Photon pair production in gluon fusion: Top quarkeffects around threshold

We present a calculation of the NLO QCD corrections to the loop-induced production of a photonpair through gluon fusion, including massive top quarks at two loops. The two-loop virtual am-plitudes are computed via projecting onto a set of Lorentz structures related to linear polarisationstates of external gauge bosons. All two-loop master integrals dependent on the top quark massare evaluated numerically. Matching the fixed order NLO result to a calculation with threshold re-summation, we obtain an accurate description of the diphoton invariant mass spectrum around thethreshold of top quark pair production. We analyse how the distribution of the diphoton invariantmass is affected by the top quark loop-induced corrections, especially around the top quark pairproduction threshold