Charmonium photoproduction in ultra-peripheral p-Pb and Pb-Pb collisions at the LHC with the ALICE experiment

Vector mesons are copiously produced in ultra-peripheral collisions. In these collisions, the impact parameter is larger than the sum of the radii of two projectiles, implying that electromagnetic processes become dominant. The cross section for this process is sensitive to the gluon distribution and can therefore probe nuclear gluon shadowing (Pb-Pb) and the gluon structure function in the nucleon (p-Pb). The ALICE Collaboration has performed the first measurement of the coherent J/$\psi$ and $\psi$(2S) photoproduction cross section in Pb-Pb collisions and that for exclusive J/$\psi$ photoproduction off protons in ultra-peripheral proton-lead collisions at the LHC. The results are compared to STARLIGHT and to QCD based models.