Speaker
Description
Heavy vector-meson photoproduction is a premier tool for imaging nuclear gluon distributions, traditionally via the momentum-transfer ($t$) dependence of exclusive cross sections. We show that in ultra-peripheral heavy-ion collisions (UPCs) the $J/\psi$ spin/decay angular structure provides a complementary handle for gluon tomography, enabled by the UPC ``two-source'' interferometer and linearly polarized photons.
We present the first evidence of spin-dependent interference in exclusive $J/\psi \to e^+e^-$ photoproduction in ultraperipheral Au+Au ($\sqrt{s_{NN}}=200$~GeV) and isobar (Ru+Ru, Zr+Zr, $\sqrt{s_{NN}}=200$~GeV) collisions at the STAR experiment. A significant negative $\cos(2\phi)$ modulation is observed for the $J/\psi$ at $p_T < 100$~MeV/$c$ with a combined significance exceeding $3\sigma$. Crucially, this sign is opposite to the positive modulation previously observed in $\rho^0 \to \pi^+\pi^-$ photoproduction. This result definitively resolves the ambiguity regarding the interference mechanism, demonstrating that the modulation sign is driven by the specific spin properties of the decay daughters (spin-1/2 fermions vs. spin-0 bosons).
Unlike the soft $\rho^0$ meson, the heavy $J/\psi$ mass scale ($M_{J/\psi} \gg \Lambda_{\mathrm{QCD}}$) allows us to probe the gluon spatial distribution at perturbative scales. We compare the measured modulation strength to Color Glass Condensate (CGC) calculations, showing that the data favor models incorporating linear photon polarization while ruling out scenarios dominated by soft-photon radiation. These findings establish $J/\psi$ spin interference as a novel, hard-scale probe of nuclear gluon structure, providing a critical baseline for future measurements at the EIC.
[1] STAR Collaboration, ``Evidence of Spin-Interference Effects in Exclusive $J/\psi \rightarrow e^+e^-$ Photoproduction in Ultraperipheral Heavy-Ion Collisions,'' arXiv:2512.02865 [nucl-ex]
| Speaker confirmation | Yes |
|---|