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

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$^{γ}_{T}$), in Pb–Pb collisions in different centrality intervals, and in pp collisions, at centre-of-momentum energy per nucleon pair of $\sqrt{s_{NN}}$ = 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$^{γ}_{T}$ 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$^{iso, ch}_{T}$ 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 Z⁰ bosons from the CMS experiment, which are all found to be in agreement.. Authors: Acharya, S. [LPC] (ORCID:0000000292135329); Agarwal, A. [Homi Bhabha National Institute]; Rinella, G. Aglieri [European Organization for Nuclear Research (CERN)] (ORCID:0000000296113696); Aglietta, L. [Dipartimento di Fisica dell’Università and Sezione INFN] (ORCID:0009000307636802); Agnello, M. [Dipartimento DISAT del Politecnico and Sezione INFN] (ORCID:0000000207605075). DOE Contract: AC05-00OR22725; FG02-96ER40982; SC0004168. Subjects: 73 NUCLEAR PHYSICS AND RADIATION PHYSICS; Angular momentum of light; Experimental Nuclear Physics; Experimental Particle Physics; Nuclear and Particle Physics; Particle Physics; Peripheral tolerance