  Quasireal photons exchanged in relativistic heavy ion interactions are
powerful probes of the gluonic structure of nuclei. The coherent J/$\psi$
photoproduction cross section in ultraperipheral lead-lead collisions is
measured as a function of photon-nucleus center-of-mass energies per nucleon
(W$^\text{Pb}_{\gamma\text{N}}$), over a wide range of 40 $\lt$
W$^\text{Pb}_{\gamma\text{N}}$ $\lt$ 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$^\text{Pb}_{\gamma\text{N}}$, and plateau above
W$^\text{Pb}_{\gamma\text{N}}$ $\approx$ 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.
