Warning: this page was set up for the 1995/96 HERA Physics Workshop. It does not fully reflect the situation in 1998, when PYTHIA 6.1 is the preferred version. Much of the information is still relevant, however.

PYTHIA/JETSET is a general-purpose generator package for e+e-, ep and pp interactions. At HERA it can be used either for DIS or photoproduction processes, though the emphasis has been put on the latter.

A complete LaTeX manual exists (also as postscript), so the information below is only intended as a brief reminder of main switches and parameters, especially those that may be of interest to optimize the performance of the program. (Several switches come with associated parameter values; this is clear from the manual but is not always indicated below.)


This section covers aspects of the photoproduction processes themselves. Parton showers and hadronization are covered below. Some interesting switches and parameters in common block /PYPARS/:

Parton showers

The parton-shower description is found in PYTHIA for spacelike initial-state radiation and in JETSET for timelike final-state one. In principle the latter should be common with e+e-, but there are local differences in boundary conditions, so parameters could be tuned separately.

Some interesting switches and parameters in common blocks /PYPARS/ and /LUDAT1/:


Hadronization (string fragmentation and secondary particle decays) is performed by the JETSET package. The same parameters ought to apply in all processes, so in principle values determined at LEP should apply unchanged for DIS and photoproduction at HERA. For a recent review, see the QCD generators report of the LEP 2 workshop, postscript.

Some interesting switches and parameters in common block /LUDAT1/:

Further information can be obtained via the PYTHIA webpage. This includes current and obsolete program versions, documentation, sample main programs, physics papers, and so on.

Torbjörn Sjöstrand (