Abstract
Here, we report the first evidence of spin interference in exclusive 𝐽/𝜓 → 𝑒+𝑒− photoproduction in ultraperipheral heavy-ion collisions at STAR at √?̅?̅_̅(̅?̅?̅̅?̅?̅)̅ = 200 GeV. In Au + Au collisions, a negative cos(2𝜙) modulation is found for 𝑝𝑇 < 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.