First study of the initial gluonic fluctuations using UPCs with ALICE

Abstract

Incoherent and dissociative J/$\psi$ photoproduction is sensitive to fluctuations of the gluonic structure of the target. Thus, the measurement of J/$\psi$ photoproduction of the colliding hadron sheds light on the initial state of QCD and provides important constraints on the initial conditions used in hydrodynamical models of heavy-ion collisions. The first measurement of the transverse momentum dependence of both coherent and incoherent J/$\psi$ photoproduction in ultraperipheral Pb-Pb collisions at midrapidity is presented. These new results provide, for the first time, a clear indication of subnucleonic fluctuations of the lead target. We also present the new measurement of dissociative J/$\psi$ photoproduction cross section as a function of energy, in p-Pb collisions at forward rapidity. Dissociative results do not show any indication of saturation and agree with previous data.