Measurements of vector meson photoproduction with ALICE in ultra-peripheral Pb-Pb collisions at $\bf{\sqrt{s_{\rm NN}} = 5.02}$ TeV

TL;DR

This paper presents ALICE measurements of vector meson photoproduction in ultra-peripheral Pb-Pb collisions at 5.02 TeV, providing insights into gluon distributions and testing the black disk regime at high energies.

Contribution

First ALICE results at 5.02 TeV for vector meson photoproduction in ultra-peripheral collisions, with detailed cross section measurements and comparisons to theoretical predictions.

Findings

Measured ho^0 and J/psi photoproduction cross sections.

Extended the kinematic range to lower Bjorken-x values.

Provided data to test the black disk regime in heavy-ion collisions.

Abstract

The intense photon fluxes of relativistic nuclei provide a possibility to study photonuclear and two-photon interactions in ultra-peripheral collisions (UPC) where the nuclei do not overlap and no strong nuclear interactions occur. The study of such collisions provides information about the initial state of nuclei. First ALICE results from the LHC Run 2 are presented for forward exclusive \jpsi production in UPC, which is sensitive to the gluon distribution in nuclei. The increased statistics and the higher collision energy allows for a more detailed study at lower values of Bjorken-$x$. The analysis of the $\gamma+A \rightarrow \rho^0+A$ process in UPC is a tool to test the so-called black disk regime, where the target nucleus appears like a black disk and the total $\rho^0+A$ cross section reaches its limit. ALICE reports new measurements of \rrho photoproduction cross sections in Pb-Pb UPC at $\sqrt{s_{\rm NN}}=5.02$ TeV at mid-rapidity, which are compared to predictions.