Ultra-peripheral collisions (UPCs) between two Pb nuclei are those in which the impact parameter is larger than the sum of their radii. They provide a useful way to study photonuclear collisions. Coherent heavy vector meson photoproduction, when the photon interacts consistently with all nucleons in a nucleus, is of particular interest because of its connection with the gluon distribution functions (PDFs) in protons and nuclei. At low Bjorken-x values, PDFs are significantly suppressed in the nucleus with respect to free proton PDFs, a phenomenon known as nuclear shadowing.
The ALICE collaboration recently submitted for publication the measurement of the coherent photoproduction of J/ψ and ψ’ at midrapidity |y|<0.8 in Pb-Pb UPCs at 5.02 TeV. This data complements the ALICE measurement of the coherent J/ψ cross section at forward rapidity -4<y<-2.5.
The nuclear gluon shadowing factor of about 0.65 at Bjorken-x values is estimated which implies moderate nuclear shadowing. The measured rapidity dependence of the coherent cross section is not completely reproduced by models in the full rapidity range. The ratio of the ψ’ to J/ψ cross sections at midrapidity is consistent with the ratio of photoproduction cross sections off protons measured by the H1 and LHCb collaborations, leading to the conclusion that shadowing effects are similar for 2S (ψ’) and 1S (J/ψ) states.
ALICE Collaboration 2021 https://arxiv.org/abs/2101.04577