highest-jet in the event. The NNPDF3.0 set [@Ball:2014uwa] is used for parton distribution functions (PDFs). The SM Higgs boson cross sections and branching fractions are taken from the LHC Higgs Cross Section Working Group report [@LHCHXSWG:YR4].

Simulated background samples are used for training multivariate discriminants, and to define selection and classification criteria. The irreducible prompt diphoton background events are generated using the v2.2.1 program [@Gleisberg:2008ta]. This program includes the tree-level matrix elements with up to three additional jets and the box diagram at leading order accuracy. The reducible background arising from $\gamma+\mathrm{jet}$ and multijet events is modeled with $\PYTHIA$.

Samples of [$\cPZ\to\Pep\Pem$]{}, [$\cPZ\to\Pgmp\Pgmm$]{}, and [$\cPZ\to\Pgmp\Pgmm\Pgg$]{}simulated events are generated with and used for comparison with data and for the derivation of energy scale and resolution corrections.

Event reconstruction {#sec:reco}
====================

Photon candidates are reconstructed from clusters of energy deposited in the ECAL and merged into superclusters [@CMS:EGM-14-001]. The reconstruction algorithm for photon clusters allows almost complete recovery of the energy from photons that convert to an electron-positron pair in the material upstream of the ECAL. A detailed description of the algorithm can be found in Ref. [@1748-0221-10-06-P06005]. Changes in the transparency of the ECAL crystals due to irradiation during the LHC 