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 (mH<2MW) and mH<2mt, 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 mH2mt, is even destructive. We also show that a judicious choice of cuts can improve the signal-to-background ratio.