Recent results on ultra-peripheral collisions with the ALICE experiment

TL;DR

This paper presents recent ALICE experiment measurements of J/ψ photoproduction in ultra-peripheral heavy-ion collisions, providing insights into gluon saturation, nuclear shadowing, and gluon matter distribution at high energies.

Contribution

It reports new measurements of J/ψ photoproduction energy dependence, nuclear gluon density suppression, and transverse gluon distribution, advancing understanding of quantum chromodynamics in nuclear environments.

Findings

Strong suppression of nuclear gluon densities at high energies.

First measurement of gluon matter distribution fluctuations.

Compatibility of coherent J/ψ photoproduction with transverse polarization.

Abstract

Recent measurements of J/$\psi$ 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 work in high-energy quantum chromodynamics. The energy evolution of coherent J/$\psi$ 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/$\psi$ production off the proton. The average distribution of the nuclear gluon matter in transverse plane, and fluctuations thereof, have been studied for the first time using the measurements of the dependence of coherent and incoherent J/$\psi$ photoproduction on the transferred momentum $|t|$. Lastly, the data from coherent photoproduction of J/$\psi$ in Pb-Pb ultra-peripheral collisions have been found to be compatible with a transverse polarization.