Breeding New Light into Old Machines (And New)

Photons produced by lasers or wigglers backscattered on high energy electron or proton beams can provide high energy, high luminosity photon-electron, photon-photon or photon-proton collisions. This allows the study of short-distance QCD processes such as high transverse momentum photon-photon and photo-production reactions, deep inelastic Compton scattering, the photon structure function, direct photon reactions, or searches for pseudo-Goldstone bosons and supersymmetry particles like the photino or goldstino. The relative reaction rates should be quite high since (1) photo-production cross sections are significantly larger than the corresponding electroproduction cross sections and (2) absence of the conventional beam-beam interaction allows significantly higher currents and smaller interaction areas. It thus seems possible to have photon luminosities much larger than for electrons. Examples are given using the PEP storage ring with the SLAC linac beam.