Four-fermion productions at a γγ collider

Abstract

Using the recently proposed ALPHA algorithm (and the resulting code) I compute the rate (at tree level) for the process γγ → v ee − u d . The bulk of the contribution is due to W pair production and decay. However, a non-negligible (∼ 10%) contribution comes from other channels, mainly production and decay of a W and a collinear charged fermion. Requiring that the reconstructed invariant u d mass lies in the intervals M W ± 5 GeV and M W ± 20 GeV one obtains a rate which is lower, by 25% and 4% respectively, than the rate obtained in the narrow width approximation, thus demonstrating the relevance of the finite W width.