Probing Small Bjorken-$x$ Nuclear Gluonic Structure via Coherent $J/\psi$ Photoproduction in Ultraperipheral Pb-Pb Collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$

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 ($W_{\gamma \mathrm{N}}^{\mathrm{Pb}}$) over a wide range of $40<W_{\gamma \mathrm{N}}^{\mathrm{Pb}}<400\mathrm{GeV}$. 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 $1.52{\mathrm{nb}}^{-1}$. The cross section is observed to rise rapidly at low $W_{\gamma \mathrm{N}}^{\mathrm{Pb}}$, and plateau above $W_{\gamma \mathrm{N}}^{\mathrm{Pb}}\approx 40\mathrm{GeV}$, up to $400\mathrm{GeV}$, entering 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.

Figure 1

A pictorial representation of the $J/\psi$ photoproduction process in Pb-Pb at leading-order QCD, showing the origin of the two-way ambiguity. The blue wavy and black helical lines represent emitted photons and gluons, respectively.

Figure 2

The differential coherent $J/\psi$ photoproduction cross section as a function of rapidity, in different neutron multiplicity classes: 0n0n, 0nXn, and XnXn (left); AnAn (right). The small vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. The horizontal bars show the bin widths. Theoretical predictions from LTA weak and strong shadowing [33], color dipole models (CD_BGK, CD_BGW, and CD_IIM) [62], and starlight [46] are shown by the curves. The right plot also displays the results from the ALICE [13, 14] and LHCb [17] experiments.

Figure 3

The total coherent $J/\psi$ photoproduction cross section as a function of $W_{\gamma \mathrm{N}}^{\mathrm{Pb}}$ from the CMS measurement in Pb-Pb UPCs at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Approximated results (implied by the asterisk) from the ALICE [13, 14] and LHCb [17] experiments are displayed for specific rapidity regions, where the two-way ambiguity effect is expected to be negligible. The $W_{\gamma \mathrm{N}}^{\mathrm{Pb}}$ values used correspond to the center of each experiment’s rapidity range. The vertical bars and the shaded and open boxes represent the statistical, experimental, and theoretical (photon flux) uncertainties, respectively. The predictions from various theoretical calculations [32, 33, 62, 63, 64, 65] are shown by the curves.

Figure 4

The nuclear gluon suppression factor $R_g^{\mathrm{Pb}}$ as a function of Bjorken $x$ extracted from the CMS measurement of the coherent $J/\psi$ photoproduction in Pb-Pb UPCs at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Approximated results (implied by the asterisk) from the ALICE [13, 14] and LHCb [17] experiments are displayed for specific rapidity regions, where the two-way ambiguity effect is expected to be negligible. A measurement of $J/\psi$ photoproduction from the E691 experiment [69, 70] is also given. The $x$ values are evaluated at the centers of their corresponding rapidity ranges. The vertical bars and shaded and open boxes represent the statistical, experimental systematic, and theoretical systematic uncertainties, respectively. The latter is due to the uncertainties in the photon flux and the IA. Predictions from various theoretical calculations [33, 62, 63, 65] are shown by the curves.

Figure 5

The invariant mass distribution (left) of ${\mu }^{+}{\mu }^{-}$ pairs in the dimuon $p_{\mathrm{T}}<0.2\mathrm{GeV}$ range from the 0n0n (upper) and XnXn (lower) neutron classes. The results of the fit are shown by the various curves, and the corresponding values of the ${\chi }^2$/number of degrees of freedom (ndf) and the fitted number of $J/\psi$ and $\psi (2S)$ candidates are given. The values of the factors $R_N$, $R$, and $f_{\mathrm{D}}$, defined below, are also shown. The transverse momentum distribution (right) of ${\mu }^{+}{\mu }^{-}$ pairs with a dimuon invariant mass in the $J/\psi$ mass window of ($2.95<m_{{\mu }^{+}{\mu }^{-}}<3.25\mathrm{GeV}$) in the 0n0n (upper) and XnXn (lower) neutron classes. Again, the results of the fit are shown by the various curves. In both plots, the vertical bars on the data points represent statistical uncertainty.