ASPECTS OF 2-PHOTON PHYSICS AT LINEAR E+E- COLLIDERS

We discuss various reactions at future e+e- and gammagamma 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 gammagamma 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 designs for future e+e- colliders, due to the different beamstrahlung spectra; in particular, both designs with much less than 1 and much greater than 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 gammagamma collider.