PHOTOS - A UNIVERSAL MONTE-CARLO FOR QED RADIATIVE-CORRECTIONS - VERSION 2.0

This paper documents an update of the PHOTOS algorithm, by E. Barberio, B. van Eijk and Z. was for the Monte Carlo simulation of QED photon radiative corrections in decays. The algorithm is implemented as an independent package written in FORTRAN 77. The program is universal, i.e. it allows for easy interfacing with ''any'' program generating decays of ''any'' particle. The program can be used to estimate the size of the QED bremsstrahlung in the leading-logarithmic (collinear) approximation, but it provides final states with their full topology, including kinematical effects due to massive particles. The proper soft-photon behaviour is also reproduced. The algorithm is designed in such a way that the inclusion (if necessary) of the exact, process-dependent, matrix element subgenerator is possible and straightforward. This update of PHOTOS provides new options to improve the performance of the program in some specific cases: the interference for two-body particle-antiparticle decays, the double bremsstrahlung, and refinements important for the decays into massive non-relativistic decay products. The comparison of the program with the exact matrix-element calculation for the process Z --> mu+mu-(gamma), W --> enu(gamma), tau --> enu(gamma), B+/- --> D0 e+/- nu(gamma) and gg --> tt(gamma)BAR, tt(gammagamma)BAR is also discussed.