Evidence of Spin-Interference Effects in Exclusive J/ψ →e⁺e^- Photoproduction in Ultraperipheral Heavy-Ion Collisions

We report the first evidence of spin interference in exclusive J/ψ→e⁺e^- photoproduction in ultraperipheral heavy-ion collisions at STAR at √s_NN=200 GeV. In Au+Au collisions, a negative cos(2φ) modulation is found for p_T<120 MeV/c with a significance of 3.2σ, while the isobar data (Ru+Ru, Zr+Zr) show a consistent negative modulation with a significance of 1.9σ, opposite in sign to that in ρ⁰→π⁺π^- 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 ρ⁰ channel. The compact J/ψ 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.