GALUGA, a Monte Carlo program for two-photon processes in $e^+ e^-$ collisions

A Monte Carlo program is presented for the computation of the most general cross section for two-photon production in e+e- collisions at fixed two-photon invariant mass W. Functions implemented for the five gamma* gamma* 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 Q_i^2 are fully kept. Special emphasis is put on a numerically stable evaluation of all variables over the full Q_i^2 range from Q_{i\min}^2 \sim m_e^2 (W/\sqrt{s})^4 \ll m_e^2 up to Q_{i\max}^2 \sim s. A comparison is made with an existing Monte Carlo program for lepton-pair production and an equivalent-photon approximation for hadronic cross sections.