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, $gg\to H\to \gamma \gamma$, with the continuum QCD background, $gg\to \gamma \gamma$, induced by quark loops. Helicity selection rules suppress the effect, which is dominantly due to the imaginary part of the two-loop $gg\to \gamma \gamma$ 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.