A Fast Monte Carlo Generator for ee + eeX Untagged Experiments*

c We describe for untagged 77 experiments a specific and very fast Monte Carlo based on the Double-Equivalent-Photon Approximation. This generator takes into account the experimental constraints in order to perform approximations involving experimentally unobservable effects and to generate events only within the experimental acceptance. It allows a very fast simulation of the events of any cross section in any invariant-mass range and, in particular, a simultaneous fit to the data of the parameters in any model of the 77 hadronic cross section. (Submitted for Publication) * Work supported by the Department of Energy, contract DE AC03 76F00515, and the French National Scientific Research Center. We have previously shown ‘s2 that for high energy machines-untagged experiments of the two-photon process can be accurately studied using the DoubleEquivalent-Photon Approximation of the Williams-Weiszgcker spectrum. For ultrarelativistic two-body production we derived analytic expressions of the differential cross section for various parameters (invariant mass, transverse momentum, visible energy), taking into account the experimental constraints.2 The expressions are simple and very fast to compute. Most of them are expressed in a beamenergy invariant manner so that data taken at different energies can be combined and simultaneously analyzed using parameters and cuts which are invariant with respect to energy.Results were checked with measurements of lepton pair (e+eor p+p-) production with the highest statistics currently available.3y4The agreement with the data is as good as that obtained from a simulation based on the QED Vermaseren program5 (see figures 1, 2, and 3). However, Monte-Carlo simulation is needed when one wants to take into account specific inefficiencies and resolution effects inherent in the apparatus. It is also needed for the study of multibody and nonrelativistic two-body production. The goal of this paper is to describe a very fast generator based on the Double-Equivalent-Photon Approximation. This program takes into account the experimental constraints in order to perform approximations involving unobservable effects and to avoid the generation of large numbers of events outside the detector acceptance. It is suitable for all experiments which require that only the prongs produced in the 77 interaction be observed in the central detector, with the following constraints: W min. 5 W I Wmax. e 2E, where E is the beam energy, 8 and pt are the polar angle and transverse momentum of the prongs with respect to the beam axis, and W is the invariant mass observed.