Two-photon physics with GALUGA 2.0

An extended version of the Monte Carlo program GALUGA is presented for the computation of two-photon production in ${\text{e}}^{+}{\text{e}}^{-}$ collisions. Functions implemented for the five ${\gamma }^{\star }{\gamma }^{\star }$ structure functions now include several ans"atze of the total hadronic cross section based on the BFKL–Pomeron and various Regge-like models. In addition, structure functions for resonance formation are included with full dependence on the two photon virtualities $Q_1^2$ and $Q_2^2$ as given in the constituent-quark model. The six lowest-lying resonances of each of the C-even mesons with $J^P=0^{-}$, $0^{+}$, $1^{+}$, $2^{+}$ and $2^{-}$ are provided. The program can also be used to calculate with exact kinematics the effective two-photon luminosity function. Special emphasis is put on a numerically stable evaluation of all variables over the full $Q_i^2$ range while keeping all dependences on the electron mass and $Q_i^2$.