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Manuscript Title: The Lund Monte Carlo for hadronic processes: PYTHIA version 4.8.
Authors: H.-U. Bengtsson, T. Sjostrand
Program title: PYTHIA 4.8
Catalogue identifier: ACCB_v3_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 46(1987)43
Programming language: Fortran.
Computer: VAX 11/780.
Operating system: VAX/VMS VERSION 4.2, SINTRAN III - VSX/500.
RAM: 90K words
Word size: 32
Keywords: Hadronic processes, Hard scattering, High-pt jets, Particle physics, Elementary, Minimum bias events, Q/g/gamma/z0/w+-/h0/r -/, H+-/z'o production, Initial state radiation, Elastic/diffractive Scattering, Beam jets, Monte carlo, Phase space, Final state radiation.
Classification: 11.2.

Subprograms used:
Cat Id Title Reference
AAFP_v2_0 JETSET 6.3 CPC 43(1986)367

Nature of problem:
The event structure in hadron-hadron collisions is very complex, and no detailed theoretical understanding exists. Hard scattering matrix elements and some other aspects are known, however.

Solution method:
Complete hadronic events are generated with Monte Carlo methods. The generation is subdivided into a number of tasks, including construction of a hard scattering (using matrix elements and structure functions), addition of initial and final state radiation, inclusion of beam jets and fragmentation of partons into hadrons. Some of these tasks are performed by JETSET, Comp. Phys. Commun 39(1986)347. Where no relevant theory exists, approximate models are applied, often developed specifically for PYTHIA.

At very high energies, the program may break down for one of two reasons, either the number of particles may exceed the memory space available, or the (mainly) single-precision kinematics may give unacceptable roundoff errors.

Unusual features:
A random number generator is required.

Running time:
Depends very much on the energy and process requested, but is roughly proportional to the number of particles produced. A pp- minimum bias event at 600 GeV CM energy, with an average of 73 particles (whereof 35 charged) takes approximately 0.5 s (ND-570 time). The necessary initialization at the beginning of the run takes 1-20 s, again depending on options selected.