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| CMS-HIN-23-009 ; CERN-EP-2025-030 | ||
| Probing gluon fluctuations in nuclei with the first energy-dependent measurement of incoherent $ \mathrm{J}/\psi $ photoproduction in ultraperipheral PbPb collisions | ||
| CMS Collaboration | ||
| 12 March 2025 | ||
| Phys. Rev. Lett. 135 (2025) 112301 | ||
| Abstract: Incoherent $ \mathrm{J}/\psi $ photoproduction in heavy ion ultraperipheral collisions (UPCs) provides a sensitive probe of localized, fluctuating gluonic structures within heavy nuclei. This study reports the first measurement of the photon-nucleon center-of-mass energy ($ W_{\gamma\mathrm{N}} $) dependence of this process in PbPb UPCs at a nucleon-nucleon center-of-mass energy of 5.02 TeV, using 1.52 nb$^{-1}$ of data recorded by the CMS experiment. The measurement covers a wide $ W_{\gamma\mathrm{N}} $ range of $ \approx $40-400 GeV, probing gluons carrying a fraction $ x $ of nucleon momentum down to an unexplored regime of 6.5 $ \times $ 10$^{-5} $. Compared to baseline predictions neglecting nuclear effects, the measured cross sections exhibit significantly greater suppression at lower $ x $. Additionally, the ratio of incoherent to coherent photoproduction is found to be constant across the probed $ W_{\gamma\mathrm{N}} $ and $ x $ range, disfavoring the establishment of the black disk limit. This study provides critical insights into the $ x $-dependent evolution of fluctuating gluonic structures within nuclei and calls for further advancements in theoretical models incorporating nuclear shadowing and gluon saturation. | ||
| Links: e-print arXiv:2503.08903 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The total differential incoherent $ \mathrm{J}/\psi $ photoproduction cross section versus $ |y| $ (left) and $ y $ (right). The right panel shows the results after disentangling the directional correlation between the produced $ \mathrm{J}/\psi $ and emitted forward neutrons. The small vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. The horizontal bars show the bin widths. Theoretical predictions from the LTA [28,29] are shown by curves. |
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Figure 1-a:
The total differential incoherent $ \mathrm{J}/\psi $ photoproduction cross section versus $ |y| $ (left) and $ y $ (right). The right panel shows the results after disentangling the directional correlation between the produced $ \mathrm{J}/\psi $ and emitted forward neutrons. The small vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. The horizontal bars show the bin widths. Theoretical predictions from the LTA [28,29] are shown by curves. |
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Figure 1-b:
The total differential incoherent $ \mathrm{J}/\psi $ photoproduction cross section versus $ |y| $ (left) and $ y $ (right). The right panel shows the results after disentangling the directional correlation between the produced $ \mathrm{J}/\psi $ and emitted forward neutrons. The small vertical bars and shaded boxes represent the statistical and systematic uncertainties, respectively. The horizontal bars show the bin widths. Theoretical predictions from the LTA [28,29] are shown by curves. |
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Figure 2:
The incoherent $ \mathrm{J}/\psi $ photoproduction cross section per $ \gamma $Pb interaction as a function of $ W_{\gamma\mathrm{N}} $ (lower axis) or $ x $ (upper axis) from the CMS measurement. The ALICE data ($ |y| < $ 0.9) measured at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 2.76 TeV [18] is also displayed. The small vertical bars, shaded and open boxes represent the statistical, experimental, and theoretical (photon flux) uncertainties, respectively. Theoretical predictions from IA and LTA [28,29], and CGC [61,62] models are shown by the curves, where the shaded bands are the theoretical uncertainties. |
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Figure 3:
The nuclear suppression factor $ \mathrm{S^{\mathrm{J}/\psi}} $ of incoherent (from this study and ALICE [18]) and coherent (from CMS [25] and ALICE [26]) $ \mathrm{J}/\psi $ photoproduction as a function of $ x $. The vertical bars and shaded and open boxes represent the statistical, experimental systematic, and theoretical systematic uncertainties, respectively. The prediction (incoherent) from the LTA [28,29] and CGC [61,62] models are shown by curves, where the shaded bands are the theoretical uncertainties. |
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Figure 4:
The ratio between incoherent and coherent $ \mathrm{J}/\psi $ photoproduction cross section as a function of $ W_{\gamma\mathrm{N}} $. The midrapidity data from the STAR [31] and ALICE [18] experiments are also displayed. The vertical bars, the shaded boxes, open boxes, and brackets of the CMS data represent the statistical, experimental, theoretical (photon flux), and total uncertainties, respectively. The vertical bar ALICE data represent the total uncertainties. The STAR error bars are invisible due to the assumption of substantial cancellation of systematic uncertainties. Theoretical predictions from the IA [27,29], LTA [28,29], and CGC [61,62] models are shown by curves, where the shaded bands are the theoretical uncertainties. |
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Figure 5:
The corrected transverse momentum spectra of $ \mu^{+}\mu^{-} $ pairs with a dimuon invariant mass in the $ \mathrm{J}/\psi $ mass window of (2.95 $ < m_{\mu^{+}\mu^{-}} < $ 3.25 GeV) in ``0nXn(Same)" (left) and ``0nXn(Opposite)" (right) events. The results of the fit are shown by the various curves. The vertical bars on the data points represent the statistical uncertainty. |
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Figure 5-a:
The corrected transverse momentum spectra of $ \mu^{+}\mu^{-} $ pairs with a dimuon invariant mass in the $ \mathrm{J}/\psi $ mass window of (2.95 $ < m_{\mu^{+}\mu^{-}} < $ 3.25 GeV) in ``0nXn(Same)" (left) and ``0nXn(Opposite)" (right) events. The results of the fit are shown by the various curves. The vertical bars on the data points represent the statistical uncertainty. |
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Figure 5-b:
The corrected transverse momentum spectra of $ \mu^{+}\mu^{-} $ pairs with a dimuon invariant mass in the $ \mathrm{J}/\psi $ mass window of (2.95 $ < m_{\mu^{+}\mu^{-}} < $ 3.25 GeV) in ``0nXn(Same)" (left) and ``0nXn(Opposite)" (right) events. The results of the fit are shown by the various curves. The vertical bars on the data points represent the statistical uncertainty. |
| Summary |
| In summary, this Letter presents the first energy-dependent measurements of incoherent $ \mathrm{J}/\psi $ photoproduction in ultraperipheral lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The data covers a photon-nucleon energy range of 40-400 GeV, probing gluons $ x $ values between 5.9 $ \times$ 10$^{-3} $ and 6.5 $ \times$ 10$^{-5} $. Compared to baseline predictions without nuclear effects, incoherent $ \mathrm{J}/\psi $ photoproduction cross sections exhibit greater suppression at lower $ x $, and this suppression is more pronounced than that of coherent $ \mathrm{J}/\psi $ production. The observed constant ratio between incoherent and coherent photoproduction cross sections suggests suppression mechanisms disfavoring simple black disk limit scenarios at small $ x $. A model that includes gluon field fluctuations at the subnucleon level does not fully describe the incoherent and coherent cross sections, but captures the general trend of their ratio across the $ W_{\gamma\mathrm{N}} $ range. This study offers important new insights into the evolution of fluctuating gluon fields within nuclei across a wide $ x $ range, indicating the need for advancements in current theoretical models. These measurements also provide valuable input for the planning of future facilities, such as the EIC [64] and LHeC [65], where photoproduction processes will be of great interest. |
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