Azimuthal asymmetry in $J/\psi+\gamma$ and $J/\psi+J/\psi$ production in ultraperipheral heavy-ion collisions at LHC

TL;DR

This paper predicts azimuthal asymmetries in heavy quarkonium production during ultraperipheral heavy-ion collisions at the LHC, providing a new way to test QCD and photon polarization effects.

Contribution

It introduces the first investigation of azimuthal asymmetries in $J/\psi+\gamma$ and $J/\psi+J/\psi$ production in UPCs using NRQCD and TMD photon distributions.

Findings

Predicts sizable $\,\cos(2\phi)$ and $\cos(4\phi)$ asymmetries.

Asymmetries are stable against higher-order corrections.

Provides a new test for quarkonium production and photon TMD structure.

Abstract

Two-photon collision in ultraperipheral heavy-ion collisions (UPCs) provides a unique and powerful platform for probing QCD with linearly polarized quasi-real photons. While photon polarization effects have been recognized in dilepton and even in light hadrons production, their consequences for heavy quarkonium production remain unexplored. In this work we investigate for the first time the $\gamma\gamma \to J/\psi+\gamma(J/\psi)$ channels in Pb-Pb UPCs at the Large Hadron Collider (LHC), by integrating the non-relativistic QCD (NRQCD) factorization approach with the transverse-momentum-dependent (TMD) photon distributions. Based on the helicity amplitudes at lowest order in strong coupling and velocity expansion, we predict sizable $\cos(2\phi)$ and $\cos(4\phi)$ azimuthal asymmetries arising from the interference of linearly polarized photon states. These azimuthal-dependent observables, defined as the ratios of weighted to unweighted cross sections, are expected to be stable against including the higher-order radiative corrections and varying nonperturbative NRQCD matrix elements, thus offering a fresh test of quarkonium production mechanism and the photon TMD structure in the ultrarelativistic limit.