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 back-scattering 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 $\gamma\gamma \rightarrow W^+ W^-$, $\gamma\gamma \rightarrow $ Higgs bosons, and higher-order loop processes, such as $\gamma\gamma \rightarrow \gamma\gamma, Z \gamma$ 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 $\gamma\gamma$ tests of quantum chromodynamics, such as the scaling of the photon structure function, $t \bar t$ production, mini-jet processes, and diffractive reactions. 
Stanley J. Brodsky, SLAC, and Peter M. Zerwas, DESY