Precision Higgs physics at a γγ collider

The loop induced coupling of an intermediate mass Higgs boson to two photons is a sensitive and unique measure for precision tests of physics beyond the Standard Model. In this work we summarize recent results on the expected precision of the partial Gamma (H --> gamma gamma) width at the gamma gamma option of a future linear collider. Heavy particles do not decouple in general and differences between the SM and MSSM predictions or 2HD-models can differ in the percentile regime. Large non-Sudakov DL corrections need to be resummed and consistency requirements demand the use of the Sterman-Weinberg jet definition in order to avoid additional DL terms from three jet final states. We find that the well understood background process, gamma gamma --> q (q) over bar allows for a C(2%) determination of Gamma (H --> gamma gamma) using conservative collider parameters. Recent improvements in the expected gamma gamma luminosity suggest that the precision for the two-photon Higgs width can be further improved and is dominated by the error in BR(H -->b (b) over bar) from the e(+/-) mode, which is presently estimated to be in the 1% regime. (C) 2001 Published by Elsevier Science B.V.