH_00 10221 62 A_0(H)0 10111 290 A_00 9000111 63 A_0(H)+ 10211 291 A_0+ 9000211 64 A_0(H)- -10211 292 A_0- -9000211 The f_1(1420) state completely replaces the f_1(1520) in the 1^3P_0 multiplet, taking over 57. The f_0(1370) (294) replaces the f_0(980) (293) in the 1^3P_0 multiplet; the latter is retained as it appears in the decays of several other states. The new a_0(1450) states (62 -64) replace the three old a_0(980) states (290 - 292) in the 1^3P_0 multiplet; the latter are kept, as they appear in f_1(1285) decays. By default production of the f_0(980) and a_0(980) states in cluster decays is vetoed. Also, the PDG numbers for the remnant particles have been changed to 98 for REMG and 99 for REMN. * Since version 6.1 contains a large number of supersymmetry processes several new particles have been added. Extra scalar bosons for the two Higgs Doublet (SUSY) scenario: IDHW RNAME IDPDG IDHW RNAME IDPDG ---- ----- ----- ---- ----- ----- 203 HIGGSL0 26 206 HIGGS+ 37 204 HIGGSH0 35 207 HIGGS- -37 205 HIGGSA0 36 Note that the lighter neutral scalar (203) is given the non-standard PDG number 26, in order to distinguish it from the minimal SM Higgs, PDG number 25. Extra sfermions and gauginos for SUSY scenarios: IDHW RNAME IDPDG IDHW RNAME IDPDG ---- ----- ----- ---- ----- ----- 401 SSDL 1000001 413 SSDR 2000001 | | | | | | 406 SST1 1000006 418 SST2 2000006 407 SSDLBR -1000001 419 SSDRBR -2000001 | | | | | | 412 SST1BR -1000006 424 SST2BR -2000006 425 SSEL- 1000011 437 SSER- 2000011 | | | | | | 430 SSNUTL 1000016 442 SSNUTR 2000016 431 SSEL+ -1000011 443 SSER+ -2000011 | | | | | | 436 SSNUTLBR -1000016 448 SSNUTRBR -2000016 449 GLUINO 1000021 454 CHGINO1+ 1000024 450 NTLINO1 1000022 455 CHGINO2+ 1000037 451 NTLINO2 1000023 456 CHGINO1 -1000024 452 NTLINO3 1000025 457 CHGINO2 -1000037 453 NTLINO4 1000035 458 GRAVTINO 1000039 The implementation of SUSY is discussed more fully below. Note that the default masses of the SUSY particles are zero and that they have no decay modes. Before a SUSY process can be simulated you must load the appropriate masses and decay modes generated using ISAWIG (see below) or an equivalent program. These new states don't interfere with the user's ability to add new particles as previously described. * It is now possible to create particle property and event listings in any combination of 3 formats - standard ASCII, LaTeX or html. These options are controlled by the new, logical variables PRNDEF [.TRUE.] PRNTEX [.FALSE.] and PRNWEB [.FALSE.]. The ASCII output is directed to stout (screen / log file) as in previous versions. When a listing of particle properties is requested (IPRINT.GE.2 or HWUDPR is called explicitly) then the following files are produced: If (PRNTEX): HW_decays.tex If (PRNWEB): HW_decays/index.html /PART0000001.html etc. The HW_decays.tex file is written to the working directory whilst the many **.html files appear in the sub-directory HW_decays/ which must have been created previously. Paper sizes and offsets for the LaTeX output are stored at the top of the block data file HWUDAT: they may need modifying to suit a particular printer. When event listings are requested (NEVHEP.LE.MAXPR.NE.0 or HWUEPR is called explicitly) the following files are created in the current working directory: If (PRNTEX): HWEV_*******.tex where *******=0000001 etc. If (PRNWEB): HWEV_*******.html is the event number Note the .html file automatically makes links to the index.html file of particle properties assumed to be in the HW_decays sub-directory. A new integer variable NPRFMT [1] has been introduced to control how many significant figures are shown in each of the 3 event outputs. Basically NPRFMT=1 gives short compact outputs whilst NPRFMT=2 gives long formats. Note that all the LaTeX files use the package longtable.sty to format the tables. Also if NPRFMT=2 or PRVTX=.TRUE. then the LaTeX files are designed to be printed in landscape mode. * There were previously some inconsistencies and ambiguities in our conventions for the mixing of flavour `octet' and `singlet' mesons. They are now: Multiplet Octet Singlet Mixing Angle --------- ----- ------- ------------ 1^1S_0 eta eta' ETAMIX=-23. 1^3S_1 phi omega PHIMIX=+36. 1^1P_1 h_1(1380) h_1(1170) H1MIX =ANGLE 1^3P_0 MISSING f_0(1370) F0MIX =ANGLE 1^3P_1 f_1(1420) f_1(1285) F1MIX =ANGLE 1^3P_2 f'_2 f_2 F2MIX =+26. 1^1D_2 eta_2(1645) eta_2(1870) ET2MIX=ANGLE 1^3D_1 MISSING omega(1600) OMHMIX=ANGLE 1^3D_3 phi_3 omega_3 PH3MIX=+28. After mixing the quark content of the physical states is given, in terms of the mixing angle, theta, by: (ddbar+uubar)/sqrt(2) ssbar --------------------- ----- Octet: cos(theta+theta_0) -sin(theta+theta_0) Singlet: sin(theta+theta_0) cos(theta+theta_0) where theta_0=ATAN(SQRT(2)). Hence, using the default value of ANGLE=ATAN(1/SQRT(2))*180/ACOS(-ONE) for theta gives ideal mixing, that is, the `octet' state = ssbar and the `singlet' = (ddbar+uubar)/sqrt(2). This choice is important to avoid large isospin violations in the 1^3P_0 and 1^3D_1 multiplets in which the octet member is unknown. * A new treatment of the colour interference terms in matrix elements has been introduced in this version. A non-planar, interference term is now shared between the planar terms corresponding to well defined colour flows in proportion to the size of the planar terms. Existing two-to-two QCD processes which have been affected are: Light Quarks Heavy Quarks ============ ============ Process IHPRO Process IHPRO ------- ----- ------- ----- q +q --> q +q 1,2 Q +g --> Q +g 10,11 q +qbar --> q +qbar 5,6 Qbar+g --> Qbar+g 21,22 qbar+q --> qbar+q 13,14 g +Q --> g +Q 23,24 qbar+qbar --> qbar+qbar 18,19 g +Qbar --> g +Qbar 25,26 g +g --> Q +Qbar 27,28 The present and previous treatments of the interference term are the same for the other two-to-two QCD processes which remain unaffected. This new procedure has been adopted for all the SUSY QCD processes. For details see: K. Odagiri, JHEP 10 (1998) 006 * A new process, direct gamma-gamma to charged particle pairs has been added. This has IPROC=16000+IQ: if IQ=1-6 then only quark flavour IQ is produced, if IQ=7,8 or 9 then only lepton flavour e, mu or tau is produced and if IQ=10 then only W pairs are produced: in these cases particle masses effects are included. Whilst if IQ=0 the natural mix of quark pairs are produced using massless MEs but including a mass threshold cut. The range of allowed transverse momenta is controlled by PTMIN & PTMAX as usual. * A new package ISAWIG has been created to work with ISAJET to produce a file of the SUSY particle masses, lifetimes and decay modes which can be read in by HERWIG. This package takes the outputs of the ISAJET SUGRA or general MSSM programs and produces a data file in a format that can be read in by the HWISSP subroutine described below. In addition to the decay modes included in the ISAJET package ISAWIG allows for the possibility of violating R-parity and includes the calculation of all 2-body squark and slepton, and 3-body gaugino and gluino R-parity violating decay modes. * A new subroutine HWISSP has been added to read the file of particle properties produced by the ISAWIG program. In principle the user can produce a similar file provided that the correct format is used. The format should be as follows. First the SUSY particle and top quark masses and lifetimes are given as, for example: 65 401 927.3980 0.74510E-13 402 925.3307 0.74009E-13 ....etc. That is, NSUSY=Number of SUSY+top particles IDHW, RMASS(IDHW) & RLTIM(IDHW) repeated NSUSY times. Next each particle's decay modes together with their branching ratios and matrix element codes are given as, for example: 6 401 0.18842796E-01 0 450 1 0 0 0 | | | | | | | | 401 0.32755006E-02 0 457 2 0 0 0 6 402 0.94147678E-02 0 450 2 0 0 0 ....etc. That is, Number of decay modes for a given particle (IDK) IDK(*), BRFRAC(*), NME(*) & IDKPRD(1-5,*) repeated for each mode. Repeated for each particle (NSUSY times). The order in which the decay products appear is significant: this is important inorder to obtain appropriate showering and hadronization. The correct ordering for each decay mode is indicated below. +----------+------------------------+------------------------------+ | Decaying | Type of Mode | Order of Decay Products: | | Particle | | 1st | 2nd | 3rd | +----------+------------------------+---------+---------+----------+ | Top | 2 body to Higgs | Higgs | Bottom | | | +------------------------+---------+---------+----------+ | | 3 body via Higgs/W | quarks or leptons | Bottom | | | | from W/Higgs | | +----------+------------------------+---------+---------+----------+ | Gluino | 2 body modes: | | | | | | without gluon | any order | | | | with gluon | gluon | colour | | | | | | neutral | | | +------------------------+---------+---------+----------+ | | 3 body modes: | colour | q or qbar | | | R-parity conserved | neutral | | +----------+------------------------+---------+---------+----------+ | Squark/ | 2 body modes: | | | | | Slepton | Gaugino/Gluino | Gaugino | quark | | | | Quark/Lepton | Gluino | lepton | | | +------------------------+---------+---------+----------+ | | 3 body modes: |sparticle| particles from | | | Weak | | W decay | +----------+------------------------+---------+---------+----------+ | Squark | 2 body modes: | | | | | | Lepton Number Violated | quark | lepton | | | | Baryon Number Violated | quark | quark | | +----------+------------------------+---------+---------+----------+ | Slepton | 2 body modes: | q or qbar | | | | Lepton Number Violated | | | | +----------+------------------------+---------+---------+----------+ | Higgs | 2 body modes: | | | | | | (s)quark-(s)qbar | (s)q or (s)qbar | | | | (s)lepton-(s)lepton | (s)l or (s)lbar | | | +------------------------+---------+---------+----------+ | | 3 body modes: | colour | q or qbar | | | | neutral | l or lbar | +----------+------------------------+---------+---------+----------+ | Gaugino | 2 body modes: | | | | | | squark-quark | q or sq | | | | slepton-lepton | l or sl | | | +------------------------+---------+---------+----------+ | | 3 body modes: | colour | q or qbar | | | R-parity conserved | neutral | l or lbar | +----------+------------------------+---------+---------+----------+ | Gaugino/ | 3 body modes: | particles in the order i,j,k | | Gluino | R-parity violating | | +----------+------------------------+---------+---------+----------+ A new matrix element code has been added for these decays: NME = 300 3 body R-parity violating gaugino and gluino decays in addition, an extra matrix element code has been reserved for use in a forthcoming version: NME = 200 3 body top quark via charged Higgs The indices i,j,k in R-parity violating gaugino/gluino decays refer to the ordering of the indices in the R-parity violating couplings in the superpotential. The convention is as in: H.Dreiner, P.Richardson and M.H.Seymour, hep-ph/9912407. Next a number of parameters derived from the SUSY Lagrangian must be given. These are: the ratio of Higgs VEVs, tan(beta), and the scalar Higgs mixing angle, alpha; the mixing parameters for the Higgses, gauginos and the sleptons; the trilinear couplings; and the Higgsino mass parameter mu. Finally the logical variable RPARTY should be set: if FALSE then R-parity is violated, and the R-parity violating couplings must also be supplied, otherwise not. Details of the FORMAT statements employed can be found by examining the subroutine HWISSP. The integer argument in the call to HWISSP(N) gives the unit number to be read from. If the data is stored in a `fort.N' file no further action is required but if the data is to be read from a file named `fname.dat' then appropriate OPEN and CLOSE statements must be added by hand: OPEN(UNIT=N,FORM='FORMATTED',STATUS='UNKNOWN',FILE='fname.dat') CALL HWISSP(N) CLOSE(UNIT=N) A number of sets of SUSY parameter files, produced using ISAWIG, for the standard LHC SUGRA and GMSB points are available from the HERWIG home page: http://hepwww.rl.ac.uk/theory/seymour/herwig/ * A large number of changes have been made to enable SUSY processes to be included in hadron-hadron collisions. The main changes are: - The subroutine HWDHQK has been replaced by HWDHOB which does both heavy quark and SUSY particle decays. - The subroutines HWBCON HWCGSP & HWCFOR have been adapted to handle the colour connections found in normal SUSY decays. - The subroutine HWBRCN has been included to deal with the inter-jet colour connections arising in R-parity violating SUSY. Also HWCBVI HWCBVT and HWCBCT have been added to handle the hadronization of baryon number violating SUSY decays and processes. If the variable RPARTY=.TRUE. [default] then the old HWBCON colour connection code is used else the new HWBRCN * The option of separate treatments for `light' and b-quark containing clusters are now available. The 3 variables, PSPLT (which controls the mass spectrum of the fragments in heavy cluster splitting) CLDIR (which controls whether perturbatively produced (anti-)quarks retain some knowledge of their direction in cluster decays to hadrons) and CLSMR (which defines to what extent the hadron and constituent quark directions are aligned), have been made two dimensional. ARRAY(1) controls clusters that do NOT contain a b quark ARRAY(2) controls clusters that do contain a b quark [ Default ARRAY(1)=ARRAY(2) equivalent to earlier versions. ] * A new variable EFFMIN [1E-3] has been introduced, it allows the user to set the minimum acceptable efficiency for event generation. * All hadron & lepton masses are now given to five significant figures whenever possible. * The treatment of the perturbative g --> qqbar vertex in the partonic showers has been improved. The total rate is unchanged, but the angular distribution now covers the full range, rather than being confined to the angular-ordered region as before. * The treatment of the intrinsic transverse momentum of partons in an incoming hadron has been improved. It is now chosen before the initial state cascade is performed, and is held fixed even if the generated cascade is rejected. This removes a correlation between the amount of perturbative and non-perturbative transverse momentum generated that existed before. * Space-time positioning of clusters is now smeared according to a Gaussian distribution of width 1/(cluster mass). * For e+e- processes with ISR a check was added requiring TMNISR to be greater than the light quark threshold. * The treatment of the W resonance in top decays has been improved. * The common block file HERWIG61.INC has had many new variables added, these are listed at the top of the file. * Corrections for bugs have been made affecting the following: - eta-eta' mixing: the parameterization was nonstandard (see above). - 4/5 body phase space generation: was not flat - affected resonance decays only. - Drell-Yan: the overall normalization was too small by a factor 2/3 also the t-channel contribution to q-qbar-> q-qbar was incorrectly normalized. - HWHV1J: the normalization of Z+jet production rate was a factor 4 too small; there was an incorrect correlation between the (signed) W and jet rapidities; the treatment of the W/Z Breit-Wigner lead a normalization error by a factor 3/pi. - HWHWPR: there was an overall normalization error of (M_ff'/M_w)^2, this only affected the line shape and normalization for the t-bbar final state for which M_ff' is large. - B_d/_s mixing: an incorrect formula was used. - VMIN2: the effective cut-off on the space-time distances travelled by light partons in a shower was incorrectly implemented. Also its default value has been increased to [0.1], which affects the colour reconnection probability. - A number of fixes to improve safety against overflowing the HEPEVT common block. - Fix to the underlying event to prevent errors with heavy quarks. - HWMODK/HWIODK: a number of corrections were made and the code made more robust. - HWURES: the minimum threshold for the decay of diquark-antidiquark clusters was incorrectly set. - The calculation of the top lifetime has been corrected and the QCD corrections included - this only affects the treatment of colour reconnection. - The space-time positioning of clusters sometimes led to them being produced outside the forward lightcone. This has been rectified. As usual, if you wish to be removed from the HERWIG mailing list, or if you know someone who wants to be added, please let one of us know. Mike Seymour, Bryan Webber, Ian Knowles, Peter Richardson, Kosuke Odagiri, Stefano Moretti, Gennaro Corcella, Pino Marchesini CERN, Edinburgh, Oxford, RAL, Rochester, Milano, etc, 16th December 1999.