Photon identification with the ATLAS detector

Good photon identification capabilities are important for many aspects of the ATLAS physics programme, including measurements of fundamental properties of the hard interaction in final states with one or more photons, possibly produced in association with jets or gauge bosons. The identification of prompt photons and the rejection of background coming mostly from photons from hadron decays relies on the high granularity of the ATLAS electromagnetic calorimeter. Several methods are used to measure with data the efficiency of the photon identification requirements, covering a broad energy spectrum. At low energy, photons from radiative $Z$ decays are used. In the medium energy range, similarities between electrons and photon showers are exploited using $Z\rightarrow ee$ decays. At high energy, inclusive photon samples are used. The results of these measurements performed with $pp$ collisions data at $\sqrt{s} = 13\,$TeV in 2015-2018 corresponding to an integrated luminosity of $140\,$fb$^{-1}$ are presented. The impact on the photon identification of the pile-up, especially large in the second part of 2017 data taking, is also discussed.