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

Abstract

We report the first evidence of spin interference in exclusive $J/\psi \to e^+e^-$ photoproduction in ultraperipheral heavy-ion collisions at STAR at $\sqrt{s_{NN}} = 200$~GeV. In Au+Au collisions, a negative $\cos(2\phi)$ modulation is found for $p_T < 120$~MeV/$c$ with a significance of $3.2\sigma$, while the isobar data (Ru+Ru, Zr+Zr) show a consistent negative modulation with a significance of $1.9\sigma$, opposite in sign to that in $\rho^{0}\!\to\!\pi^+\pi^-$ 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 $J/\psi$ 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.