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

TL;DR

The paper addresses the need for precise LO predictions for $\gamma\gamma \to 4f$ and $\gamma\gamma \to 4f\gamma$ at photon colliders, including SM extensions via anomalous gauge couplings and an effective $\gamma\gamma H$ interaction. It develops a Monte Carlo generator with full polarization, a realistic backscattered-photon spectrum, and compact SM helicity amplitudes based on Weyl–van-der-Waerden spinors, and extends to ATGC, AQGC, and Higgs couplings. The authors validate against Whizard/Madgraph, analyze W-pair signal definitions, gauge-width schemes, and the impact of beam spectra, and provide estimates of sensitivity to anomalous couplings at TESLA-like colliders. They show that the double-pole approximation attains about 1–3% accuracy away from thresholds and that naive gauge-variant signal definitions can incur sizable deviations, highlighting the importance of gauge-invariant treatments for $\gamma\gamma$-driven four-fermion production. Overall, the work delivers a validated LO tool for gamma-gamma collider phenomenology and charts a path toward precision radiative-correction calculations in the double-pole framework for constraining new physics in the gauge sector.

Abstract

We have constructed a Monte Carlo generator for lowest-order predictions for the processes gamma gamma -> 4f and gamma gamma -> 4f+gamma in the Standard Model and extensions thereof by an effective gamma gamma Higgs 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 -> 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 -> W W -> 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.