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| CMS-HIN-23-009 ; CERN-EP-2025-030 | ||
| Small-x evolution of gluon fields from incoherent J/ψ photoproduction in ultraperipheral PbPb collisions | ||
| CMS Collaboration | ||
| 12 March 2025 | ||
| Submitted to Phys. Rev. Lett. | ||
| Abstract: Incoherent J/ψ photoproduction in heavy ion ultraperipheral collisions (UPCs), in which the photon interacts with localized, fluctuating gluonic hotspots rather than the entire nucleus, provides a unique probe of those fluctuations. This study presents the first measurement of the dependence of this photoproduction yield on the photon-nucleon center-of-mass energy (WγN), using PbPb UPCs at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The data corresponds to an integrated luminosity of 1.52 nb−1, recorded by the CMS experiment. The measurement covers a wide WγN range of ≈40-400 GeV, probing gluons carrying a fraction of nucleon momentum x in the range between 5.9 × 10−3 and 6.5 × 10−5. The measured incoherent J/ψ photoproduction cross section is suppressed relative to theoretical predictions without nuclear effects. However, the ratio of incoherent to coherent photoproduction remains constant across the probed WγN and x range. Together, these results pose significant challenges to current theoretical models which include gluon saturation or nuclear shadowing effects. | ||
| Links: e-print arXiv:2503.08903 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The total differential incoherent J/ψ meson photoproduction cross section versus |y| (left) and y (right) in PbPb UPCs at √sNN= 5.02 TeV. The right panel shows the results after disentangling the directional correlation between the produced J/ψ meson and emitted forward neutrons. The values at −y and +y correspond to cases where J/ψ and neutrons have the same and opposite rapidity signs, respectively. Data from the ALICE experiment [9] is also displayed. 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 [61,62] are shown by curves. |
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Figure 1-a:
The total differential incoherent J/ψ meson photoproduction cross section versus |y| (left) and y (right) in PbPb UPCs at √sNN= 5.02 TeV. The right panel shows the results after disentangling the directional correlation between the produced J/ψ meson and emitted forward neutrons. The values at −y and +y correspond to cases where J/ψ and neutrons have the same and opposite rapidity signs, respectively. Data from the ALICE experiment [9] is also displayed. 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 [61,62] are shown by curves. |
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Figure 1-b:
The total differential incoherent J/ψ meson photoproduction cross section versus |y| (left) and y (right) in PbPb UPCs at √sNN= 5.02 TeV. The right panel shows the results after disentangling the directional correlation between the produced J/ψ meson and emitted forward neutrons. The values at −y and +y correspond to cases where J/ψ and neutrons have the same and opposite rapidity signs, respectively. Data from the ALICE experiment [9] is also displayed. 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 [61,62] are shown by curves. |
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Figure 2:
The incoherent J/ψ meson photoproduction cross section per γPb interaction as a function of WγN (lower axis) or x (upper axis) from the CMS measurement. The ALICE data (|y|< 0.9) measured at √sNN= 2.76 TeV [21] is also displayed. The small vertical bars, the shaded and open boxes represent the statistical, experimental, and theoretical (photon flux) uncertainties, respectively. Theoretical predictions from IA and LTA [61,62], and CGC [63,64] models are shown by the curves, where the shaded bands are the theoretical uncertainties. |
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Figure 3:
The nuclear suppression factor SJ/ψ of incoherent J/ψ meson photoproduction as a function of x. The ALICE mid-rapidity (|y|< 0.9) data [21] is also displayed. The SJ/ψ of coherent J/ψ meson photoproduction from CMS [28] and ALICE [29] experiments are also displayed. The vertical bars and shaded and open boxes represent the statistical, experimental systematic, and theoretical systematic uncertainties, respectively. The prediction of SJ/ψ (incoherent) from the LTA [61,62] and CGC [63,64] 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 J/ψ meson photoproduction cross section as a function of WγN. The midrapidity data in AuAu UPCs at √sNN= 200 GeV from the STAR experiment [31] and in PbPb UPCs at √sNN= 2.76 TeV from the ALICE experiment [21] 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 not visible due to the assumption of substantial cancellation of systematic uncertainties. Theoretical predictions from the IA [30,62], LTA [61,62], and CGC [63,64] 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 μ+μ− pairs with a dimuon invariant mass in the J/ψ mass window of (2.95 <mμ+μ−< 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 μ+μ− pairs with a dimuon invariant mass in the J/ψ mass window of (2.95 <mμ+μ−< 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. |
png pdf |
Figure 5-b:
The corrected transverse momentum spectra of μ+μ− pairs with a dimuon invariant mass in the J/ψ mass window of (2.95 <mμ+μ−< 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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