Multiphoton Monte Carlo event generator for Bhabha scattering at small angles

Abstract

We describe in this paper the application of the theory of Yennie, Frautschi, and Suura (YFS) to construct a Monte Carlo (MC) event generator with multiple-photon production for Bhabha scattering at low angles. The respective generator provides the four-momenta of the electron and positron and of all soft and hard photons with a proper treatment of the phase space and conservation of the total four-momentum. The final-state electron and positron are assumed to be visible above some minimum angle with respect to the beams (double tag). The QED matrix element in the algorithm is taken according to the YFS exponentiation scheme. The Monte Carlo program will be helpful in luminosity determination at experiments at the SLAC Linear Collider and the CERN collider LEP; it takes into account QED O(α) and the leading higher-order corrections. The important difference with the existing MC procedures is that the minimum energy above which photons are generated may be set arbitrarily low. Sample Monte Carlo data are illustrated in our discussion.