a boson decaying to a pair of electrons, or to a pair of electrons or muons in association with a photon [@CMS:EGM-14-001]. After applying corrections, based on control samples in data, to the input of the photon identification classifier, the efficiencies measured in data are found to be 3 (5)% lower than in simulation for photons in the barrel (endcap) regions with $\RNINE < 0.85$ ($0.9$), where $\RNINE$ is defined as the sum of the energy measured in a $3\times3$ crystal matrix, centered on the crystal with the highest energy in the ECAL cluster of the candidate, divided by the energy of the candidate. Photon candidates undergoing a conversion before reaching the ECAL have wider shower profiles and lower values of $\RNINE$. A correction factor is applied to simulated events to take into account the discrepancy in the efficiency between data and simulation. For the remaining photons, the predicted efficiencies are compatible with the ones measured in data.

Mass resolution estimator {#sec:sigmaM}
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The selected photon pairs are categorized according to their estimated relative mass resolution. For the typical energy range of the photons used in this analysis, corresponding to tens of GeV, the energy resolution estimator depends on the 