Orateur
Description
The presence of an unexpected significant excess of low $p_{T}$ $J/\psi$ over the expected hadronic $J/\psi$ production was confirmed in peripheral Pb$-$Pb collisions and observed for the first time in semi-central Pb$-$Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV with ALICE at LHC. The measurements were performed in the dimuon decay channel at forward rapidity ($2.5 < y < 4$) and in the dielectron decay channel at mid-rapidity. Surprisingly, a large increase in the $J/\psi$ nuclear modification factor was observed in peripheral Pb$-$Pb collisions at low $p_{T}$, below 0.3 GeV/c. Most of the excess is believed to originate from a coherent photoproduction mechanism, which was unexpected until few years ago in peripheral collisions with nuclear overlap and is well known in ultraperipheral Pb$-$Pb collisions. The coherent photoproduction implies the interaction of a quasi-real photon coherently with the full Pb target nucleus, which remains intact. Many theoretical models developed for ultraperipheral collisions have tried to extend the description of the coherent $J/\psi$ photoproduction mechanism to Pb$-$Pb collisions with nuclear overlap. In this presentation, I will show the results obtained with ALICE, and will compare them to theoretical calculations. And I will introduce the most recent analysis of the rapidity differential coherent $J/\psi$ photoproduction cross section measurement at forward rapidity in ALICE.
