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Evidence of Spin-Interference Effects in Exclusive J/ψ→e^{+}e^{-} Photoproduction in Ultraperipheral Heavy-Ion
B E Aboona1, J Adam2, L Adamczyk3
1Texas A&M University, College Station, Texas 77843.
None:
We report the first evidence of spin interference in exclusive J/ψ→e^{+}e^{-} photoproduction in ultraperipheral heavy-ion collisions at STAR at sqrt[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 ρ^{0}→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 ρ^{0} 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.
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