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Manuscript Title: BHAGENE3, a Monte Carlo event generator for lepton pair production and wide angle Bhabha scattering in e+e- collisions near the Z-peak.
Authors: J.H. Field, T. Riemann
Program title: BHAGENE3
Catalogue identifier: ADCS_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 94(1996)53
Programming language: Fortran.
Computer: IBM 3090.
Operating system: VM/CMS.
RAM: 678K words
Word size: 32
Keywords: Particle physics, Elementary, Event simulation, Radiative corrections, Monte carlo simulation, Wide angle bhabha Scattering, Muon pair production, Photons, Electrodynamics quantum, Electroweak theory.
Classification: 11.2.

Nature of problem:
Calculation of all 1-loop electroweak corrections and of QED corrections up to O(alpha**3) for wide angle Bhabha scattering and muon pair production in the vicinity of the Z peak.

Solution method:
The 1-loop electroweak virtual corrections are calculated analytically. The O(alpha) QED radiative corrections use the exact matrix element. The higher order QED corrections are given by an improved soft photon approximation. Event configurations containing real photons are first generated according to an approximate model and are then reweighed according to the theoretical distributions. The weight throwing technique is used to produce unit weight events in the full final state phase space.

Restrictions:
Some terms in ml/E (l=e, mu) are neglected, so that fermion threshold effects are not properly taken into account. The approximation used for O(alpha**2) and higher order real photon radiation may be unreliable in events with multiple hard photon radiation. For cross section accuracy at the % level it is not recommended to use the program for Bhabha scattering angles less than 10 degrees.

Unusual features:
Extensive use is made of one and two dimensional look-up tables for fast and flexible generation of Monte Carlo variables. The space required for these tables means that the fast memory requirements may be greater than in other comparable programs. These look-up tables as well as average event weights are created in the initialisation phase of the program which is, in consequence, relatively time-consuming.

Running time:
For Bhabha events: initialisation about 140 sec, generation 709 unit weight events per second. For muon pairs the initialisation time is shorter by a factor of about 3 and the event generation rate is roughly doubled. The time unit is an IBM 3090 CPU second.

References:
[1] J.H. Field, Phys. Lett. B323 (1994) 423.
[2] M. Bohm, A. Denner and W. Hollik, Nucl. Phys. B304 (1988) 687.
[3] F.A. Berends, R. Kleiss and W. Hollik, Nucl. Phys. B304 (1988) 712.
[4] D. Bardin, W. Hollik and T. Riemann, Z. Phys. C Particles and Fields 49 (1991) 485.