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Manuscript Title: Improved Lund Monte Carlo for high-pT physics.
Authors: H.-U. Bengtsson, G. Ingelman
Program title: PYTHIA 3.4
Catalogue identifier: ACCB_v2_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 34(1985)251
Programming language: Fortran.
Computer: IBM 3081.
Operating system: MVS.
RAM: 800K words
Word size: 32
Keywords: Particle physics, Elementary, Phase space, Hard hadron-hadron Collisions, High-pt jets, Prompt photons, Colour flux field Topology, Jet fragmentation, 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:
Hadron-hadron collisions with a large momentum transfer is generally understood as a hard scattering between essentially free, pointlike partons (quarks and gluons) within the colliding hadrons. This underlying reaction can be described by QCD, the candidate theory of strong interactions. However, the hadronization, i.e. the transformation from the scattered partons to the observable hadrons, is not well understood but must nevertheless be taken into account to compare theory with experimental results. We present a Monte Carlo program for high-pT jet physics and prompt photon production in hard hadron-hadron collisions, based on the Lund string model for jet fragmentation. All partonic subprocesses of order alphas**2 and alphas alphaem are included with their proper colour field topologies. A prominent part of the paper is dedicated to a manual on how to use the Fortran 77 program, PYTHIA version 3.3, successor to and synthesis of earlier high-pT programs.

Solution method:
Matrix elements obtained by second order perturbative QCD are used to simulate the underlying partonic scattering process. The separated colour charges, i.e. high-p tranverse partons and hadron remnants, are then connected by colour flux tubes according to the Lund string model which through its Monte Carlo implementation is used to simulate the hadronization process. In this way, complete events are generated which can directly be compared with experimental data, e.g. to study the colour field topologies implied by the model. The present program is a further development and a more complete manual is available.

All subprocesses to order alpha2s and alpha s * aplha em are included but higher order corrections are neglected. This means that initial state bremsstrahlung is only taken into account through Q**2-dependent structure functions and final state gluon emission is neglected in the present version.

Unusual features:
A random number generator and the ordinary gamma function are required.

Running time:
The time needed to generate one event is around 0.05 s to 0.08 s (on IBM 3081) depending on the collision energy and the minimum transverse momentum required for the high-p transverse jets.