J/ψ photoproduction in Pb–Pb and p–Pb ultra-peripheral collisions with ALICE at LHC

Abstract

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 s NN = 2.76 TeV , performed with the ALICE detector. The J / ψ 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 / ψ production and found to be in good agreement with models which include nuclear gluon shadowing. The cross section measurement for incoherent J / ψ and γ γ → e + e − at mid-rapidity is reported. Finally we present the first results on p–Pb UPC at s NN = 5.02 TeV .