THE COUPLING OF THE LIGHTEST SUSY HIGGS-BOSON TO 2 PHOTONS IN THE DECOUPLING REGIME

We analyze the contribution of the SUSY particles to the coupling of t he lightest Higgs boson to two photons in supersymmetric theories. We discuss to what extent these contributions can be large enough to allo w for a discrimination between the lightest SUSY and the standard Higg s particles in the decoupling limit where all other Higgs bosons are v ery heavy and no supersymmetric particle has been discovered at future colliders. We find that only chargino and top squark loops can genera te a sizeable difference between the standard and the SUSY Higgs-photo n couplings. For masses above 250 GeV, the effect of chargino loops on the two-photon width is however smaller than similar to 10% in the en tire SUSY parameter space. Top squarks heavier than 250 GeV can induce deviations larger than 10% only if their couplings to the Higgs boson are large. Since top squark contributions can be sizeable, we derive the two-loop QCD correction to squark loops and show that they are wel l under control.