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Manuscript Title: POLDIS, a Monte Carlo for POLarized (semi-inclusive) Deep Inelastic Scattering.
Authors: A. Bravar, K. Kurek, R. Windmolders
Program title: POLDIS, version 2.0
Catalogue identifier: ADGI_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 105(1997)42
Programming language: Fortran.
Computer: Sun UltraSPARC 1 station.
Operating system: Solaris 2.5, OSF-32.
Word size: 32
Keywords: Particle physics, Elementary, Polarized deep inelastic, Lepton scattering, Spin-dependent cross, Sections, Spin asymmetries, Perturbative quantum, Chromodynamics, Qcd improved parton, Model, Polarized structure, Functions.
Classification: 11.5.

Subprograms used:
Cat Id Title Reference
ACLI_v1_0 PDFLIB version 3.10 CPC 75(1993)396
ADFE_v1_0 LEPTO version 6.5 CPC 101(1997)108
ADEY_v1_0 AROMA version 2.2 CPC 101(1997)135
ACTU_v1_0 PYTHIA 5.7 AND JETSET 7.4 CPC 82(1994)74

Nature of problem:
Deep inelastic scattering (DIS) of polarized charged leptons off polarized nuclear targets allows to access the spin dependent nucleon structure. When interpreted in the framework of the quark-parton model the experimental results show that the quark spins account only for a rather small fraction of the nucleon spin, thus implying an appreciable contribution either of gluons or possibly of orbital angular momentum [1]. The study of semi-inclusive DIS allows, in addition, to distinguish the role of each individual parton component and to separate the contributions of different constituents to the nucleon spin. For instance, the study of polarized open charm lepto-production allows to access the gluon polarization DeltaG in a polarized nucleon.

Solution method:
Unpolarized DIS scattering events are generated with existing lepto- production event generators LEPTO [2] for DIS and AROMA [3] for Heavy Flavor production. THe hadronization is based on the LUND string model and is performed with JETSET [4]. Spin asymmetries are calculated at partonic level to first order in alpha s for each event, and are then convoluted with the ratio between the corresponding polarized and unpolarized parton distribution functions (i.e. parton polarization). Polarization weights are thus obtained for each generated event. The average of these polarization weights gives the polarized cross section spin-asymmetry for the generated sample (the spin-dependent cross sections can be obtained from this asymmetry and the spin-independent cross section). These calculations are performed in a set of subroutines to be linked with LEPTO and/or AROMA. No modification to these programs is required. The unpolarized parton densities are obtained from the PDFLIB library [5]; various polarized parton densities can be selected from a collection of some existing parametrizations which are provided with this program.

Processes mediated by one photon exchange are implemented. The present code can be used over a wide kinematical range, where the effects of the weak interaction can be neglected (i.e. Q**2 <= 100 GeV**2 << MZ0**2).

Unusual features:
CERNLIB packlib, mathlib and kernlib libraries are used.

Running time:
6 ms/event CPU time on a DEC Alpha station, and 7.5 ms/event on a Sun UltraSPARC 1 station.

[1] EM Coll. J. Ashman et al., Nucl. Phys. B 328, 1 (1989); for a theoretical review see M. Anselmino, A. Efremov and E. Leader, Phys. Rep. 261, 1 (1995); and references therein; for an experimental review see U. Stiegler, Phys. Rep. 277, 1 (1996); and references therein.
[2] G. Ingelman, A. Edin and J. Rathsman, LEPTO 6.5, Comp. Phys. Comm. 101, 108 (1997).
[3] G. Ingelman, J. Rathsman and G.A. Schuler, AROMA 2.2, Comp. Phys. Comm. 101, 135 (1997).
[4] T. Sjostrand, JETSET, Comp. Phys. Comm. 82, 74 (1994).
[5] H. Plothow-Besch, PDFLIB, Comp. Phys. Comm. 75, 396 (1993).