Photon pair production and the intermediate-mass Higgs boson

We study the production of photon pairs at future hadron colliders from quark-antiquark annihilation and gluon fusion via quark loops. We show that gluon fusion is just as important as quark-antiquark annihilation for photon pairs of invariant mass less than 200 GeV, and that the gluon-fusion cross section is twice as large for a light top quark as for a heavy top quark. Photon pairs are an irreducible background to the two-photon decay mode of the Higgs boson, which is important if the Higgs boson is of intermediate mass (mH2MW) and mH2mₜ, i.e., if its decay to top quarks is kinematically forbidden. If that is the case, we show that the top-quark contribution to the gluon-fusion process is small and, if mH{}2mₜ, is even destructive. We also show that a judicious choice of cuts can improve the signal-to-background ratio.