Measurement of the photon identification efficiencies with the ATLAS detector using LHC Run 2 data collected in 2015 and 2016

TL;DR

This paper measures photon identification efficiencies in the ATLAS detector using LHC Run 2 data, comparing data-driven results with simulations, and providing correction factors for improved photon detection accuracy.

Contribution

It presents the first detailed measurement of photon identification efficiencies across various conditions in ATLAS Run 2 data, including conversion status and pseudorapidity regions.

Findings

Efficiency ratios between data and simulation are within 0.5% to 5%.

Photon reconstruction and conversion efficiencies are quantified with uncertainties.

Impact of isolation and pile-up on photon identification is analyzed.

Abstract

The efficiency of the photon identification criteria in the ATLAS detector is measured using 36.1 fb$^{-1}$ to 36.7 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}$ = 13 TeV collected in 2015 and 2016. The efficiencies are measured separately for converted and unconverted isolated photons, in four different pseudorapidity regions, for transverse momenta between 10 GeV and 1.5 TeV. The results from the combination of three data-driven techniques are compared with the predictions from simulation after correcting the variables describing the shape of electromagnetic showers in simulation for the average differences observed relative to data. Data-to-simulation efficiency ratios are determined to account for the small residual efficiency differences. These factors are measured with uncertainties between 0.5% and 5% depending on the photon transverse momentum and pseudorapidity. The impact of the isolation criteria on the photon identification efficiency, and that of additional soft $pp$ interactions, are also discussed. The probability of reconstructing an electron as a photon candidate is measured in data, and compared with the predictions from simulation. The efficiency of the reconstruction of photon conversions is measured using a sample of photon candidates from $Z\to\mu\mu\gamma$ events, exploiting the properties of the ratio of the energies deposited in the first and second longitudinal layers of the ATLAS electromagnetic calorimeter.