Photoproduction of heavy vector mesons in ultra-peripheral Pb–Pb collisions

E.L Kryshen

doi:10.1016/j.nuclphysa.2017.05.083

Abstract

Ultra-peripheral Pb-Pb collisions, in which the two nuclei pass close to each other, but at an impact parameter greater than the sum of their radii, provide information about the initial state of nuclei. In particular, heavy vector meson production, where the particle mass sets a hard scale, proceeds in such collisions by photon-gluon interactions, and gives access to nuclear PDFs. The ALICE collaboration has published measurements of J/ψ and ψ(2S) photoproduction in ultra-peripheral collisions in LHC Run 1 at forward (J/ψ) and mid-rapidity, and has obtained a substantially larger data set in 2015 from LHC Run 2, allowing much more detailed studies of the production mechanism to be performed. In particular, the increased energy and more detailed measurements in the forward region in Run 2 give access to significantly lower values of Bjorken-x than in previous studies. In this talk, the latest available results from Run 2 will be given.

Highlights

Lead nuclei, accelerated at the LHC, are sources of strong electromagnetic fields, which are equivalent to a flux of quasi-real photons, Pb–Pb collisions can be used to measure γPb interactions in a new kinematic regime
These interactions are usually studied in ultra-peripheral collisions (UPC), characterised by impact parameters larger than the sum of the radii of the incoming nuclei, in which hadronic interactions are strongly suppressed [1, 2]
Coherent heavy quarkonium photoproduction is of particular interest since, in leading order perturbative QCD, its cross section is proportional to the squared gluon density of the target [3]

Summary

Introduction

Lead nuclei, accelerated at the LHC, are sources of strong electromagnetic fields, which are equivalent to a flux of quasi-real photons, Pb–Pb collisions can be used to measure γPb interactions in a new kinematic regime. Coherent heavy quarkonium photoproduction is of particular interest since, in leading order perturbative QCD, its cross section is proportional to the squared gluon density of the target [3]. Quarkonium photoproduction in Pb–Pb UPC provides a direct tool to study nuclear gluon shadowing effects [4], which are poorly known and play a crucial role in the initial stages of heavy-ion collisions. 2. Charmonium photoproduction in Pb-Pb UPC at √sNN = 5.02 TeV

Results

Conclusion