A new version of the Monte Carlo program HERWIG (version 6.4) is now available, and can be obtained from the following web site: http://hepwww.rl.ac.uk/theory/seymour/herwig/ More complete information on HERWIG can be found in the publication JHEP 0101:010 G. Corcella et. al. and also in the documentation for the previous version, which are available at the same site, together with other useful files and information. Here we merely give the new features relative to 6.3. If you use HERWIG, please refer to it something along the lines of: G. Corcella, I.G. Knowles, G. Marchesini, S. Moretti, K. Odagiri, P. Richardson, M.H. Seymour and B.R. Webber, JHEP 0101 (2001) 010. When running MSSM processes starting from version 6.1, please add reference to: S. Moretti, K. Odagiri, P. Richardson, M.H. Seymour and B.R. Webber, preprint Cavendish-HEP-98/06, CERN-TH/2001-177, in preparation. *** NEW FEATURES OF THIS VERSION *** *---------------------------------------------------------------* | The main new features are: spin correlations between the | | production and decay of heavy fermions, ie top, tau and SUSY | | particles, polarization effects in SUSY production processes | | in lepton-lepton collisions, an interface to TAUOLA for tau | | decays and MSSM Higgs processes in lepton-lepton collisions. | *---------------------------------------------------------------* * [N.B. Default values for input variables shown in square brackets.] * Spin correlation effects have been added in processes where top quarks, tau leptons and SUSY particles are produced, as described in JHEP 0111 (2001) 029. At the moment the effects are only calculated for the production of these particles in the following processes: IPROC=100-199, IPROC=700-799, IPROC=1300-1399, IPROC=1400-1499, IPROC=1500-1599,IPROC=1700-1799, IPROC=2000-2099, IPROC=2800-2825, IPROC=3000-3030. However, if these particles are produced in other processes, the spin correlation algorithm will still be used to perform their decays. The correlations are also included for the decay of the MSSM Higgs bosons, regardless of how they are produced. * The spin correlations are controlled by the variable SYSPIN [.TRUE.] which switches the correlations on (ie .TRUE.). If required the correlations are initialized by the new routine HWISPN. This routine initializes the two, three and four body matrix elements. * The three and four body matrix elements can be used separately to generate the decay distributions without spin correlation effects. These are switched on by the switches THREEB [.TRUE.] for three body decay and FOURB [.FALSE.] for four body decays. The four body decays are only important in SUSY Higgs studies, and have small branching ratios. However, they take some time to initialize and are therefore switched off by default. * The initialization of the spin correlations and/or decay matrix elements can be time consuming and we have therefore included an option to read/write the information. The information is written to unit LWDEC [88] and read from LRDEC [0]. If either are zero the data is not written/read. If IPRINT=2 then information on the branching ratios for the decay modes and the maximum weights for the matrix elements is outputted. * The effect of polarization for incoming leptonic beams in MSSM SUSY processes has also been included. These effects are included both in the production of SUSY particles and via the spin correlation algorithm in their decays. * An interface to the TAUOLA decay package has been added. To use this interface, the dummy subroutines DEXAY, INIETC, INIMAS, INIPHX, INITDK, PHOINI, PHOTOS must be deleted and the parameter TAUDEC ['HERWIG'] must be set to 'TAUOLA'. You should then link to both TAUOLA and PHOTOS. The easiest way to do this is to obtain the TAUOLA/PHOTOS versioning system from http://wasm.home.cern.ch/wasm/test.html, and produce a version with the correct size of the HEPEVT common block. This system will provide a JETSET demo. The code for this, apart from the JETSET main program and interface, can then be linked to HERWIG (with the above changes) instead of JETSET. * The following MSSM Higgs production processes in l+l- collisions have been added (l=e,mu): IPROC=910 l+ l- -> nue nue-bar h0 + e+ e- h0 IPROC=920 l+ l- -> nue nue-bar H0 + e+ e- H0 IPROC=960 l+ l- -> Z0 h0 IPROC=970 l+ l- -> Z0 H0 IPROC=955 l+ l- -> H+ H- IPROC=965 l+ l- -> A0 h0 IPROC=965 l+ l- -> A0 H0 For the last three, a new subroutine has been introduced, HWHIHH, whereas the first four make use of the implementation of their SM counterparts, which are based on the subroutines HWHIGW and HWHIGZ. * Corrections for bugs have been made affecting the following: - gaugino pair production in hadron collisions: a bug affecting the sign of the centre-of-mass collision angle was corrected. - gaugino/squark production: a bug a the matrix element was corrected. - MSSM Higgs production in hadron-hadron via vector-vector fusion: a bug inducing the use of the mass of the SM Higgs as hard scale of the subprocess as well as its label in the IDHW array has now been taken care of. - Gauge Boson pair production. A bug in testing the error code after the initial state shower has been corrected. - Gauge Boson pair production. The scale has been changed to the parton-parton centre-of-mass energy from the average of the produced boson masses which was used in the previous version. - Gauge Boson pair production. The W+ and W- were interchanged in hadronic W+W- production. This has been corrected. - Pion Beams. The default MRS nucleon structure functions were used without warning in pion scattering. If you wish to simulate pion scattering either the older NSTRU=1,2 pion sets or PDFLIB should by used. 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 Cambridge, CERN, Durham, Edinburgh, KEK, Manchester, Milano, Munich. 21st January 2002.