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 -> gamma gamma provides a substantial contribution. In particular, the amplitudes mediated by the top quark become important at the t (t) over bar threshold and above. In this paper we present the first complete computation of the next-to-leading order (NLO) corrections (up to alpha(3)(S)) 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 arc very large at low diphoton invariant mass m(gamma gamma) and close to the t (t) over bar 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 (gamma gamma) distribution at t (t ) over bar threshold than at LO and an enhancement at high invariant mass with respect to the massless calculation.