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| CMS-PAS-HIN-25-006 | ||
| Spin-dependent azimuthal asymmetry of coherent $ \mathrm{J}/\psi $ photoproduction in peripheral PbPb collisions at $ \sqrt{s_{_{NN}}} = $ 5.36 TeV | ||
| CMS Collaboration | ||
| 2026-03-19 | ||
| Abstract: We report on the measurement of azimuthal asymmetry in muons from the decay of coherently photoproduced $ \mathrm{J}/\psi $ vector mesons in peripheral 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 $ \mathrm{nb}^{-1} $. The asymmetry is quantified from the coherent $ \mathrm{J}/\psi $ yield as a function of the azimuthal angle difference between the decay muons and the second-order event plane. The amplitude of the $ \cos 2\Delta\phi $ modulation, $ A_{2} $, is measured as a function of collision centrality in the rapidity range 1.6 $ < |y| < $ 2.4, and as a function of rapidity in the 50-80% centrality class. For 50-80% centrality and 1.6 $ < |y| < $ 2.4, $ A_{2} = - $ 0.194 $ \pm $ 0.022 (stat) $ \pm $ 0.018 (syst), observed to be negative for the first time. The $ A_{2} $ values show no clear dependence on rapidity or centrality and are consistent with theoretical expectations, which attribute the modulation to the linear polarization of photons transferred to the coherently produced $ \mathrm{J}/\psi $. These results provide the first direct test of the spin-state dependence of azimuthal asymmetry in coherent vector meson photoproduction and demonstrate a strong correlation between the decay products and the event plane, suggesting a novel method for reaction-plane determination. | ||
| Links: CDS record (PDF) ; CADI line (restricted) ; | ||
| Figures | |
png pdf |
Figure 1:
The invariant mass distribution of muon pairs (left), the efficiency-corrected $ \mathrm{J}/\psi p_{\mathrm{T}} $ spectrum for 0.4 $ \pi < \Delta\phi < 0.5\pi $ (middle), and the coherent $ \mathrm{J}/\psi $ yield as a function of $ \Delta\phi $ (right) in PbPb collisions at 50-80% centrality and 1.6 $ < |y| < $ 2.4. The invariant mass distribution is fitted with a double-sided Crystal Ball function plus a third-order Chebyshev polynomial. The $ p_{\mathrm{T}} $ spectrum is fitted with Eq. 4 for $ p_{\mathrm{T}} > $ 1.5 GeV to estimate the hadronic and partial incoherent contaminations for $ p_{\mathrm{T}} < $ 0.2 GeV. The region $ p_{\mathrm{T}} < $ 0.2 GeV is chosen where coherent $ \mathrm{J}/\psi $ production is known to dominate, following the same selection used in previous CMS analyses [12,20]; the pronounced low-$ p_{\mathrm{T}} $ excess observed in the data is consistent with this expectation. The right panel shows the efficiency-corrected coherent $ \mathrm{J}/\psi $ yields as a function of $ \Delta\phi $, prior to correcting for event-plane resolution. Vertical bars represent statistical uncertainties in all three panels. |
png pdf |
Figure 1-a:
The invariant mass distribution of muon pairs (left), the efficiency-corrected $ \mathrm{J}/\psi p_{\mathrm{T}} $ spectrum for 0.4 $ \pi < \Delta\phi < 0.5\pi $ (middle), and the coherent $ \mathrm{J}/\psi $ yield as a function of $ \Delta\phi $ (right) in PbPb collisions at 50-80% centrality and 1.6 $ < |y| < $ 2.4. The invariant mass distribution is fitted with a double-sided Crystal Ball function plus a third-order Chebyshev polynomial. The $ p_{\mathrm{T}} $ spectrum is fitted with Eq. 4 for $ p_{\mathrm{T}} > $ 1.5 GeV to estimate the hadronic and partial incoherent contaminations for $ p_{\mathrm{T}} < $ 0.2 GeV. The region $ p_{\mathrm{T}} < $ 0.2 GeV is chosen where coherent $ \mathrm{J}/\psi $ production is known to dominate, following the same selection used in previous CMS analyses [12,20]; the pronounced low-$ p_{\mathrm{T}} $ excess observed in the data is consistent with this expectation. The right panel shows the efficiency-corrected coherent $ \mathrm{J}/\psi $ yields as a function of $ \Delta\phi $, prior to correcting for event-plane resolution. Vertical bars represent statistical uncertainties in all three panels. |
png pdf |
Figure 1-b:
The invariant mass distribution of muon pairs (left), the efficiency-corrected $ \mathrm{J}/\psi p_{\mathrm{T}} $ spectrum for 0.4 $ \pi < \Delta\phi < 0.5\pi $ (middle), and the coherent $ \mathrm{J}/\psi $ yield as a function of $ \Delta\phi $ (right) in PbPb collisions at 50-80% centrality and 1.6 $ < |y| < $ 2.4. The invariant mass distribution is fitted with a double-sided Crystal Ball function plus a third-order Chebyshev polynomial. The $ p_{\mathrm{T}} $ spectrum is fitted with Eq. 4 for $ p_{\mathrm{T}} > $ 1.5 GeV to estimate the hadronic and partial incoherent contaminations for $ p_{\mathrm{T}} < $ 0.2 GeV. The region $ p_{\mathrm{T}} < $ 0.2 GeV is chosen where coherent $ \mathrm{J}/\psi $ production is known to dominate, following the same selection used in previous CMS analyses [12,20]; the pronounced low-$ p_{\mathrm{T}} $ excess observed in the data is consistent with this expectation. The right panel shows the efficiency-corrected coherent $ \mathrm{J}/\psi $ yields as a function of $ \Delta\phi $, prior to correcting for event-plane resolution. Vertical bars represent statistical uncertainties in all three panels. |
png pdf |
Figure 1-c:
The invariant mass distribution of muon pairs (left), the efficiency-corrected $ \mathrm{J}/\psi p_{\mathrm{T}} $ spectrum for 0.4 $ \pi < \Delta\phi < 0.5\pi $ (middle), and the coherent $ \mathrm{J}/\psi $ yield as a function of $ \Delta\phi $ (right) in PbPb collisions at 50-80% centrality and 1.6 $ < |y| < $ 2.4. The invariant mass distribution is fitted with a double-sided Crystal Ball function plus a third-order Chebyshev polynomial. The $ p_{\mathrm{T}} $ spectrum is fitted with Eq. 4 for $ p_{\mathrm{T}} > $ 1.5 GeV to estimate the hadronic and partial incoherent contaminations for $ p_{\mathrm{T}} < $ 0.2 GeV. The region $ p_{\mathrm{T}} < $ 0.2 GeV is chosen where coherent $ \mathrm{J}/\psi $ production is known to dominate, following the same selection used in previous CMS analyses [12,20]; the pronounced low-$ p_{\mathrm{T}} $ excess observed in the data is consistent with this expectation. The right panel shows the efficiency-corrected coherent $ \mathrm{J}/\psi $ yields as a function of $ \Delta\phi $, prior to correcting for event-plane resolution. Vertical bars represent statistical uncertainties in all three panels. |
png pdf |
Figure 2:
The amplitude of the $ \cos2\Delta\phi $ modulation, $ A_2 $, of muons decayed from coherently photoproduced $ \mathrm{J}/\psi $ decays as a function of centrality (left) and rapidity (right) in PbPb collisions at $ \sqrt{s_{_{NN}}} = $ 5.36 TeV. The error bars represent the statistical uncertainties, and the shaded boxes represent the systematic uncertainties, respectively. The black open squares [32] and blue open diamonds [52] represent theoretical predictions considering the correlation between the vector meson polarization and the event plane. |
png pdf |
Figure 2-a:
The amplitude of the $ \cos2\Delta\phi $ modulation, $ A_2 $, of muons decayed from coherently photoproduced $ \mathrm{J}/\psi $ decays as a function of centrality (left) and rapidity (right) in PbPb collisions at $ \sqrt{s_{_{NN}}} = $ 5.36 TeV. The error bars represent the statistical uncertainties, and the shaded boxes represent the systematic uncertainties, respectively. The black open squares [32] and blue open diamonds [52] represent theoretical predictions considering the correlation between the vector meson polarization and the event plane. |
png pdf |
Figure 2-b:
The amplitude of the $ \cos2\Delta\phi $ modulation, $ A_2 $, of muons decayed from coherently photoproduced $ \mathrm{J}/\psi $ decays as a function of centrality (left) and rapidity (right) in PbPb collisions at $ \sqrt{s_{_{NN}}} = $ 5.36 TeV. The error bars represent the statistical uncertainties, and the shaded boxes represent the systematic uncertainties, respectively. The black open squares [32] and blue open diamonds [52] represent theoretical predictions considering the correlation between the vector meson polarization and the event plane. |
| Summary |
| In summary, we present the first measurement of azimuthal asymmetry in muons from coherently photoproduced $ \mathrm{J}/\psi $ vector mesons with respect to the second-order event plane in peripheral PbPb collisions at nucleon-nucleon center-of-mass energy of 5.36 TeV. The $\cos 2\Delta\phi$ coefficient $A_{2}$ is found to be negative for the first time and shows no significant dependence on $ \mathrm{J}/\psi $ rapidity or collision centrality, consistent with theoretical expectations linking the linear polarization of the $ \mathrm{J}/\psi $ to the reaction plane. Compared to previously measured positive $A_{2}$ values for spin-0 final states (pions from coherent $\rho^0$), these results provide a direct test of the spin-state dependence of azimuthal asymmetries and demonstrate feasibility of a complementary probe of the reaction plane through the intrinsic polarization of coherently photoproduced vector mesons. Furthermore, they demonstrate the potential of using such an intrinsic polarization as a complementary reaction-plane estimator in heavy-ion collisions, which may offer improved performance in small systems such as proton-nucleus collisions where event-by-event fluctuations and nonflow contributions are substantial. |
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