Aspects of two-photon physics at linear e⁺e^- colliders

Abstract

We discuss various reactions at future e⁺e^- and γγ colliders involving real (beamstrahlung or backscattered laser) or quasi-real (bremsstrahlung) photons in the initial state and hadrons in the final state. The production of two central jets with large transverse momentum p_T is described in some detail; we give distributions for the rapidity and p_T of the jets as well as the di-jet invariant mass, and discuss the relative importance of various initial state configurations and the uncertainties that arise from the at present rather poor knowledge of the parton content of the photon. We also present results for mono-jet production where one jet goes down a beam pipe, for the production of charm, bottom and top quarks, and for single production of W and Z bosons. Where appropriate, the two-photon processes are compared with annihilation reactions leading to similar final states. We also argue that the behaviour of the total inelastic γγ cross section at high energies will probably have little impact on the severity of background problems caused by soft and semi-hard (minijet) two-photon reactions. We find very large differences in cross sections for all two-photon processes between existing desings for future e⁺e^- colliders, due to the different beamstrahlung spectra; in particular, both designs with <<1 and >>1 events per bunch crossing exist. The number of hadronic two-photon events is expected to rise quickly with the beam energy. Hadronic backgrounds will be even worse if the e⁺e^- collider is converted into a γγ collider.