PYTHIA 6.4 Physics and Manual

as showers, fragmentation and decays, require no specific initialization (except for theone implied by the presence of BLOCK DATA PYDATA, see above), i.e. each event and eachtask stands on its own. Current common-block values are used to perform the tasks inspecific ways, and those rules can be changed from one event to the next (or even withinthe generation of one and the same event) without any penalty. The random-numbergenerator is initialized at the first call, but usually this is transparent.In the core process generation machinery (e.g. selection of the hard process kinematics),on the other hand, a sizable amount of initialization is performed in the PYINIT call, andthereafter the events generated by PYEVNT all obey the rules established at that point.This improves the efficiency of the generation process, and also ties in with the MonteCarlo integration of the process cross section over many events. Therefore common-blockvariables that specify methods and constraints to be used have to be set before the PYINITcall and then not be changed afterwards, with few exceptions. Of course, it is possibleto perform several PYINIT calls in the same run, but there is a significant time overheadinvolved, so this is not something one would do for each new event. The two older separateprocess generation routines PYEEVT (and some of the routines called by it) and PYONIAalso contain some elements of initialization, where there are a few advantages if events aregenerated in a coherent fashion. The cross section is not as complicated here, however, sothe penalty for reinitialization is small, and also does not require any special user calls.Apart from writing a title page, giving a brief initialization information, printing errormessages if need be, and responding to explicit requests for listings, all tasks of theprogram are performed ‘silently’. All output is directed to unit MSTU(11), by default 6,and it is up to you to set this unit open for write. The only exceptions are PYRGET, PYRSETand PYUPDA where, for obvious reasons, the input/output file number is specified at eachcall. Here you again have to see to it that proper read/write access is set.The programs are extremely versatile, but the price to be paid for this is having alarge number of adjustable parameters and switches for alternative modes of operation.No single user is ever likely to need more than a fraction of the available options. Sinceall these parameters and switches are assigned sensible default values, there is no reasonto worry about them until the need arises.Unless explicitly stated (or obvious from the context) all switches and parameters canbe changed independently of each other. One should note, however, that if only a fewswitches/parameters are changed, this may result in an artificially bad agreement withdata. Many disagreements can often be cured by a subsequent retuning of some otherparameters of the model, in particular those that were once determined by a comparisonwith data in the context of the default scenario. For example, for e + e − annihilation, sucha retuning could involve one QCD parameter (α s or Λ), the longitudinal fragmentationfunction, and the average transverse momentum in fragmentation.The program contains a number of checks that requested processes have been implemented,that flavours specified for jet systems make sense, that the energy is sufficient toallow hadronization, that the memory space in PYJETS is large enough, etc. If anythinggoes wrong that the program can catch (obviously this may not always be possible), anerror message will be printed and the treatment of the corresponding event will be cutshort. In serious cases, the program will abort. As long as no error messages appear onthe output, it may not be worthwhile to look into the rules for error checking, but if butone message appears, it should be enough cause for alarm to receive prompt attention.Also warnings are sometimes printed. These are less serious, and the experienced usermight deliberately do operations which go against the rules, but still can be made tomake sense in their context. Only the first few warnings will be printed, thereafter theprogram will be quiet. By default, the program is set to stop execution after ten errors,after printing the last erroneous event.It must be emphasized that not all errors will be caught. In particular, one tricky questionis what happens if an integer-valued common-block switch or subroutine/function30