Progress in Photon Colliders

Abstract

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 e+e- and the second for gamma-gamma, gamma-e and e^-e^- 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 gamma-gamma luminosity exceeding 10^{35} cm^-2s^-1. 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 gamma-gamma, gamma-e colliders with the luminosity comparable to that in e^+e^- collisions are beyond the competition in study of many phenomena of particle physics.