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2026 article

Spin-dependent azimuthal asymmetry of coherent J/$ψ$ photoproduction in hadronic PbPb collisions at $\sqrt{s_\mathrm{NN}}$ = 5.36 TeV

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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.

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Les sujets associés

High-Energy Particle Collisions ResearchQuantum Chromodynamics and Particle InteractionsParticle physics theoretical and experimental studies

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