A new version of the Monte Carlo program HERWIG (version 5.9) is now available, and can be obtained from the following web site: http://hepwww.rl.ac.uk/theory/seymour/herwig/ *** NEW FEATURES OF THIS VERSION *** * The common block file HERWIG59.INC has been significantly rearranged and tidied up. * Many new hadrons have been added. All S & P wave mesons are present including the 1^P_0 & 3^P_1 states and many new, excited B^**, B_c & quarkonium states. Also all D wave kaons and some `light' I=3 states [pi_2, rho(1700) & rho_3]. All the baryons (singlet/octet/decuplet) containing up to one heavy (c,b) quark are included. --- Consequently the default parameters require retuning --- * New 8-character particle names have been introduced and the revised 7 digit PDG numbering scheme, as advocated in the LEP2 report, has been adopted. * The layout of HWUDAT has been altered to make it easier to identify and modify particle propeties. Three new arrays have been introduced RLTIM, RSPIN & IFLAV. These are: the particle's lifetime (ps), spin, and a code which specifies the flavour content of each hadron - used (in HWURES) to create sets of iso-flavour hadrons for cluster decay. Using the standard numbering of quark flavours the convention is: mesons: n_q n_qbar Eg. pi^+: 21, pi^-: 12 baryons: +/-n_q1 n_q2 n_q3 Eg. Xi^0: 332, Xi^0bar: -332 etc. (-ve for antibaryons; digits in decreasing order) Light, neutral mesons are identified as: 11 if I=1: pi^0,rho^0,... 33 if I=0: eta, eta'.. etc. Some parts of the program have been automated so that it is possible for the user to add new particles by specifying their properties via the arrays in /HWPROP/ & /HWUNAM/ and increasing NRES appropriately: this should be done before a call to HWUINC. As an example following lines add an isoscalar, spin pi state 'STAN' and a (very light) stable toponium state 'BEER' with the decay mode: STAN ---> BEER+BEER+BEER. NRES=NRES+1 RNAME(NRES)='STAN ' IDPDG(NRES)=666 IFLAV(NRES)=11 ICHRG(NRES)=0. RMASS(NRES)=0.5 RLTIM(NRES)=1.000D-10 RSPIN(NRES)=3.142 NRES=NRES+1 RNAME(NRES)='BEER ' IDPDG(NRES)=66 IFLAV(NRES)=66 ICHRG(NRES)=0. RMASS(NRES)=0.1 RLTIM(NRES)=1.000D+30 RSPIN(NRES)=0.0 CALL HWMODK(666,1.D0,0,66,66,66,0,0) * The mixing angles of all the light, I=0 mesons can now be set using: ETAMIX: eta <-> eta' F0MIX: f_0(1300) <-> f_0(980) PHIMIX: omega <-> phi, F1MIX: f_1(1285) <-> f_1(1510) H1MIX: h_1(1170) <-> h_1(1380) F2MIX: f_2 <-> f_2' * Using the logical arrays VTOCDK & VTORDK the production of specified particles can be stopped in both cluster decays and via the decay of other unstable resonances. * A priori weights for the relative production rates in cluster decays of mesons and baryons differing only via their S & L quantum numbers can be supplied using SNGWT & DECWT for singlet (i.e. Lambda-like) & decuplet baryons and REPWT for mesons. The old VECWT now corresponds to REPWT(0,1,0) and TENWT to REPWT(0,2,0). * The default masses of the c and b quarks have been lowered to 1.55 & 4.95 repectively: this corresponds to the mass of the lightest meson minus the u/d quark mass. This increases the number of heavy mesons, and hence total multiplicities, and slightly softens their momentum spectrum. The rate of photoproduced charm states increases and B-pi momentum correlations become smoother. * The resonance decay tables supplied in the program have been largely revised. Measured/expected modes with branching fraction at or above 1 per mille are given, including 4 & 5 body decays. To print the new tables call HWUDPR. * The arrays FBTM, FTOP & FHVY which stored the branching fractions of the bottom, top & heavier quarks' `partonic' decays are now nolonger used. Such decays are specified in the same way as all other decay modes: this permits different decays to be given to individual heavy hadrons. Partonic decays of charm hadrons and quarkonium states are also now supported. The products' order in a partonic decay mode is significant. For example if the decay is: Q --> W+q --> (f+fbar')+q, occuring inside a Q-sbar hadron the required ordering is: Q+sbar --->(f+fbar')+(q+sbar) or (q+fbar')+(f+sbar) `colour rearranged' In both cases the (V-A)^2 ME^2 is proportional to: p_0.p_2 * p_1*p_3 * The structure of the program has been altered so that secondary hard subrocess and subsequent fragmentation associated with each partonic heavy hadron decay appear separately. Thus pre-hadronization t quark decays are treated individually as are any subsequent bottom hadron partonic decays. * Additionally decays of heavy hadrons to exclusive non-partonic final states are supported. No check against double counting from partonic modes is included. However this isn't expected to be a major problem for the semi-leptonic and 2-body hadronic modes supplied. * An array NME has been introduced to enable a possible matrix element to be specified for each decay mode. NME = 0 Isotropic decay 100 Free particle (V-A)*(V-A): p_0.p_2 * p_1.p_3 101 Bound quark (V-A)*(V-A): p_0.p_2 * p_1*[p_3 - xs*p_0] xs = m_Q/M_0 - spectator quark momentum fraction 130 Ore & Powell ortho-positronium ME^2: onium --> gg+g/gamma. The list of matrix elements presently supported is modest, users are urged to contact an author to have other MEs implimentated. * The decay tables can be written to/read from a file by using HWIODK, adopting the format advocated in the LEP2 report. In addition to the PDG numbering of particles the HERWIG numbers or character names can be used. This permits easy alteration of the decay tables. In HWUINC a call is made to HWUDKS which sets up HERWIGs internal pointers and performs some basic checks of the decay tables. Each decay mode must conserve charge and be kinematically allowed and not contain vetoed decay products. The sum of a particles branching ratios is set to 1. Also a warning is printed if an antiparticle does not have all the charge conjugate decays modes of the particle. * HWMODK enables changes to the decay tables to be made by alterating/ adding single decay modes including on an event by event basis. This can be done before HWUINC, in which case when altering the BR and/or ME code of an existing mode a warning is given of a duplicate second mode which supercedes the first. BRs set below 10^-6 are eliminated, whilst if one mode is within 10^-6 of 1 all other modes are removed. Note that some forethought is required if the BRs of 2 modes of the same particle are changed since the operation of rescaling to 1 the BR sum causes a non-commutativity in the order of the calls. * Production vertex information is now made available, using VHEP, for all partons, clusters and final state particles: set PRVTX=.TRUE. to print them. The vertices of partons and clusters are given wrt local coordinates associated with their individual hard sub-process. * All partonic and resonace rest frame lifetimes are generated with an exponential distribution: exp(-t/)/. The average lifetime, , is given in terms of the particles mass, width and virtuality by: hbar.sqrt(q^2) (q^2) = ----------------------------- \/(q^2-M^2)^2 + (Gamma.q^2/M)^2 = hbar/Gamma for an on-shell particle ~ hbar.q/(q^2-M^2) a highly virtual particle For partons an effective width = sqrt(VMIN2), to act as a cut-off on lifetimes, is introduced. * The space-time picture for cluster formation and splitting is partly ad hoc and partly string inspired - no physics depends upon it. * All particles with lifetimes greater than PLTCUT are set stable. * If PIPSMR=.TRUE. the primary interaction point's spatial position is is smeared according to the triple Gaussian in HWRPIP: this position is assigned to the CMF track. * If MAXDKL=.TRUE. then each putative decay is tested in HWDXLM to see that it occurs within a specified volume (cylinder/sphere for IOPDKL =1/2): if not it is set stable. * If MIXING=.TRUE. then B^0_d,s mesons are allowed to oscillate: XMIX and YMIX contain Delta-M/Gamma and Delta-gamma/2*Gamma respectively. A new particle, ISTHEP=200, is introduced giving the flavour of the neutral B meson at production in addition to the `decaying' track. * A multiple intra & inter-jet colour rearrangement model is available for CLRECO=.TRUE. The q-qbar pairings in two non-adjacent clusters are interchanged with probability PRECO if the distances between the production vertices of both q-qbar pairs when added in quadrature is reduced. EXAG can be used to artificially scale the lifetimes of any weak bosons. * A number of bugs have been corrected: in HWEPRO for weighted events; in HWSBRN affecting the reconstruction of the photon beam remnant; and in HWHEPG stopping event generation. Plus minor modifications to HWBGEN; in the use of HWHIGM by HWHIGJ; and small changes in HWHDIS & HWHEGG. * A significant bug in HWDHQK, affecting top quark decays, was present in version 5.8 ONLY. The scale of the top decay had been set to the b-quark mass, stopping gluon radiation from the b and restricting that from the W decay products to have transverse momentum less than the b mass. The scales are now correctly set for top decays. * Improved efficiency of photon generation in HWEGAM. * New hard sub-process have been added: - Compton scattering, gamma + q --> gamma + q, IPROC=5300. - Two-to-two parton scattering via exchange of a colour singlet IPROC=2400 Mueller-Tang pomeron: the fixed alpha_s and omega_0 are given by ASFIXD and OMEGA0 respectively. IPROC=2450 photon exchange, for like flavour qqbar pairs including the t-channel component of the interference with q-qbar -> q-qbar. - Drell-Yan has been extended to the production of all fermion pairs IPROC=1399; 1300 gives all quark flavours 1300+IQ a specific quark flavour, 1350 all leptons (including neutrinos) 1350+IL a specific lepton flavour. The s-channel component of the interference with like flavour q-qbar scattering is included here. - Z+jet production is included as IPROC=2150 (HWHW1J becomes HWHV1J) * Running coupling now used for prompt J/PSI production in DIS. * The phase-space limits for the momentum fraction of incoming photons in the Weizsacker-Williams approximation is now set by the variables YWWMIN & YWWMAX, allowing different ranges for the tagged and untagged photons in two-photon DIS. * Interfaced to the Schuler-Sjostrand parton distribution functions, version 2. These appear as PDFLIB sets with author group 'SaSph', but are actually implemented via a call to their SASGAM code. The value in MODPDF specifices the set (1-4 for 1D [recommended set],1M, 2D,2M), whether the Bethe-Heitler process is used for heavy flavours (add 10), whether the P^2-dependence is included (add 20), and which of their P^2 models is used (add 100 times their IP2 parameter). * New variables ANOMSC(1 or 2,IBEAM) record the evolution scale and Pt at which an anomalous (gamma* --> q+qbar) splitting was generated in the backward evolution of beam IBEAM. set 0 if no such splitting was generated. This is implemented in HWBGEN and HWSBRN. * In preparation for multiple interactions, several routines have been added or modified. New are: HWHREM for identifying and cleaning up the beam remnants; HWHSCT to administer the extra scatters. Minor modifications to: HWBGEN & HWSBRN, don't report energy conservation errors when ISLENT = -1; HWSSPC, improved approximation for remnant mass at high energies; and HWUPCM, improved safety against negative square roots. * Photon Initial State Radiation in e+e- annihilation events allowed. TMNISR sets the minimum s-hat/s value, ZMXISR sets the (arbitrary) separation between unresolved and resolved emission; using ZMXISR=0 switches off photon ISR. * Numerical integral in HWBDED now done analytically removing the need to reintegrate for each new energy; in principle allowing use in 5- jet WW events, but this is not yet implemented. * New phase-space variable WHMIN added. This sets the minimum allowed hadronic mass and affects photoproduction reactions (gamma-hadron & gamma-gamma) and DIS. In lepton-hadron DIS it is largely irrelevant since there is already a cut on Bjorken y which at fixed s is almost the same but for lepton-gamma DIS it makes a big difference. * A new treatment of running Higgs width and non-resonant diagrams, as suggested in M.H. Seymour, Phys. Lett. B354 (1995) 409. Selected by setting IOPHIG=2 or 3 (default); previous options 2 and 3 have been withdrawn. Note that including the non-resonant diagrams changes the meaning of what is generated: IOPHIG = 0 or 1, gives the s-channel diagram, an unphysical choice of part of the amplitude; IOPHIG = 2 or 3, gives the I=0 & J=0 part of the excess over the cross section expected for a zero mass Higgs boson, a physical choice of part of the cross section. The inclusion of non-resonant diagrams causes the cross section to increase below and decrease above resonance. * New treatment of the splitting in two of clusters containing hadron (or photon) remnants. Previous versions gave the 2 fragments a mass spectrum typical of soft processes: dn/dm**2 = Gaussian. In the new version the child containing the remnant is treated as before but the other cluster, containing a perturbative parton, is treated as a normal clusters: dn/dm = m**psplt. IOPREM controls this behaviour: 0 = old version, 1 = new (default). * Direct gamma+gamma* -> q+qbar is included in the hard correction for lepton-gamma DIS; plus minor bug fixed in HWBDIS. * The dummy routine IUCOMP has been removed, this avoids errors when the program is linked to CERNLIB. * It has been noticed that differences in the way quark masses are treated in different processes can cause inconsistencies between different ways of generating the same process. The most noticeable example is in direct photoproduction, where one can use process 9130 or 5000. See the note at the end of Section 5 of the documentation for more information on the strategies used in different processes. Version 5.1 of HERWIG was described in detail in the main reference, Comput. Phys. Commun. 67 (1992) 465. The new features of each version released since may be found in the documentation file HERWIG59.DOC. Ian Knowles Mike Seymour Bryan Webber CERN July 22nd 1996