Coherent J/$\psi$ and $\psi'$ photoproduction at midrapidity in ultra-peripheral Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The coherent photoproduction of J/$\psi$ and $\psi'$ mesons was measured in ultra-peripheral Pb-Pb collisions at a center-of-mass energy $\sqrt{s_{\rm NN}}$ = 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 ($e^{+} e^{-}$) and proton-antiproton decay channels, while for the $\psi'$, the dilepton and the $e^{+} e^{-} \pi^{+} \pi^{-}$ 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 $\psi'$ photoproduction. The coherent cross section is found to be in a good agreement with models incorporating moderate nuclear gluon shadowing of about 0.65 at a Bjorken-$x$ of around $6\times 10^{-4}$, 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 $\psi'$ to J/$\psi$ coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.