$J/\psi$ photoproduction in Pb-Pb and p-Pb ultra-peripheral collisions with ALICE at LHC

The photoproduction of vector mesons in Ultra-Peripheral Collisions (UPC) is a powerful tool to probe the nuclear gluon distribution, for which there is considerable uncertainty in the low-x region. We present the first measurements in Pb-Pb collisions at $\sqrt{s_{NN}}$=2.76 TeV, performed with the ALICE detector. The J/psi is identified via its dimuon decay in the forward rapidity region and via dimuon and dielectron decay at midrapidity. The results are compared to theoretical models for coherent J/psi production and found to be in good agreement with models which include nuclear gluon shadowing. The cross section measurement for incoherent J/psi and @c@c->e^+e^- at mid-rapidity is reported. Finally we present the first results on p-Pb UPC at $\sqrt{s_{NN}}$=5.02 TeV.