PYTHIA at HERA

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.)

Photoproduction

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/:

• MSTP(13): choice of Q2 range over which electrons are assumed to radiate photons.
• MSTP(14): structure of incoming photon beam (direct/resolved/VMD/anomalous). Use the non-default value 10 to obtain a mixture of the various event classes.
• MSTP(33): possibility to introduce K factors in hard-scattering matrix elements.
• MSTP(51): choice of parton-distribution set of the proton. Default is CTEQ 2L.
• MSTP(52): instead select external PDFLIB library for proton. Also requires removal of two dummy routines in PYTHIA code.
• MSTP(55): choice of parton-distribution set of the photon. Default is SaS 1D.
• MSTP(56): instead select external PDFLIB library for photon. Also requires removal of two dummy routines in PYTHIA code.
• MSTP(82): structure of multiple interactions.
• MSTP(93): primordial kT distribution in photon remnant.
• MSTP(94): energy partitioning in hadron or resolved photon remnant.
• MSTP(101): structure (e.g. multiplicity) of diffractive systems.
• MSTP(171): possibility of variable energy from one event to the next.

• PARP(15): lower pT cut-off for anomalous gamma -> q + qbar branchings.
• PARP(81, PARP(82): pTmin cutoff (p0 turnoff) for jet production in multiple interactions.
• PARP(161) - PARP(164): VMD couplings of the rho, omega, phi and J/psi vector mesons.

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/:

• MSTP(3): automatic selection of Lambda value to match parton distributions used.

• PARP(61): Lambda value for spacelike showers.
• PARP(62): Q0 cut-off of spacelike showers.
• PARP(67): matching between hard-interaction Q2 scale and spacelike shower maximum scale.
• PARP(71): matching between hard-interaction Q2 scale and timelike shower maximum scale.
• PARP(72): Lambda value for timelike showers in ep and pp, specifically for anything but the decay of an s-channel resonance. (Rather new.)
• PARJ(81): Lambda value for timelike showers in e+e-; specifically in the decay of an s-channel resonance. (No longer needed.)
• PARJ(82): Q0 cut-off of timelike showers.

Hadronization

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/:

• MSTJ(11): fragmentation functions (e.g. switch to Peterson et al for heavy flavours).

• PARJ(1): baryon production fraction.
• PARJ(2): s/u quark suppression.
• PARJ(3): strange diquark suppression.
• PARJ(4): spin 1 diquark suppression.
• PARJ(5) - PARJ(7): popcorn baryon production.
• PARJ(11) - PARJ(17): relative pseudoscalar/vector/scalar/pseudovector/axial meson composition.
• PARJ(21): fragmentation pT.
• PARJ(25), PARJ(26): additional eta and eta' suppression.
• PARJ(41), PARJ(42): a and b parameters of the Lund symmetric fragmentation function.
• PARJ(54), PARJ(55): epsilon parameters of the Peterson fragmentation function for c and b quarks.

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.