Speaker
Description
Linearly polarized quasireal photons from relativistic heavy ions can transfer their polarization to coherently photoproduced vector mesons, producing a second-harmonic modulation in their decay angular distributions. A positive modulation has been observed for spin-0 pions from coherent $\rho^{0}$ decays, using the $\rho^{0}$ transverse momentum direction as the polarization reference, while a negative one is predicted for spin-$\frac{1}{2}$ leptons from $J/\psi$ decays. Since the correlation between the meson transverse momentum and polarization directions induced by two-source interference weakens at forward rapidity or in asymmetric systems, and the reconstructed direction of a low-$p_{\mathrm{T}}$ coherent $J/\psi$ is sensitive to detector resolution and final-state radiation, the second-order event plane, which approximates the impact-parameter direction, provides a more robust polarization reference.
In this talk, we present the first observation of the $\cos(2\Delta\phi)$ modulation of positive muons from coherently photoproduced $J/\psi\to\mu^{+}\mu^{-}$ decays relative to the second-order event plane, including its centrality and rapidity dependence. The measurement directly tests the predicted spin-state dependence, evaluates whether decay product directions from coherently photoproduced vector mesons can provide a complementary estimate of the reaction-plane orientation, particularly in small systems affected by nonflow and geometry fluctuations, and can constrain models of coherent photonuclear production in hadronic collisions.
