Physics Opportunities at a Photon-Photon Collider

The advent of back-scattered laser beams for electron-positron colliders will allow detailed studies of a large array of high energy photon-photon and photon-electron collision processes with polarized beams. These include tests of electroweak theory in photon-photon annihilation such as $\gamma \gamma \to W^+ W^-$, $\gamma \gamma \to$ Higgs bosons, and higher-order loop processes, such as $\gamma \gamma \to \gamma \gamma, Z \gamma, H^0 Z^0$ and $Z Z.$ Methods for measuring the anomalous magnetic and quadrupole moments of the $W$ and $Z$ gauge bosons to high precision in polarized electron-photon and photon-photon collisions are discussed. 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 $W W$ collisions and annihilation. I also review high energy $\gamma \gamma$ and $e \gamma$ tests of quantum chromodynamics, including the production of two gluon jets in photon-photon collisions, deeply virtual Compton scattering on a photon target, and leading-twist single-spin asymmetries for a photon polarized normal to a production plane. Exclusive hadron production processes in photon-photon collisions provide important tests of QCD at the amplitude level, particularly as measures of hadron distribution amplitudes which are also important for the analysis of exclusive semi-leptonic and two-body hadronic B-decays.