First study of initial gluonic fluctuations using UPCs with ALICE

Incoherent photoproduction is sensitive to fluctuations in the gluon structure of the target. Thus, the photoproduction measurement off colliding hadrons sheds light on QCD at high energies and provides important constraints on the initial conditions used in hydrodynamical models of heavy-ion collisions. In this contribution, the first measurement of the transverse momentum dependence of both coherent and incoherent photoproduction in ultra-peripheral collisions (UPCs) of lead ions at mid-rapidity is presented. These new results provide, for the first time, an indication of subnucleonic fluctuations in the color structure of lead ions. Recent results on the energy dependence of photoproduction with proton dissociation in proton–lead UPCs are also shown, measured for the first time at a hadron collider.