Probing small Bjorken-x nuclear gluonic structure via coherent J=ψ photoproduction in ultraperipheral Pb-Pb collisions at √s 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 / ψ photoproduction cross section in ultraperipheral lead-lead collisions is measured as a function of photon-nucleus center-of-mass energies per nucleon ( W Pb γ N ) over a wide range of 40 < W Pb γ N < 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 Pb γ N , and plateau above W Pb γ N ≈ 40 GeV , up to 400 GeV , entering a new regime of small Bjorken- x ( ≈ 6 × 10 − 5 ) gluons being probed in a heavy nucleus. The observed energy dependence is not predicted by current quantum chromodynamic models.