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Manuscript Title: SABSPV: a Monte Carlo integrator for small-angle Bhabha scattering.
Authors: M. Cacciari, G. Montagna, O. Nicrosini, F. Piccinini
Program title: SABSPV
Catalogue identifier: ADCG_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 90(1995)301
Programming language: Fortran.
Computer: DEC VAX.
Operating system: VMS, UNIX.
RAM: 40K words
Word size: 32
Keywords: Particle physics, Elementary, High energy electron Positron collisions, Small-angle bhabha Scattering, Luminosity determination, Qed corrections, Electron structure Functions, Experimental cuts, Monte carlo integration.
Classification: 11.4.

Nature of problem:
The precise determination of the theoretical Bhabha scattering cross section in the small-angle regime is a key ingredient for precision luminometry at LEP. To this aim, QED radiative corrections to the tree-level Standard Model Cross section have to be taken into account, together with vacuum polarization effects. Particular care has to be devoted to higher-order corrections. The theoretical formulation must allow the computation of the cross section for a wide variety of cuts on the final-state particles.

Solution method:
A suitable matching of a fixed-order calculation with the structure- function techniques for resumming large initial- and final-state leading logarithmic corrections [2] is performed. A Monte Carlo integration with weighted events has been implemented in order to mimic as close as possible the experimental triggering conditions. The importance- sampling techinque [3] is employed to take care of the peaking behaviour of the integrand.

The O(alpha) QED corrections are computed exactly only for the dominant contribution to the small-angle cross section, namely the square of gamma-exchange in the t-channel; all the other contributions to the cross sections are corrected at the leading-logarithmic level. The contribution of additional hadronic or leptonic pairs is at present neglected. Starting from O(alpha**2), QED corrections are implemented at the leading-logarithmic level.

Unusual features:
None (Routines from the CERN Program Library are used.)

Running time:
On a HP 9000/735 the code takes about 2 * 10**-4 seconds per event, with standard cuts. A one per mille relative error can be achieved with about 10**7 events.

[1] F. James, Comput. Phys. Commun. 79(1994)111.
[2] M. Cacciari, G. Montagna, O. Nicrosini and F. Piccinini, in "Reports of the Working Group on Precision Calculations for the Z Resonance", eds. D. Bardin, W. Hollik and G. Passarino (CERN Report 95-03, Geneva, 1995), p.389, and references therein.
[3] F. James, Rep. Prog. Phys. 34(1980)1145.