Photon collider at TESLA

High energy photon colliders (gamma gamma, gammae) based on backward Compto n scattering of laser light is a very natural addition to e(+)e(-) linear c olliders. In this report, we consider this option for the TESLA project. Re cent study has shown that the horizontal emittance in the TESLA damping rin g can be further decreased by a factor of four. In this case, the gamma gam ma luminosity in the high energy part of spectrum can reach about (1/3)Le-e . Typical cross-sections of interesting processes in gamma gamma collisions are higher than those in e+e- collisions by about one order of magnitude, so the number of events in gamma gamma collisions will be more than that in e(+)e(-) collisions. Photon colliders can, certainly, give additional info rmation and they are the best for the study of many phenomena. The main que stion is now the technical feasibility. The key new element in photon colli ders is a very powerful laser system. An external optical cavity is a promi sing approach for the TESLA project. A free electron laser is another optio n. However, a more straightforward solution is "an optical storage ring (op tical trap)" with a diode pumped solid state laser injector which is today technically feasible. This paper briefly reviews the status of a photon col lider based on the linear collider TESLA, its possible parameters and exist ing problems. (C) 2001 Elsevier Science B.V. All rights reserved.