HIGH-ENERGY PHOTON-PHOTON COLLISIONS

The collisions of high energy photons produced at an electron-positron collider provide a comprehensive laboratory for testing QCD, electroweak interactions, and extensions of the standard model. The luminosity and energy of the colliding photons produced by backscattering laser beams is expected to be comparable to that of the primary e+ e- collisions. In this overview, we shall focus on tests of electroweak theory in photon-photon annihilation, particularly gamma gamma --> W+ W-, gamma gamma --> Higgs bosons, and higher-order loop processes, such as gammagamma --> gammagamma, Zgamma and ZZ. Since each photon can be resolved into a W+ W- pair, high energy photon-photon collisions can also provide a remarkably background-free laboratory for studying WW collisions and annihilation. We also review high energy gammagamma tests of quantum chromodynamics, such as the scaling of the photon structure function, tt production, mini-jet processes, and diffractive reactions.