Coherent vector meson photoproduction and azimuthal anisotropy studies in ion collisions at the LHC

At CERN’s Large Hadron Collider (LHC), ultra-relativistic ions generate electromagnetic fields that act as intense sources of quasi-real photons. When two ions pass each other at an impact parameter larger than the sum of their radii, hadronic interactions are strongly suppressed and ultra-peripheral collisions (UPCs) occur. UPCs provide a clean environment for studying photoninduced reactions and nuclear structure. In particular, coherent vector meson photoproduction is sensitive to the gluon distribution in the target nucleus, while light and heavy vector mesons probe complementary non perturbative and perturbative regimes of quantum chromodynamics. This thesis presents two studies of coherent vector meson photoproduction performed with the ALICE experiment. The main result is the measurement of the impact parameter dependence of the azimuthal anisotropy in coherent ρ0 photoproduction in Pb–Pb UPCs at √ sNN = 5.02 TeV. The ρ0 mesons are reconstructed at midrapidity through the ρ0 → π+π− decay channel. The anisotropy is studied using the angle ϕ between the sum and difference of the pion transverse momentum vectors. A cos(2ϕ) modulation of the ρ0 yield arises from the interplay between the linear polarization of the exchanged photons and the quantum mechanical interference between two indistinguishable production amplitudes, in which either nucleus can emit the photon while the other acts as the target. This mechanism is analogous to the double-slit experiment. The two nuclei act as spatially separated production sources, while the final state does not identify which nucleus emitted the photon and which served as the target. The corresponding amplitudes therefore interfere at the wave-function level, with the impact parameter playing a role analogous to the slit separation. Events are separated into three neutron emission classes using the Zero Degree Calorimeters, providing experimental access to different impact parameter ranges. The measured modulation amplitude increases by about one order of magnitude from from large to small impact parameters, in agreement with theoretical calculations. This measurement demonstrates quantum interference at the femtometre scale and opens new possibilities for studying the gluon structure of nuclei. The thesis also presents an exploratory study of coherent J/ψ photoproduction at forward rapidity in OO UPCs at √ sNN = 5.36 TeV, using the J/ψ → μ+μ− channel. A clear J/ψ signal and a pronounced low transverse momentum coherent peak are observed in the dimuon channel. The study establishes the strategy for completing the cross section measurement, which can provide new constraints on nuclear gluon shadowing, saturation, and the spatial structure of the oxygen nucleus.