PYTHIA 6.4 Physics and Manual

WDTPM, WDTEM, XLAM and IDLAM, have been expanded from dimension 300 to 400.• Lepton- and baryon-number-violating decay channels have been included for supersymmetricparticles [Ska01, Sjö03]. Thus the decay tables have grown considerablylonger.• The string hadronization scheme has been improved and expanded better to handlejunction topologies, where three strings meet. This is relevant for baryon-numberviolatingprocesses, and also for the handling of baryon beam remnants. Thus newroutines have been introduced, and also e.g. new K(I,1) status codes.• A runtime interface to Isasusy has been added, for determining the SUSY massspectrum and mixing parameters more accurately than with the internal Pythiaroutines.• The Technicolor scenario is updated and extended. A new common block, PYTCSM,is introduced for the parameters and switches in Technicolor and related scenarios,and variables are moved to it from a few other common blocks. New processes381–388 are introduced for standard QCD 2 → 2 interactions with Technicolor (orother compositeness) extensions, while the processes 11, 12, 13, 28, 53, 68, 81 and82 now revert back to being pure QCD.• The PYSHOW time-like showering routine has been expanded to allow showering insidesystems consisting of up to 80 particles, which can be made use of in some resonancedecays and in user-defined processes.• The PYSSPA space-like showering routine has been expanded with a q → qγ branching.• The PYSIGH routine has been split into several, in order to make it more manageable.(It had increased to a size of over 7000 lines, which gave some compilersproblems.) All the phase-space and parton-density generic weights remain, whereasthe process-specific matrix elements have been grouped into new routines PYSGQC,PYSGHF, PYSGWZ, PYSGHG, PYSGSU, PYSGTC and PYSGEX.• Some exotic particles and QCD effective states have been moved from temporaryflavour codes to a PDG-consistent naming, and a few new codes have been introduced.• The maximum number of documentation lines in the beginning of the event recordhas been expanded from 50 to 100.• The default parton distribution set for the proton is now CTEQ 5L.• The default Standard Model Higgs mass has been changed to 115 GeV.Changes in version 6.3: The major changes in 6.3 was the introduction of a newunderlying–event framework for hadron collisions, together with new transverse-momentum-orderedinitial- and final-state parton showers. Other changes include an interfaceto SUSY spectrum and decay calculators conformant to the SUSY Les Houches Accord.Early releases of Pythia 6.3, up to 6.312, were still experimental, and did not include thenew parton showers. Details on these versions can be found in the update notes availableon the web. We here concentrate on the structure of the program after (and including)version 6.312.Firstly, there are no incompatibilities between Pythia 6.2 and Pythia 6.3 at thelevel of commonblock sizes or subroutine arguments (except for the PYSUGI interface toIsajet, see below). That is, any program that ran with Pythia 6.2 also ought to runwith Pythia 6.3, without any change required. Note, however, the following news:• The old (6.2) master event-generation routine PYEVNT is still available; it uses the oldunderlying–event machinery (PYMULT) and parton showers (PYSSPA and PYSHOW). A‘Tune A’–like set of parameters [Fie02] has been adopted as default. It reproducesTune A at the Tevatron, but implies a slower energy rescaling, to give a moreconservative estimate of the underlying activity at the LHC.26