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Manuscript Title: A Monte Carlo generation method with importance sampling for high energy collisions of hadrons.
Authors: W. Kittel, L. Van Hove, W. Wojcik
Program title: GENIS
Catalogue identifier: AAUA_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 1(1970)425
Programming language: Fortran.
Computer: CDC 6600.
Operating system: CDC SCOPE 2.0.
RAM: 1K words
Word size: 60
Keywords: High energy, Hadron, Phase space, Collision, Elementary Particle physics, Monte carlo Integration, Importance Sampling, Restricted, Transverse momenta.
Classification: 11.2.

Nature of problem:
In the theoretical analysis of hadron collisions at high energies the numerical evaluation of integrals over complete phase space is wasteful of computer time because experiment shows that only a small region of the integration domain is relevant for most physical purposes. This region is characterised by small values of the transverse momenta of the particles produced in the collision.

Solution method:
We use a Monte Carlo integration method with importance sampling, taking as integration variables the momenta of the outgoing particles in the centre-of-mass system and distinguishing between their longitudinal and transverse components. The former increase with energy, the latter grow only at low energies and remain essentially constant at high energies.

The program is designed for a number n of outgoing particles of up to 18. This limit can be increased easily by the user if required.

Unusual features:
GENIS is a subprogram to be used with any main program feeding it with the information on the incoming laboratory momentum and total energy in the centre of mass as well as the masses of incoming and outgoing particles. It is in particular suited to be used with FOWL.

Running time:
The numbers in table 1 apply for the CDC 6600 computer at CERN. The first column gives the number of outgoing particles, the second the approximate computing time per generated point for points fulfilling the kinematic constraint (eq. (14) of the long write-up). Except at low energy, practically all generated points satisfy the constraint. The rest of the table shows the percentage of generated points which violate it for two values of the energy, the latter being expressed in terms of the laboratory momentum pLAB of the incident beam particle.