Photon identification in 2015 ATLAS data

This document summarizes the method to identify isolated photons in the ATLAS experiment, and the techniques to measure the photon identification efficiencies. The strategy developed during LHC Run-I is repeated on the proton-proton collisions collected during 2015 at $\sqrt{s}$ = 13 TeV, with an integrated luminosity of 3.2 fb$^{−1}$. Three independent analyses have been exploited. One uses photons from radiative $Z \to ll\gamma$ decays. The second extracts the shower shape properties of electrons from $Z \to ee$ decays and extrapolates them to photons. The third directly measures the efficiency on samples of reconstructed photons, after determining and subtracting the hadronic background with a technique based on track isolation. The results from all analyses are then compared with each other and with the prediction from the simulation.