- ph ] 2 5 Ju n 19 97 Budker INP 97 – 54 Progress in Photon Colliders ∗

Last two years were very important in history of a photon colliders. This option is included now in conceptual design reports of the NLC, JLC and TESLA/SBLC projects. All the designs foresee two interaction regions: one for ee and the second for γγ, γe and ee collisions. This paper is focused on three aspects: 1) arguments for photon colliders; 2) parameters of current projects; 3) ultimate luminosities and energies, new ideas. Recent studies have shown that the main collision effect coherent pair creation is suppressed at photon colliders with the energy (2E < 2 TeV) due to the beam repulsion, and one can achieve, in principle, the γγ luminosity exceeding 10 cms . The required electron beams with very small emittances can be obtained, for example, using a laser cooling of electron beams. This new method requires a laser with a power by one order of magnitude higher than that required for the “conversion” of electrons to photons. Such lasers are not available today, but hopefully they will appear by the time when linear colliders will be built. High energy γγ, γe colliders with the luminosity comparable to that in ee collisions are beyond the competition in study of many phenomena of particle physics.