Geometry-induced azimuthal anisotropy in coherent $J/ψ$ photoproduction

TL;DR

This work addresses how azimuthal anisotropies in heavy-ion collisions can originate from the initial-state geometry of nuclear electromagnetic fields rather than hydrodynamic flow. It develops a framework combining light-cone perturbation theory in the dipole picture with the IP-Sat saturation model, leveraging the radial linear polarization of coherently emitted photons to produce a $\cos 2\phi$ modulation in the leptons from $J/\psi\to \ell^+\ell^-$ decays; the modulation is mapped to centrality via an optical Glauber description of $b_\perp$. The authors demonstrate that the calculated asymmetry reproduces STAR measurements at RHIC and provide predictions for Pb+Pb collisions at the LHC, showing the angular signal grows with centrality and saturates in peripheral events. This observable serves as a robust baseline to distinguish initial-state geometric effects from collective medium dynamics and offers insights into the transverse structure of nuclear photons and small-$x$ gluon dynamics.$

Abstract

Azimuthal anisotropies in heavy-ion collisions are conventionally interpreted as signatures of hydrodynamic flow. We demonstrate that in peripheral collisions, a significant $\cos 2φ$ asymmetry in the decay leptons of coherently photoproduced $J/ψ$ mesons arises purely from the initial-state geometry of the nuclear electromagnetic field. This modulation originates from the linear polarization of coherent photons, which is radially aligned in impact parameter space and transferred to the vector meson. By employing light-cone perturbation theory within the dipole formalism, we calculate the centrality dependence of this asymmetry for collisions at RHIC and LHC energies. Our predictions quantitatively reproduce STAR data. This observable thus provides a rigorous benchmark for distinguishing electromagnetic initial-state effects from collective medium dynamics.

Figures (4)

• Figure 1: Schematic illustration of coherent $J/\psi$ photoproduction in peripheral heavy ion collisions. In both panels, $b_{\perp}$ denotes the impact parameter, $x_{\perp}$ marks the transverse production point, and $\epsilon_{V\perp}$ indicates the transverse polarization vector of the $J/\psi$.
• Figure 2: The centrality $c(b_\perp)$ computed using the optical Glauber model is plotted as a function of impact parameter $b_\perp$.
• Figure 3: Centrality dependence of the $\cos 2\phi$ asymmetry in coherent $J/\psi$ production at 200 GeV, compared with STAR measurements STAR:2017enhWang:2023yis. $J/\psi$ is reconstructed via its decay into an $e^+ e^-$ pair.
• Figure 4: Predicted centrality dependence of the $\cos 2\phi$ asymmetry in coherent $J/\psi$ production at 5.36 TeV. $J/\psi$ is reconstructed via its decay into a $\mu^+ \mu^-$ pair. The upper, less-negative edge of the band corresponds to the uniform-disc geometry, whereas the lower, more-negative edge is given by the GBW dipole model.