Photon collider at TESLA

High energy photon colliders (γγ, γe) based on backward Compton scattering of laser light is a very natural addition to e+e− linear colliders. In this report, we consider 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 in the high energy part of spectrum can reach about (1/3)Le+e−. Typical cross-sections of interesting processes in γγ collisions are higher than those in e+e− collisions by about one order of magnitude, so the number of events in γγ collisions will be more than that in e+e− collisions. Photon colliders can, certainly, give additional information and they are the best for the study of many phenomena. The main question is now the technical feasibility. The key new element in photon colliders is a very powerful laser system. An external optical cavity is a promising approach for the TESLA project. A free electron laser is another option. However, a more straightforward solution is “an optical storage ring (optical trap)” with a diode pumped solid state laser injector which is today technically feasible. This paper briefly reviews the status of a photon collider based on the linear collider TESLA, its possible parameters and existing problems.