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 (WPb
γN) over a wide range
of 40 < WPb
γN < 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 WPb
γN, and plateau above WPb
γN ≈ 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.
