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Manuscript Title: BBBREM, Monte Carlo simulation of radiative Bhabha scattering in the very forward direction.
Authors: R. Kleiss, H. Burkhardt
Program title: BBBREM
Catalogue identifier: ACTW_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 81(1994)372
Programming language: Fortran.
RAM: 200K words
Word size: 32
Keywords: Particle physics, Elementary, Qed, Bhabha scattering, Bremsstrahlung, Radiative processes, Forward scattering, Collinear singularities, Monte carlo simulation, Experimental cuts, Beam lifetimes, Luminosity monitoring.
Classification: 11.4.

Nature of problem:
Radiative Bhabha scattering, e+e- -> e+e-gamma, has a very large cross section at small scattering angles, and plays various roles in existing and future e+e- colliders. It can be an important background to several two-photon scattering processes; it forms the major ingredient in the finite lifetime of colliding beams; and it is a possible process by which the luminosity can be measured, by observing electrons or photons emerging at zero scattering angle. Accurate knowledge of its cross section is therefore important.

Solution method:
Due to the extremely singular structure of the matrix elements, and the possibility of complicated or unusual experimental cuts, a straight- forward integration of the cross section over the allowed phase space is impractical. We therefore construct a Monte Carlo algorithm that generates (random) events in phase space, with a distribution that matches the actual cross section as closely as possible. These events are assigned a weight which corrects for discrepancies between the actual and the approximate matrix elements: the average value of the weight in a sample of generated events is the Monte Carlo estimate of the cross section. Since each generated event is a complete description of the momenta of the produced particles, any conceivable experimental cut can be implemented, in addition to the single a-priori constraint, namely, a minimum value for the energy of the Bremsstrahlung photon. This value can be set (to essentially arbitrarily small values) by the user. By setting the weight of events that fail a particular set of cuts to zero, one obtains the cross section under those cuts.

Unusual features:

Running time:
about 185 musec per generated event on SUN SPACR 10; about 40 musec per generated event on IBM 9000.