Measurement of the inclusive isolated-photon production cross section in pp and Pb–Pb collisions at √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_T^γ), in Pb–Pb collisions in different centrality intervals, and in pp collisions, at centre-of-momentum energy per nucleon pair of √s_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_T^γ than previously published results by the ATLAS and CMS experiments at the same collision energy. The covered pseudorapidity range is |η^γ| < 0.67. The isolation selection is based on a charged particle isolation momentum threshold p_T^(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.