Four-fermion production at $\gamma\gamma$ colliders: 1. Lowest-order predictions and anomalous couplings

Abstract

We have constructed a Monte Carlo generator (the corresponding FORTRAN code can be obtained from the authors upon request) for lowest-order predictions for the processes $\gamma\gamma\to 4f$ and $\gamma\gamma\to 4f\gamma$ in the standard model and extensions thereof by an effective $\gamma\gamma H$ coupling as well as anomalous triple and quartic gauge-boson couplings. Polarization is fully supported, and a realistic photon beam spectrum can be taken into account. For the processes $\gamma\gamma\to 4f$ all helicity amplitudes are explicitly given in a compact form. The presented numerical results contain, in particular, a survey of cross sections for representative final states and their comparison to results obtained with the program package Whizard/Madgraph. The impact of a realistic beam spectrum on cross sections and distributions is illustrated. Moreover, the size of various contributions to cross sections, such as from weak charged- or neutral-current, or from strong interactions, is analyzed. Particular attention is paid to W-pair production channels $\gamma\gamma\to W W\to 4f(\gamma)$ where we investigate the impact of background diagrams, possible definitions of the W-pair signal, and the issue of gauge-invariance violation caused by finite gauge-boson widths. Finally, the effects of triple and quartic anomalous gauge-boson couplings on cross sections as well as the possibility to constrain these anomalous couplings at future $\gamma\gamma$ colliders are discussed.