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Manuscript Title: PEPSI: a Monte Carlo generator for polarized leptoproduction.
Authors: L. Mankiewicz, A. Schafer, M. Veltri
Program title: PEPSI version 1.1
Catalogue identifier: ACJF_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 71(1992)305
Programming language: Fortran.
Computer: RISC DEC STATION.
Operating system: UNIX.
RAM: 57K words
Word size: 32
Keywords: Particle physics, Elementary, Event simulation, Perturbative quantum, Chromodynamics (qcd), Improved qcd parton Model, Polarized deep Inelastic scattering, Structure functions, Cross sections.
Classification: 11.2.

Subprograms used:
Cat Id Title Reference
AAFP_v2_0 JETSET 6.3 CPC 43(1987)367

Nature of problem:
Deep inelastic scattering of longitudinally polarized leptons off polarized nucleons or nuclei are the ideal tools to determine the spin distributions of partons inside such targets. Like the momentum distribution determined by unpolarized deep inelastic scattering, the partonic spin distributions contain an important dynamical information which strongly influence our understanding of the nucleon structure, and hence of non-perturbative Quantum Chromodynamics. The surprising result of a recent EMC experiment has simulated strong theoretical and experimental efforts to further investigate the problem of the spin structure of the nucleon. In the case of these investigations it has become clear that additional experimental information is needed to interpret the data unambiguously. The present document contains a description of a Monte Carlo program created to simulate these experiments and polarized deep inelastic scattering in general.

Solution method:
The existing Monte Carlo program LEPTO 4.3 simulation in unpolarized deep inelastic scattering has been modified to include polarization. The relevant cross-sections for hard QCD processes mediated by the virtual photon exchange have been calculated and included in the procedure which generates the probabilities of various events and their kinematics. The subsequent fragmentation into hadrons is performed using the standard JETSET 6.3 fragmentation code. For user's convenience, three unitary sets of unpolarized and polarized parton distributions are provided.

Restrictions:
At present only processes mediated by the virtual gamma exchange are implemented. It restricts the scope of the code to the kinematic domain where efects due to weak interactions can be neglected. A possible polarization-dependence of the fragmentation functions is not taken into account. It means that, e.g., the asymmetries in the semi-inclusive spectra can be reliably simulated for spinless particles only. Also, the Q**2 dependence of the polaized parton distributions is not included and therefore care has to be taken when simulating a DIS experiment in a wide Q**2 range.

Running time:
Generation of a complete event takes approximately 0.11 seconds on a RISC DEC station 3100.