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

Abstract

We report a measurement of the azimuthal asymmetry of muons from the decay of coherently photoproduced 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 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/$\psi$ meson rapidity or the degree of overlap of the colliding ions. A theoretical model in which the coherently photoproduced 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.

Submitted 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)