Two-photon bremsstrahlung

We present compact expressions for the matrix elements for double initial-state bremsstrahlung in the process e+e-→fanti f at LEP energies. Special attention is given to kinematical situations where one or both of the bremsstrahlung photons are collinear with the incoming electrons: this entails a careful description of the transition region between collinear and noncollinear kinematics. The resulting expressions allow for simple and straightforward computer implementation. We describe a Monte Carlo computation based on these matrix elements, which contains two-loop corrections to the initial state and one-loop corrections to the final state. The results are compared with other analytical and Monte Carlo studies. (orig.)