UPC : a powerful tool for \ensuremath{{\mathrm J}/\psi}\xspace photoproduction analysis in ALICE

Abstract

Ultra-peripheral collisions (UPC) occur when the interacting nuclei or protons have an impact parameter larger than the sum of their radii. They are mediated by virtual photon exchange. The photoproduction of heavy vector mesons is especially interesting because they couple to the photon. The ALICE Collaboration has analysed both \mbox{p--Pb}\xspace and \mbox{Pb--Pb}\xspace UPC at the centre-of-mass energy of $\sqrt{s_{\rm NN}} = 5.02\text{ TeV}$ which correspond to $\gamma$p and $\gamma$-Pb interactions, respectively. This poster reports the exclusive photoproduction of \ensuremath{{\mathrm J}/\psi}\xspace off proton and Pb targets, which shed light on the occurrence of saturation and nuclear shadowing, respectively. In more detail, the \mbox{p--Pb}\xspace results measure the growth of the cross section for exclusive production over a wide range in Bjorken-$x$ from \raisebox{0.5ex}{\texttildelow}$10^{-2}$ to \raisebox{0.5ex}{\texttildelow}$10^{-5}$, while \mbox{Pb--Pb}\xspace results demonstrate the presence of nuclear shadowing at high energies and low scales of the order of the mass of the \ensuremath{{\mathrm J}/\psi}\xspace.