Recent results on ultra-peripheral collisions with the ALICE experiment

Abstract

Recent measurements of J/ψ photoproduction based on data from ultra-peripheral Pb–Pb and p–Pb collisions recorded by the ALICE experiment during Run 2 of the LHC are presented. Photoproduction, as a photon-induced process, is sensitive to the structure of hadrons, and the results are of great importance for a better understanding of how gluon saturation and nuclear shadowing function in high-energy quantum chromodynamics. The energy evolution of coherent J/ψ photoproduction has been measured, indicating a strong suppression of nuclear gluon densities at high energies. The energy dependence has also been measured for exclusive and dissociative J/ψ production off the proton. The average distribution of nuclear gluon matter in the transverse plane, along with its fluctuations, has been studied for the first time using measurements of the dependence of coherent and incoherent J/ψ photoproduction on the transferred momentum t. Lastly, the data from coherent photoproduction of J/ψ in Pb–Pb ultra-peripheral collisions have been found to be compatible with a transverse polarization.