Top-quark effects in diphoton production through gluon fusion at next-to-leading order in QCD

At hadron colliders, the leading production mechanism for a pair of photons is from quark-antiquark annihilation at the tree level. However, due to large gluon-gluon luminosity, the loop-induced process gg→γγ provides a substantial contribution. In particular, the amplitudes mediated by the top quark become important at the tt threshold and above. In this paper we present the first complete computation of the next-to-leading order (NLO) corrections (up to αS3) to this process, including contributions from the top quark. These entail two-loop diagrams with massive propagators whose analytic expressions are unknown and have been evaluated numerically. We find that the NLO corrections to the top-quark induced terms are very large at low diphoton invariant mass m(γγ) and close to the tt threshold. The full result including five massless quarks and top quark contributions at NLO displays a much more pronounced change of slope in the m(γγ) distribution at tt threshold than at LO and an enhancement at high invariant mass with respect to the massless calculation.