Coherent $J/\psi$ photoproduction and polarization in peripheral Pb$-$Pb collisions

Abstract

Photonuclear reactions are induced by the strong electromagnetic field generated by ultrarelativistic heavy-ion collisions. These processes have been extensively studied in ultraperipheral collisions, where the impact parameter is larger than twice the nuclear radius. In recent years, the observation of coherent $J/\psi$ photoproduction at very low transverse momentum has been claimed in nucleus$-$nucleus (A$-$A) collisions with nuclear overlap, based on the measurement of an excess in the $J/\psi$ yield with respect to hadroproduction expectations. Such quarkonium measurements can help to constrain the nuclear gluon distribution at low Bjorken-$x$ and high energy. The photoproduced quarkonium is expected to preserve the polarization of the incoming photon due to the $s$-channel helicity conservation. In this contribution, we report on the new preliminary measurements of the $y$-differential cross section and the first polarization analysis of coherently photoproduced $J/\psi$ in peripheral Pb$-$Pb collisions with the ALICE detector at forward rapidity in the dimuon decay channel. Comparison with models will be shown when available.