Study of exclusive photoproduction of charmonium in ultra-peripheral lead-lead collisions

TL;DR

This paper measures the cross-sections of exclusive J/psi and psi(2S) meson photoproduction in ultra-peripheral lead-lead collisions at the LHC, providing data that align with theoretical models.

Contribution

First measurement of differential and integrated cross-sections for exclusive charmonium photoproduction in ultra-peripheral PbPb collisions at 5.02 TeV.

Findings

Measured coherent J/psi cross-section: 5.965 mb

Measured coherent psi(2S) cross-section: 0.923 mb

Cross-section ratio psi(2S)/J/psi: 0.155

Abstract

The cross-sections of exclusive (coherent) photoproduction $J/\psi$ and $\psi(\mathrm{2S})$ mesons in ultra-peripheral PbPb collisions at a nucleon-nucleon centre-of-mass energy of $5.02\,\mathrm{TeV}$ are measured using a data sample corresponding to an integrated luminosity of $228\pm10\,\mathrm{\mu b}^{-1}$, collected by the LHCb experiment in 2018. The differential cross-sections are measured separately as a function of transverse momentum and rapidity in the nucleus-nucleus centre-of-mass frame for $J/\psi$ and $\psi(\mathrm{2S})$ mesons. The integrated cross-sections are measured to be \mbox{$\sigma^\mathrm{coh}_{J/\psi} = 5.965 \pm 0.059 \pm 0.232 \pm 0.262\,\mathrm{mb}$} and \mbox{$\sigma^\mathrm{coh}_{\psi(\mathrm{2S})} = 0.923 \pm 0.086 \pm 0.028 \pm 0.040\,\mathrm{mb}$}, where the first listed uncertainty is statistical, the second systematic and the third due to the luminosity determination. The cross-section ratio is measured to be \mbox{$\sigma^\mathrm{coh}_{\psi(\mathrm{2S})}/\sigma^\mathrm{coh}_{J/\psi} = 0.155 \pm 0.014 \pm 0.003$}, where the first uncertainty is statistical and the second is systematic. These results are compatible with theoretical predictions.