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Manuscript Title: The Lund Monte Carlo for high-pT physics.
Authors: H.-U. Bengtsson
Program title: PYTHIA
Catalogue identifier: ACCB_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 31(1984)323
Programming language: Fortran.
Computer: ND-50.
Operating system: SINTRAN III/VS.
RAM: 5K words
Word size: 32
Keywords: Particle physics, Elementary, Phase space, Scattering, High-pt, Colour flow, String structure, Hadronization, Monte carlo simulation.
Classification: 11.2.

Subprograms used:
Cat Id Title Reference
AAVJ_v1_0 JETSET 4.3G CPC 27(1982)243

Nature of problem:
In high pT reactions, QCD, the candidate theory of strong interactions, can be used to calculate perturbatively the subprocesses on the parton level: the probability for two quarks or gluons reacting, their momentum distributions, etc. However, what is experimentally observed is the final state hadrons, not the partons themselves. Hence, to be able accurately to predict the outcome of an experiment, one needs to take into account the hadronization process. We present a Monte Carlo program for high-pT physics and prompt photon production, based on the Lund Monte Carlo for Jet Fragmentation. The theoretical background is treated in detail, including a complete derivation of the cross- sections for the different final state configurations that result from the Lund model which forms the framework of the program. A prominent part of the paper is dedicated to a manual on how to use the Fortran 77 program, PYTHIA, successor to and synthesis of earlier high-pT programs.

Solution method:
The Lund model provides a phenomenlogical description of hadronization. Colour charges are assumed to be connected by colour flux tubes, approximated with relativistic massless strings, the breaking up of which produces the final state hadrons. The actual string configuration is thus important for the particle distributions. Every final state string configuration can be uniquely associated with a colour amplitude; when this is used to calculate the probabilities for each configuration, a Monte Carlo program can then generate the strings and let them fragment, giving the final state hadrons.

Restrictions:
Only high-p transverse processes of order alphaem alphas and alpha**2s are considered.

Unusual features:
A random number generator is required.

Running time:
An average event takes around 2 seconds to generate, including fragmentation of strings and decay of unstable particles.