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 $\cos (2\varphi )$ 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.

Comments: Accepted for publication in Phys. Rev. Lett