0 Introduction and recent developments in γγ , γ e colliders

High energy photon colliders (γγ, γe) based on backward Compton scattering of laser light is a very natural addition to ee linear colliders. In this report we consider mainly this option for the TESLA project. Recent study has shown that the horizontal emittance in the TESLA damping ring can be further decreased by a factor of four. In this case the γγ luminosity luminosity in the high energy part of spectrum can reach 0.3–0.5 Le+e− . Typical cross sections of interesting processes in γγ collisions are higher than those in eecollisions by about one order of magnitude, so the number of events in γγ collisions will be more that in ee collisions. The key new element in photon colliders is a very powerful laser system. The most straightforward solution is “an optical storage ring (optical trap)” with diode pumped laser injector which is today technically feasible. This paper briefly review the status of a photon collider based at TESLA, its possible parameters.