$J/\psi$ Photoproduction Results from ALICE

Diffractive photoproduction of J/ψ vector mesons is studied at the LHC with the ALICE detector in p–Pb and Pb–Pb ultra-peripheral collisions (UPCs), where the Pb ions act as powerful sources of quasi-real photons. In this contribution, the first measurement at the LHC of dissociative photoproduction of J/ψ off protons is presented; this process is sensitive to quantum fluctuations of the structure of the target at the subnucleon level. In addition, cross sections for the exclusive channel and for continuum dimuon production at small masses were obtained in the same kinematic region. This latter process probes our simultaneous understanding, in a new kinematic region, of the photon flux emitted by protons and by lead ions. The transverse momentum dependence of J/ψ photoproduction on lead targets in Pb–Pb collisions at midrapidity, which is sensitive to the gluonic structure of Pb in the impact-parameter plane, is also presented. The measurement of J/ψ photoproduction off hadrons sheds light onto the initial state of QCD targets and provides important constraints on the initial conditions used in hydrodynamical models of heavy-ion collisions.