Coherent J/ψ photoproduction at midrapidity in P b − P b collisions at s N N = 5.02 TeV

The coherent J/ ψ photoproduction cross section is measured for the first time at midrapidity in peripheral to semicentral Pb−Pb collisions at sNN=5.02 TeV. The centrality differential cross section d σ /d y is reported for the centrality range 40–90%, together with the doubly-differential cross section d2 σ /d y d p T, extracted in two peripheral centrality classes. The J/ ψ mesons are reconstructed in the dielectron channel, in the rapidity interval | y | < 0.9 using the ALICE central barrel detectors. The J/ ψ cross section at midrapidity is statistically compatible to the earlier ALICE measurement at forward rapidity and at the same centre-of-mass energy, and shows only a mild centrality dependence over the covered range. Several sets of theoretical calculations taking into account the hadronic overlap in the collisions but ignoring possible final-state effects from a hot expanding medium are found to give a fairly good description of the current measurements within uncertainties.