Probing Small Bjorken-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>x</mml:mi></mml:math> Nuclear Gluonic Structure via Coherent <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>ψ</mml:mi></mml:mrow></mml:math> Photoproduction in Ultraperipheral Pb-Pb Collisions at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msqrt><mml:mrow><mml:msub><mml:mrow><mml:mi>s</mml:mi></mml:mrow><mml:mrow><mml:mi>NN</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:msqrt><mml:mo>=</mml:mo><mml:mn>5.02</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>TeV</mml:mi></mml:mrow></mml:math>

Quasireal photons exchanged in relativistic heavy ion interactions are powerful probes of the gluonic structure of nuclei. The coherent $J/\ensuremath{\psi}$ photoproduction cross section in ultraperipheral lead-lead collisions is measured as a function of photon-nucleus center-of-mass energies per nucleon (${W}_{\ensuremath{\gamma}\mathrm{N}}^{\mathrm{Pb}}$) over a wide range of $40&lt;{W}_{\ensuremath{\gamma}\mathrm{N}}^{\mathrm{Pb}}&lt;400\text{ }\text{ }\mathrm{GeV}$. 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 $1.52\text{ }\text{ }{\mathrm{nb}}^{\ensuremath{-}1}$. The cross section is observed to rise rapidly at low ${W}_{\ensuremath{\gamma}\mathrm{N}}^{\mathrm{Pb}}$, and plateau above ${W}_{\ensuremath{\gamma}\mathrm{N}}^{\mathrm{Pb}}\ensuremath{\approx}40\text{ }\text{ }\mathrm{GeV}$, up to $400\text{ }\text{ }\mathrm{GeV}$, entering a new regime of small Bjorken-$x$ ($\ensuremath{\approx}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$) gluons being probed in a heavy nucleus. The observed energy dependence is not predicted by current quantum chromodynamic models.

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