Measurement of the inclusive isolated-photon production cross section in pp and Pb–Pb collisions at sNN=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 ($p_{\text{T}}^{\gamma }$), in Pb–Pb collisions in different centrality intervals, and in pp collisions, at centre-of-momentum energy per nucleon pair of $\sqrt{s_{\text{NN}}}=5.02$ 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 $p_{\text{T}}^{\gamma }$ 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 $p_{\text{T}}^{\mathrm{iso},\mathrm{ch}}=1.5$ 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.