The coherent photoproduction of $\rm{ J/\psi}$ and $\rm{ \psi'}$ mesons was measured in ultra-peripheral Pb-Pb collisions at a center-of-mass energy $\sqrt{ s_{ \mathrm{ NN}}}~=~5.02$ TeV with the ALICE detector. Charmonia are detected in the central rapidity region for events where the hadronic interactions are strongly suppressed. The $\rm{ J/\psi}$ is reconstructed using the dilepton ($l^{ +} l^{ -}$) and proton-antiproton decay channels, while for the $\rm{ \psi'}$, 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 $\rm{ J/\psi}$ and $\rm{ \psi'}$ 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 $\rm{ J/\psi}$ cross section including ALICE measurements at forward rapidity. The ratio of $\rm{ \psi'}$ to $\rm{ J/\psi}$ coherent photoproduction cross sections was also measured and found to be consistent with the one for photoproduction off protons.
Categories: nucl-ex hep-ex