Photon-induced $J/\psi$ as a linearly polarized probe of collision geometry in relativistic heavy-ion collisions

Abstract

In relativistic heavy-ion collisions, the linear polarization of quasi-real photons can correlate the spin orientation of photon-induced vector mesons with the collision geometry. To explore this sensitivity, we present the first measurement of the decay angular distribution of photon-induced $J/\psi$ with respect to the event plane in heavy-ion collisions with hadronic overlap. The analysis uses Ru+Ru and Zr+Zr collisions at a nucleon--nucleon center-of-mass energy of $\sqrt{s_{\mathrm{NN}}}=200$ GeV recorded by STAR. After correcting for the finite event-plane resolution and subtracting the residual hadronic contribution, we extract a negative second-order modulation, $A_2^{\mathrm{phot}}=-0.39\pm 0.11(stat.)\pm 0.04(sys.)$, for $|y^{ee}|<0.8$, $p_{\mathrm{T}}^{ee}<0.2\mathrm{GeV}/c$, and 30--80% centrality. This modulation provides evidence that the $J/\psi$ polarization is correlated with the event plane, as expected for linearly polarized photons. The observed modulation establishes polarized $J/\psi$ photoproduction as a direct probe of the transverse orientation of the initial collision geometry.

arXiv comment7 pages, 3 figures