Probing small Bjorken-$x$ nuclear gluonic structure via coherent J/$\ddot{I}ˆ$ photoproduction in ultraperipheral PbPb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV

Abstract

Quasireal photons exchanged in relativistic heavy ion interactions are powerful probes of the gluonic structure of nuclei. The coherent J/$\ddot{I}ˆ$ photoproduction cross section in ultraperipheral lead-lead collisions is measured as a function of photon-nucleus center-of-mass energies per nucleon (W${}_{{\hat{I}}^3\text{N}}^{\text{Pb}}$), over a wide range of 40 $<$ W${}_{{\hat{I}}^3\text{N}}^{\text{Pb}}$ $<$ 400 GeV. Results are obtained using data at the nucleon-nucleon center-of-mass energy of 5.02 TeV collected by the CMS experiment at the CERN LHC, corresponding to an integrated luminosity of 1.52 nb${}^{-1}$. The cross section is observed to rise rapidly at low W${}_{{\hat{I}}^3\text{N}}^{\text{Pb}}$, and plateau above W${}_{{\hat{I}}^3\text{N}}^{\text{Pb}}$ $\approx$ 40 GeV, up to 400 GeV, a new regime of small Bjorken-$x$ ($\approx$ 6 $\times$ 10${}^{-5}$) gluons being probed in a heavy nucleus. The observed energy dependence is not predicted by current quantum chromodynamic models.

Replaced with the published version. Added the journal reference and the DOI. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-22-002 (CMS Public Pages)