\midheading{$g(-p_1)+g(-p_2) \to  \gamma(p_3)+\gamma(p_4)$}
This process computes the contribution of light-quark loops
to the process $gg \to \gamma\gamma$.  It is calculable at leading LO
and next-to-leading order NLO.
The calculation and a phenomenological analysis are described
in Ref.~\cite{Campbell:2016yrh}.
This process includes the one-loop gluon-gluon contribution as
given in ref.~\cite{Bern:2002jx}.