\midheading{$g(-p_1)+g(-p_2) \to  \gamma(p_3)+\gamma(p_4)$, (top loops at LO)}
This process calculable at leading LO and next-to-leading order NLO.

This process computes the contribution of quark loops ($n_f = 5$ light
quarks and top quarks) to the process $gg \to \gamma\gamma$.  It is
calculable at leading LO and next-to-leading order NLO, although the
effect of top-quark loops are only included at LO.  The calculation
and a phenomenological analysis are described in
Ref.~\cite{Campbell:2016yrh}.