Evidence of Spin-Interference Effects in Exclusive $J\mathrm{/}\psi \to e^{+}{e}^{-}$ Photoproduction in Ultraperipheral Heavy-Ion Collisions

We report the first evidence of spin interference in exclusive $J\mathrm{/}\psi \to e^{+}{e}^{-}$ photoproduction in ultraperipheral Au+Au and isobar (Ru+Ru, Zr+Zr) collisions ($\sqrt{s_{NN}}=200$~GeV) at STAR. A negative $\cos (2\phi )$ modulation is observed for $p_T<100$ MeV/$c$, opposite in sign to that in ${\rho }^0\text{\hspace{0.17em}⁣}\to \text{\hspace{0.17em}⁣}{\pi }^{+}{\pi }^{-}$ photoproduction. This establishes for the first time that the interference sign is controlled by the spin structure of the final-state daughters, resolving the ambiguity present in the all-boson ${\rho }^0$ channel. The compact $J\mathrm{/}\psi$ probes gluon distributions at perturbative scales, resulting in a weaker modulation and providing stringent constraints on Color Glass Condensate calculations. These findings demonstrate that spin-dependent interference in heavy vector mesons provides a new, experimentally accessible handle on gluon structure beyond traditional cross-section measurements.