Photoproduction of low-pT J/ψ from peripheral to central Pb–Pb collisions at 5.02 TeV

Abstract

An excess of J/ψ yield at very low transverse momentum (pT < 0.3 GeV/c), originating from coherent photoproduction, is observed in peripheral and semicentral hadronic Pb–Pb collisions at a center-of-mass energy per nucleon pair of √sNN = 5.02 TeV. The measurement is performed with the ALICE detector via the dimuon decay channel at forward rapidity (2.5 < y < 4). The nuclear modification factor at very low pT and the coherent photoproduction cross section are measured as a function of centrality down to the 10% most central collisions. These results extend the previous study at √sNN = 2.76 TeV, confirming the clear excess over hadronic production in the pT range 0 − 0.3 GeV/c and the centrality range 70–90%, and establishing an excess with a significance greater than 5σ also in the 50–70% and 30–50% centrality ranges. The results are compared with earlier measurements at √sNN = 2.76 TeV and with different theoretical predictions aiming at describing how coherent photoproduction occurs in hadronic interactions with nuclear overlap.