Resonance-continuum interference in the diphoton Higgs signal at the LHC

A low mass standard model Higgs boson should be visible at the Large Hadron Collider through its production via gluon-gluon fusion and its decay to two photons. We compute the interference of this resonant process, gg-->H-->gammagamma, with the continuum QCD background, gg-->gammagamma, induced by quark loops. Helicity selection rules suppress the effect, which is dominantly due to the imaginary part of the two-loop gg-->gammagamma scattering amplitude. The interference is destructive, but only of order 5% in the standard model, which is still below the 10%-20% present accuracy of the total cross section prediction. We comment on the potential size of such effects in other Higgs models.