High energy photon colliders (γγ,γe) are based on e-e-linear colliders where high energy photons are produced using Compton scattering of laser light on high energy electrons just before the interaction point. This paper is a part of the Technical Design Report of the linear collider TESLA.1Physics program, possible parameters and some technical aspects of the photon collider at TESLA are discussed.

Perturbative QCD one-loop corrections to the cross sections of the bb(cc) quark pair production in polarized photon-photon collisions, as well as cross sections of the radiative processes γγ → bbg, ccg leading to two-and three-jet final states are calculated. It is shown that the signal from the intermediate mass Higgs boson is observable and precise measurements of the Higgs boson twophoton width are possible at a photon-photon collider, although the statistical significance is substantially reduced with respect to the tree level calculations. We demonstrate that virtual corrections are of the same order or larger than the Born contribution in the J z = 0 channel at high energy for small values of the cutoff y cut , separating two-jet from three-jet topologies. The nature of these large corrections is elucidated. For bb pair production at √ s γγ ∼ 100 GeV and for small values of y cut ≤ 0.04 the higher order resummation of double logarithmic terms O α s m 2 b /s ln 2 (s/m 2 b ) is needed.

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We have studied the production of a light Higgs boson with a mass of 120 GeV in photon-photon collisions at a Compton collider. The event generator for the backgrounds to a Higgs signal due to bb and cc heavy quark pair production in polarized γγ collisions is based on a complete next-to-leading order (NLO) perturbative QCD calculation. For Jz = 0 the large double-logarithmic corrections up to four loops are also included. It is shown that the two-photon width of the Higgs boson can be measured with high statistical accuracy of about 2% for integrated γγ luminosity in the hard part of the spectrum of 40 fb −1 . As a result the total Higgs boson width can be calculated in a model independent way to an accuracy of about 14%.