Four Fermions Productions at a $\gamma \gamma$ Collider

Abstract

Using the recently proposed ALPHA algorithm (and the resulting code) I compute the rate (at tree level) for the process $\gamma \gamma \to {\bar{\nu }}_e{e}^{-}u\bar{d}$. The bulk of the contribution is due to W pair production and decay. However a non negligible ($\sim 10\%$) contribution comes from other channels, mainly the production and decay of a W and a collinear charged fermion. Requiring that the reconstructed invariant $u\bar{d}$ mass lies in the intervals $M_W\pm 5$ GeV and $M_W\pm 20$ 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.

Comments: Plain Latex (10 pages) plus 8 postcript figures (NOT included in the text and to be printed separately)