GALUGA, a Monte Carlo program for two-photon processes in ee collisions

A Monte Carlo program is presented for the computation of the most general cross section for two-photon production in ee collisions at fixed twophoton invariant mass W . Functions implemented for the five γγ structure functions include three models of the total hadronic cross section and the lepton-pair production cross section. Prospects of a structure-function determination through a study of the azimuthal dependence between the two scattering planes are outlined. All dependences on the electron mass and the photon virtualities Q2i are fully kept. Special emphasis is put on a numerically stable evaluation of all variables over the full Q2i range from Q2imin ∼ m2e(W/ √ s) ≪ m2e up to Q2imax ∼ s. A comparison is made with an existing Monte Carlo program for lepton-pair production and an equivalent-photon approximation for hadronic cross sections. a Heisenberg Fellow. CERN–TH/96–313 October 1996