Higgs Decay to Photons at Two Loops *

The standard model (SM) predicts the existence of one scalar particle, the Higgs boson (H). The Higgs boson is the only particle of the SM which has not been found until now. Electroweak precision data mainly collected at CERN LEP and SLAC SLC in combination with the direct top-quark mass measurement at the Tevatron would favour a light Higgs boson with a mass below about 200 GeV at the 95% confidence level, while the direct search at LEP leads to a lower bound of 114 GeV at the 95% confidence level [1]. This mass range is compatible with the so-called intermediatemass range, defined by MW ≤ MH ≤ 2MW , MW and MH being the mass of the W boson and the Higgs boson, respectively. In this mass regime the decay of the Higgs boson into a pair of photons is an important detection channel at the Large Hadron Collider (LHC) due to its clear signature, though the branching fraction does not exceed 0.3%. Furthermore, this decay channel is useful in determining the properties of the Higgs boson. At a future International Linear Collider (ILC) precision measurements would be possible. In particular, at the ILC the two photon mode could be made possible, which allows for the production of Higgs bosons via the fusion of two photons. This way a precise measurement of Γ(H → γγ), with a