CMS-PAS-HIN-25-006

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Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

CMS Publications