Coherent $J/\psi $ and ${\psi }^{\prime }$ photoproduction at midrapidity in ultra-peripheral Pb–Pb collisions at $\sqrt{{s}_{\mathrm{NN}}}\phantom{\rule{3.33333pt}{0ex}}=\phantom{\rule{3.33333pt}{0ex}}5.02$ TeV

The coherent photoproduction of $J/\psi $ and ${\psi }^{\prime }$ mesons was measured in ultra-peripheral Pb–Pb collisions at a center-of-mass energy $\sqrt{{s}_{\mathrm{NN}}}\phantom{\rule{3.33333pt}{0ex}}=\phantom{\rule{3.33333pt}{0ex}}5.02$ TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The $J/\psi $ is reconstructed using the dilepton ( ${l}^{+}{l}^{-}$ ) and proton–antiproton decay channels, while for the ${\psi }^{\prime }$ the dilepton and the ${l}^{+}{l}^{-}{\pi }^{+}{\pi }^{-}$ decay channels are studied. The analysis is based on an event sample corresponding to an integrated luminosity of about 233 $\mu {b}^{-1}$ . The results are compared with theoretical models for coherent $J/\psi $ and ${\psi }^{\prime }$ photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate 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 dependence of the coherent $J/\psi $ cross section including ALICE measurements at forward rapidity. The ratio of ${\psi }^{\prime }$ to $J/\psi $ coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.