These are mostly collimated photons from neutral-hadron decays (, [$\eta^0$]{}). The algorithm is implemented with a BDT trained on simulated events. The input variables of the BDT are: the pseudorapidity and energy of the supercluster corresponding to the reconstructed photon, several variables characterizing the shape of the electromagnetic shower, and the isolation energy sums computed with the particle-flow algorithm [@CMS-PRF-14-001]. Further information on the photon identification BDT can be found in [@CMS-PAS-HIG-16-040].

Event selection {#sec:selection}
===============

Each photon of the candidate pair entering the analysis is required to have a supercluster within $\abs{\eta}<2.5$, excluding the region $1.4442<\abs{\eta}<1.566$, which corresponds to the ECAL barrel-endcap transition region, and to satisfy selection criteria, described in Ref. [@CMS-PAS-HIG-16-040], slightly more stringent than the trigger requirements, based on transverse momentum, isolation, and shower shape variables. The transverse momentum scaled by the invariant mass of the diphoton candidate ($\pt/{\ensuremath{m_{\gamma\gamma}}\xspace}$) has to be greater than 1/3 (1/4) for the -leading (-subleading) photon. The use of thresholds in $\pt/{\ensuremath{m_{\gamma\gamma}}\xspace}$, rather than fixed thresholds in , prevents the distortion of the low end of the [$m_{\gamma\gamma}$]{}spectrum. Furthermore, each photon must fulfill a requirement based on the output of the photon identification classifier, chosen as explained in Section \[sec:categorization\].

Jets 