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Manuscript Title: The Lund Monte Carlo for jet fragmentation.
Authors: T. Sjostrand
Program title: JETSET 4.3 G
Catalogue identifier: AAVJ_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 27(1982)243
Programming language: Fortran.
Computer: ND-50.
Operating system: SINTRAN III/VS.
RAM: 28K words
Word size: 32
Keywords: Elementary, Particle physics, Event simulation, Jet fragmentation, Hadronization, Quark jet, Gluon jet, Multiparticle production, Monte carlo simulation.
Classification: 11.2.

Other versions:
Cat Id Title Reference
AAVM_v1_0 JETSET 4.3 E CPC 28(1983)229
AAFP_v1_0 JETSET 6.2 CPC 39(1986)347
AAFP_v2_0 JETSET 6.3 CPC 43(1987)367
ACTU_v1_0 PYTHIA 5.7 AND JETSET 7.4 CPC 82(1994)74

Nature of problem:
In high energy collisions normally most of the particles in the final state appear within a few rather narrow cones. Each of these collections of particles, called jets, are assumed to be coming from the hadronization of an outgoing quark or gluon. QCD, the candidate theory of strong interactions, can be used perturbatively to describe the scattering or creation of these partons at small distance scales (high Q**2), and there are reasons to believe that the large distance behaviour of QCD makes quarks and gluons confined inside hadrons. Exactly how the confinement forces transform e.g. a quark into a jet of particles is however not known at present.

Solution method:
The Lund model provides a phenomenological description of hadronization. Colour charges are assumed to be connected by colour flux tubes, kinematically described by the massless relativistic string with no transverse excitations. A quark then corresponds to an endpoint on a string and a gluon to a kink on it. The breakup of these strings is described in an essentially iterative fashion, string->hadron+remainder- string, for fast particles corresponding to an iteration jet->hadron+ remainder-jet, but at the same time providing a natural joining of the jets in the central region.

Restrictions:
Each string piece must have a certain minimum energy so as to be able to fragment into at least two particles. This corresponds to a minimum invariant mass between each two partons connected by a colour flux tube. Also, in the present implementation a jet system may contain at most ten connected partons and a total of at most 250 particles produced (including unstable particles which subsequently decay), but this is easily changed.

Unusual features:
A random number generator is required. Energy, momentum and flavour are conserved step by step in the fragmentation process.

Running time:
An event with a CM energy of 40 GeV and an average of 30 particles (of which 14 are charged) in the final state takes approximately 0.2 s. Generation time is roughly proportional to the multiplicity.