Fe b 20 06 Photon colliders : The first 25 years ∗

In this invited talk at the " historical " session of PHOTON2005, I was asked to recount the history and the development, from its earliest days to the present, of the idea of photon colliders based on conversion of high energy electrons to high-energy photons at a future high-energy linear e + e − collider. Described in this talk are the general features and schemes of a photon collider, the evolution in understanding of what the parameters of a realistic photon collider are, possible solutions of various technical problems, the physics motivation, and the present status of photon-collider development. For a more detailed description of the photon collider at the ILC and a discussion of the associated technical issues, please refer to my talks at PLC2005, the conference that immediately followed PHOTON2005 (to be published in Acta Physica Polonica B as well). 1 Prehistory and the idea of the photon collider Photon colliders do not exist yet, but already have a rich 25-year history. The early history of γγ physics, studied mainly in collisions of virtual photons at e + e − storage rings, has been presented at PHOTON2005 by S. Brodsky [1] and I. Ginzburg [2]. Hence, I begin my narration by describing the circumstances that led to the birth of the idea of the high-energy photon collider. This is the first time I share an account of these events with the public; this conference, subtitled " The Photon: its First Hundred Years and the Future, " provides an appropriate venue for such historical reviews. I will also mention the story of the observation of C = + resonances in γγ collisions at SLAC in 1979, which is also an important event in the γγ history. Two-photon physics had been talked about since 1930s, but as an active research field is began in early 1970s, when production of e + e − pairs was discovered in collisions of virtual photons at the e + e − storage ring VEPP-2 in Novosibirsk and theorists realized that this method can be used to study a variety of two-photon processes. To study two-photon physics at a greater depth, we in Novosibirsk decided to build MD-1, a dedicated detector with a transverse magnetic field and a tagging system for scattered electrons. Before experiments at the VEPP-4 collider started, in 1978–79 I had the privilege of having been able to visit SLAC for four …