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| CMS-HIN-25-006 ; CERN-EP-2026-241 | ||
| Spin-dependent azimuthal asymmetry of coherent $ \mathrm{J}/\psi $ photoproduction in hadronic PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV | ||
| CMS Collaboration | ||
| 3 September 2026 | ||
| Submitted to Physical Review Letters | ||
| Abstract: We report a measurement of the azimuthal asymmetry of muons from the decay of coherently photoproduced $ \mathrm{J}/\psi $ mesons with respect to the second-order event plane in hadronic lead-lead (PbPb) collisions. The analysis is based on data collected by the CMS experiment at the LHC at a nucleon-nucleon center-of-mass energy of 5.36 TeV, corresponding to an integrated luminosity of 1.26$ \text{nb}^{-1}$. The asymmetry is quantified using the amplitude $ A_{2} $ of the second-order harmonic modulation of the decay-muon azimuthal distribution. A negative $ A_{2} = - $ 0.194 $ \pm $ 0.022 (stat) $ \pm $ 0.018 (syst) is observed with a significance greater than five standard deviations for the first time. The $ A_{2} $ values show no significant dependence on the $ \mathrm{J}/\psi $ meson rapidity or the degree of overlap of the colliding ions. A theoretical model in which the coherently photoproduced $ \mathrm{J}/\psi $ meson inherits the linear polarization of the incident photon is consistent with the data. When considered together with recent measurements of coherent ${\rho}{0} \to \pi^{+}\pi^{-} $ photoproduction in ultraperipheral collisions, these results provide a direct observation of spin dependence in the azimuthal asymmetry of decay products from coherently photoproduced vector mesons. In addition, the observed correlation with the event plane may enable a novel approach to reaction plane determination in the future. | ||
| Links: e-print arXiv:2609.04376 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; HepData record ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
The invariant mass distribution of muon pairs (left) and the acceptance- and efficiency-corrected $ \mathrm{J}/\psi $ meson $ p_{\mathrm{T}} $ spectrum (middle), both for 0.4 $ \pi < \Delta\phi < 0.5\pi $, and the coherently photoproduced $ \mathrm{J}/\psi $ meson yield as a function of $ \Delta\phi $ (right) in PbPb collisions at 50--80% centrality and in the $ \mathrm{J}/\psi $ rapidity range 1.6 $ < |y| < $ 2.4. Vertical bars represent statistical uncertainties in all three panels, while horizontal bars in the middle and right panels indicate the bin widths. |
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Figure 1-a:
The invariant mass distribution of muon pairs (left) and the acceptance- and efficiency-corrected $ \mathrm{J}/\psi $ meson $ p_{\mathrm{T}} $ spectrum (middle), both for 0.4 $ \pi < \Delta\phi < 0.5\pi $, and the coherently photoproduced $ \mathrm{J}/\psi $ meson yield as a function of $ \Delta\phi $ (right) in PbPb collisions at 50--80% centrality and in the $ \mathrm{J}/\psi $ rapidity range 1.6 $ < |y| < $ 2.4. Vertical bars represent statistical uncertainties in all three panels, while horizontal bars in the middle and right panels indicate the bin widths. |
png pdf |
Figure 1-b:
The invariant mass distribution of muon pairs (left) and the acceptance- and efficiency-corrected $ \mathrm{J}/\psi $ meson $ p_{\mathrm{T}} $ spectrum (middle), both for 0.4 $ \pi < \Delta\phi < 0.5\pi $, and the coherently photoproduced $ \mathrm{J}/\psi $ meson yield as a function of $ \Delta\phi $ (right) in PbPb collisions at 50--80% centrality and in the $ \mathrm{J}/\psi $ rapidity range 1.6 $ < |y| < $ 2.4. Vertical bars represent statistical uncertainties in all three panels, while horizontal bars in the middle and right panels indicate the bin widths. |
png pdf |
Figure 1-c:
The invariant mass distribution of muon pairs (left) and the acceptance- and efficiency-corrected $ \mathrm{J}/\psi $ meson $ p_{\mathrm{T}} $ spectrum (middle), both for 0.4 $ \pi < \Delta\phi < 0.5\pi $, and the coherently photoproduced $ \mathrm{J}/\psi $ meson yield as a function of $ \Delta\phi $ (right) in PbPb collisions at 50--80% centrality and in the $ \mathrm{J}/\psi $ rapidity range 1.6 $ < |y| < $ 2.4. Vertical bars represent statistical uncertainties in all three panels, while horizontal bars in the middle and right panels indicate the bin widths. |
png pdf |
Figure 2:
The amplitude of the $ \cos(2\Delta\phi) $ modulation, $ A_2 $, of muons from the decays of coherently photoproduced $ \mathrm{J}/\psi $ mesons as a function of collision centrality (left) and $ \mathrm{J}/\psi $ rapidity (right) in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. The error bars represent the statistical uncertainties, and the shaded boxes represent the systematic uncertainties. The black open squares [34] and blue open diamonds [70] represent theoretical predictions that account for the correlation between the vector meson polarization and the event plane. The open boxes around the theoretical predictions indicate their uncertainties. |
png pdf |
Figure 2-a:
The amplitude of the $ \cos(2\Delta\phi) $ modulation, $ A_2 $, of muons from the decays of coherently photoproduced $ \mathrm{J}/\psi $ mesons as a function of collision centrality (left) and $ \mathrm{J}/\psi $ rapidity (right) in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. The error bars represent the statistical uncertainties, and the shaded boxes represent the systematic uncertainties. The black open squares [34] and blue open diamonds [70] represent theoretical predictions that account for the correlation between the vector meson polarization and the event plane. The open boxes around the theoretical predictions indicate their uncertainties. |
png pdf |
Figure 2-b:
The amplitude of the $ \cos(2\Delta\phi) $ modulation, $ A_2 $, of muons from the decays of coherently photoproduced $ \mathrm{J}/\psi $ mesons as a function of collision centrality (left) and $ \mathrm{J}/\psi $ rapidity (right) in PbPb collisions at $ \sqrt{\smash[b]{s_{_{\mathrm{NN}}}}} = $ 5.36 TeV. The error bars represent the statistical uncertainties, and the shaded boxes represent the systematic uncertainties. The black open squares [34] and blue open diamonds [70] represent theoretical predictions that account for the correlation between the vector meson polarization and the event plane. The open boxes around the theoretical predictions indicate their uncertainties. |
| Summary |
| In summary, we present a measurement of the azimuthal asymmetry of muons from the decay of coherently photoproduced$ \mathrm{J}/\psi $ vector mesons with respect to the second-order event plane in hadronic lead-lead collisions at a nucleon-nucleon center-of-mass energy of 5.36 TeV. The asymmetry amplitude $A_{2}$ is found to be negative and shows no significant dependence on$ \mathrm{J}/\psi $ rapidity or collision centrality. These observations are consistent with theoretical expectations linking the linear polarization of the$ \mathrm{J}/\psi $ to the reaction plane. Therefore, it may be feasible to use the intrinsic polarization of coherently photoproduced vector mesons as a complementary reaction-plane estimator in heavy ion collisions. Such an approach could be particularly advantageous in small systems, such as proton-nucleus collisions. Together with previously measured positive amplitudes for spin-0 pion final states from coherently photoproduced ${\rho}{0} $ mesons, these results provide the first observation of spin-state dependence in azimuthal asymmetries. |
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Compact Muon Solenoid LHC, CERN |
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