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

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 $J\mathrm{/}\psi$ 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 $\sqrt{s_{\mathrm{N}\mathrm{N}}}=200$ 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, $A_2^{\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}}=-0.39\pm 0.11(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.04(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{.})$, for $\mathrm{\mid }{y}^{ee}\mathrm{\mid }<0.8$, $p_{\mathrm{T}}^{ee}<0.2\mathrm{G}\mathrm{e}\mathrm{V}\mathrm{/}c$, and 30–80% centrality. This modulation provides evidence that the $J\mathrm{/}\psi$ polarization is correlated with the event plane, as expected for linearly polarized photons. The observed modulation establishes polarized $J\mathrm{/}\psi$ photoproduction as a direct probe of the transverse orientation of the initial collision geometry.