Speaker
Description
The study of $\rm{J}/\psi$ photoproduction in ultra-peripheral collisions (UPCs) offers a powerful probe of gluon dynamics in heavy nuclei. In UPCs, photons can interact with the nucleus coherently (involving the entire nucleus) or incoherently (with individual constituents). While coherent processes provide insight into the average gluon density, incoherent photoproduction is uniquely sensitive to the local gluon density fluctuations at both nucleonic or subnucleonic levels. Thus, the systematic measurement of energy-dependent $\rm{J}/\psi$ photoproduction could
explore the evolution of gluonic structures within nuclei in detail, with potential implications for uncovering the onset of gluon saturation at sufficient small-x regime.
In this talk, we present the first energy-dependent measurement of the incoherent $\rm{J}/\psi$ photoproduction cross section in PbPb UPCs at the LHC with the CMS experiment, using the forward neutron tagging to cover a wide photon-nucleon center-of-mass energy range from 40 to 400 GeV. This allows us to explore fluctuating gluon fields over a broad Bjorken-x range, extending into the uncharted region of x<10^-4. Furthermore, we report the ratio of incoherent to coherent \rm{J}/\psi$ production and evaluate the nuclear suppression factor for the incoherent production as a function of x. By comparing state-of-the-art theoretical predictions, these results offer unique insights into gluon fluctuations and have important implications for understanding nuclear gluon distributions and saturation phenomena.
