Measurement of the inclusive isolated-photon production cross section in pp and Pb–Pb collisions at \(\mathbf {\sqrt{\textit{s}_{NN }} = 5.02}\) TeV

Abstract

The ALICE Collaboration at the CERN LHC has measured the inclusive production cross section of isolated photons at midrapidity as a function of the photon transverse momentum (\(p_{\textrm{T}}^{\gamma }\)), in Pb–Pb collisions in different centrality intervals, and in pp collisions, at centre-of-momentum energy per nucleon pair of \(\sqrt{s_{\textrm{NN}}}~=~5.02\) TeV. The photon transverse momentum range is between 10–14 and 40–140 GeV/\(c\), depending on the collision system and on the Pb–Pb centrality class. The result extends to lower \(p_{\textrm{T}}^{\gamma }\) than previously published results by the ATLAS and CMS experiments at the same collision energy. The covered pseudorapidity range is \(|\eta ^{\gamma } | <0.67\). The isolation selection is based on a charged particle isolation momentum threshold \(p_{\textrm{T}}^\mathrm{iso,~ch} = 1.5\) GeV/\(c\) within a cone of radii \(R=0.2\) and 0.4. The nuclear modification factor is calculated and found to be consistent with unity in all centrality classes, and also consistent with the HG-PYTHIA model, which describes the event selection and geometry biases that affect the centrality determination in peripheral Pb–Pb collisions. The measurement is compared to next-to-leading order perturbative QCD calculations and to the measurements of isolated photons and Z\(^{0}\) bosons from the CMS experiment, which are all found to be in agreement.