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

An excess of J/ψ yield at very low transverse momentum (p_T<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 s_NN=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 p_T 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 s_NN=2.76 TeV, confirming the clear excess over hadronic production in the p_T 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 s_NN=2.76 TeV and with different theoretical predictions aiming at describing how coherent photoproduction occurs in hadronic interactions with nuclear overlap.