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CMS-HIN-22-002 ; CERN-EP-2023-031
Probing small Bjorken-$ x $ nuclear gluonic structure via coherent J/$\psi $ photoproduction in ultraperipheral Pb-Pb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV
Phys. Rev. Lett. 131 (2023) 262301
Abstract: 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^{\mathrm{Pb}}_{\gamma\mathrm{N}} $) over a wide range of 40 $ < W^{\mathrm{Pb}}_{\gamma\mathrm{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^{\mathrm{Pb}}_{\gamma\mathrm{N}} $, and plateau above $ W^{\mathrm{Pb}}_{\gamma\mathrm{N}} \approx $ 40 GeV, up to 400 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.
Figures & Tables Summary References CMS Publications
Figures

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Figure 1:
A pictorial representation of the J/$\psi$ photoproduction process in PbPb 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.

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Figure 2:
The differential coherent J/$\psi$ photoproduction cross section as a function of rapidity, in different neutron multiplicity classes (left): 0n0n, 0nXn and XnXn; (right): AnAn. 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/strong shadowing [34], color dipole models (CD_BGK,CD_BGW, and CD_IIM) [57], and STARLIGHT [47] are shown by the curves. The right plot also displays the results from the ALICE [13,15] and LHCb [17] experiments.

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Figure 2-a:
The differential coherent J/$\psi$ photoproduction cross section as a function of rapidity, in 0n0n, 0nXn and XnXn neutron multiplicity classes. 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/strong shadowing [34], color dipole models (CD_BGK,CD_BGW, and CD_IIM) [57], and STARLIGHT [47] are shown by the curves.

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Figure 2-b:
The differential coherent J/$\psi$ photoproduction cross section as a function of rapidity, with neutron multiplicity selection (AnAn). 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/strong shadowing [34], color dipole models (CD_BGK,CD_BGW, and CD_IIM) [57], and STARLIGHT [47] are shown by the curves. The plot also displays the results from the ALICE [13,15] and LHCb [17] experiments.

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Figure 3:
The total coherent J/$\psi$ photoproduction cross section as a function of $ W^{\mathrm{Pb}}_{\gamma\mathrm{N}} $ from the CMS measurement in PbPb UPCs at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.02 TeV. Approximated results (implied by the asterisk) from the ALICE [13,15] and LHCb [17] experiments are displayed for specific rapidity regions, where the two-way ambiguity effect is expected to be negligible. The $ W^{\mathrm{Pb}}_{\gamma\mathrm{N}} $ 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 [33,59,60,34,58,57] are shown by the curves.

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Figure 4:
The nuclear gluon suppression factor $ R^{\mathrm{Pb}}_{\mathrm{g}} $ as a function of Bjorken $ x $ extracted from the CMS measurement of the coherent J/$\psi$ photoproduction in PbPb UPCs at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}}= $ 5.02 TeV. Approximated results (implied by the asterisk) from the ALICE [13,15] 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 [64,65] 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 [60,34,58,57] are shown by the curves.

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Figure 5:
The invariant mass distribution (left) of $ \mu^{+}\mu^{-} $ pairs in the dimuon $ p_{\mathrm{T}} < $ 0.2 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 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.

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Figure 5-a:
The invariant mass distribution of $ \mu^{+}\mu^{-} $ pairs in the dimuon $ p_{\mathrm{T}} < $ 0.2 GeV range from the 0n0n neutron class. 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 vertical bars on the data points represent statistical uncertainty.

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Figure 5-b:
The transverse momentum distribution of $ \mu^{+}\mu^{-} $ pairs with a dimuon invariant mass in the J/$\psi $ mass window of (2.95 $ < m_{\mu^{+}\mu^{-}} < $ 3.25 GeV) in the 0n0n neutron class. The results of the fit are shown by the various curves. The vertical bars on the data points represent statistical uncertainty.

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Figure 5-c:
The invariant mass distribution of $ \mu^{+}\mu^{-} $ pairs in the dimuon $ p_{\mathrm{T}} < $ 0.2 GeV range from the XnXn neutron class. 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 vertical bars on the data points represent statistical uncertainty.

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Figure 5-d:
The transverse momentum distribution of $ \mu^{+}\mu^{-} $ pairs with a dimuon invariant mass in the J/$\psi $ mass window of (2.95 $ < m_{\mu^{+}\mu^{-}} < $ 3.25 GeV) in the XnXn neutron class. The results of the fit are shown by the various curves. The vertical bars on the data points represent statistical uncertainty.
Tables

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Table 1:
The measured values of the incoherent fractions ($ f_{\mathrm{I}} $) and their uncertainties for each of the neutron multiplicity class.

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Table 2:
The measured values of the feed-down fractions ($ f_{\mathrm{D}} $) and their uncertainties for each of the neutron multiplicity class.
Summary
In summary, the first measurement of the coherent J/$\psi$ photoproduction cross section off lead nuclei, as a function of the photon-nuclear center-of-mass energy per nucleon ($ W^{\mathrm{Pb}}_{\gamma\mathrm{N}} $) has been presented over a broad energy range. In a coherent process, the J/$\psi$ is produced by the photon interacting with the nucleus as a whole. Results are obtained with ultraperipheral PbPb collision data by applying the forward neutron tagging technique. The cross section is observed to rise rapidly at low $ W^{\mathrm{Pb}}_{\gamma\mathrm{N}} $ but appears to plateau above $ W^{\mathrm{Pb}}_{\gamma\mathrm{N}} \approx $ 40 GeV, up to 400 GeV, where a new regime of gluon momentum fraction (Bjorken $ x \approx $ 6 $\times$ 10$^{-5} $) in a heavy nucleus is probed. This observed trend is not predicted by current theoretical models. This can be interpreted either as the first direct evidence for the saturation of nuclear gluonic density or that the scattering cross section is near the black-disk limit.
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