Coherent J/ψ and ψ' photoproduction at midrapidity in ultra-peripheral Pb–Pb collisions at √sNN = 5.02 TeV

Abstract

The coherent photoproduction of J/ψ and ψ' mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy √sNN = 5.02 TeV with the ALICE detector. Charmonia are detected in the central rapid ity region for events where the hadronic interactions are strongly suppressed. The J/ψ is reconstructed using the dilepton (l +l −) and proton–antiproton decay channels, while for the ψ' the dilepton and the l +l −π+π− decay channels are studied. The analysis is based on an event sample correspond ing to an integrated luminosity of about 233μb−1. The results are compared with theoretical models for coherent J/ψ and ψ' photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moder ate nuclear gluon shadowing of about 0.64 at a Bjorken-x of around 6×10−4, such as the EPS09 parametrization, however none of the models is able to fully describe the rapidity depen dence of the coherent J/ψ cross section including ALICE measurements at forward rapidity. The ratio of ψ' to J/ψ coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.