AI 中文总结
CMS 实验在 LHC 5.36 TeV 能下，通过 PbPb 碰撞数据首次观测到相干 J/ψ 光生衰变产物方位角不对称的自旋依赖，为矢量介子自旋相关研究提供直接证据。
AI 中文摘要
我们报告了对 PbPb 强子碰撞中，相干光生 J/ψ 介子衰变产生的μ子相对于二阶事件平面的方位角不对称性的测量结果。该分析基于 CMS 实验在 LHC 上采集的质心系能量为 5.36 TeV 的数据，对应积分亮度为 1.26 nb⁻¹。该不对称性用衰变μ子方位角分布的二阶谐波调制振幅 A₂ 量化。我们首次观测到 A₂ = -0.194 ± 0.022（统计）± 0.018（系统），显著性大于 5 倍标准偏差。A₂ 值未表现出对 J/ψ 介子快度或碰撞离子重叠程度的显著依赖。入射光子的线偏振被相干光生 J/ψ 介子继承的理论模型与数据吻合。结合近期超周边碰撞中相干 ρ⁰ → π⁺π⁻ 光生的测量结果，这些结果为相干光生矢量介子衰变产物方位角不对称性中的自旋依赖提供了直接观测证据。此外，观测到的与事件平面的关联未来可能为反应平面确定提供新方法。
英文摘要
We report a measurement of the azimuthal asymmetry of muons from the decay of coherently photoproduced J/$ψ$ 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 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 J/$ψ$ meson rapidity or the degree of overlap of the colliding ions. A theoretical model in which the coherently photoproduced J/$ψ$ meson inherits the linear polarization of the incident photon is consistent with the data. When considered together with recent measurements of coherent $ρ^0$ $\to$ $π^+π^-$ 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.
CommentsSubmitted to Physical Review Letters. All figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-25-006 (CMS Public Pages)
