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

TL;DR

This paper presents the first experimental evidence of spin interference effects in exclusive $J/mbda o e^+e^-$ photoproduction during ultraperipheral heavy-ion collisions, revealing spin-dependent interference patterns that inform gluon structure studies.

Contribution

It demonstrates for the first time that spin interference sign depends on the final-state spin structure, resolving previous ambiguities and offering new insights into gluon distributions.

Findings

Negative $\,\cos(2\phi)$ modulation observed at low $p_T$

Interference sign controlled by final-state spin structure

Provides constraints on Color Glass Condensate models

Abstract

We report the first evidence of spin interference in exclusive $J/\psi \to e^+e^-$ 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 $\rho^{0}\!\to\!\pi^+\pi^-$ 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.