Azimuthal asymmetry in $J/ψ+γ$ and $J/ψ+J/ψ$ production in ultraperipheral heavy-ion collisions at LHC

TL;DR

The paper addresses azimuthal asymmetries in exclusive $\gamma\gamma\to J/\psi+\gamma(J/\psi)$ production in ultraperipheral Pb-Pb collisions by merging QED TMD factorization with NRQCD. Using LO NRQCD helicity amplitudes and a convolution over photon TMDs, it shows that linearly polarized photons produce $\cos(2\phi)$ and $\cos(4\phi)$ modulations through interference of helicity states, an effect missed by the EPA. The study predicts sizable asymmetries under LHC kinematics (e.g., $|\langle \cos(2\phi) \rangle| \lesssim 0.13$ for $J/\psi+\gamma$ near threshold) and provides integrated cross sections of about $43.4$ nb for $J/\psi+\gamma$ and $13.4$ nb for $J/\psi+J/\psi$ at $\sqrt{s_{\rm NN}}=5.02$ TeV. These observables are argued to be robust against higher-order corrections and LDME uncertainties, offering a novel probe of quarkonium production mechanisms and photon TMD structure in the ultrarelativistic limit.

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 $γγ\to J/ψ+γ(J/ψ)$ 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 $\cos(2φ)$ and $\cos(4φ)$ 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.

Figures (4)

• Figure 1: Representative lowest-order diagrams for $\gamma\gamma \to J/\psi + \gamma$ (left) and $\gamma\gamma \to J/\psi + J/\psi$ (right).
• Figure 2: The spectra of the invariant mass of $J/\psi+\gamma(J/\psi)$ for exclusive $J/\psi + \gamma$ and $J/\psi + J/\psi$ production in Pb+Pb UPCs at $\sqrt{s_{\rm NN}} = 5.02$ TeV, under the cuts $1.6 < |y_{1,2}| < 2.4$, $P_\perp > 200$ MeV, with $q_\perp$ integrated up to 200 MeV.
• Figure 3: The azimuthal distribution $d\sigma / d\phi$ for $J/\psi + \gamma(J/\psi)$ production in Pb+Pb UPCs at $\sqrt{s_{\rm NN}} = 5.02$ TeV, integrated over $Q$ from threshold to 20 GeV and $q_\perp$ from 0 to 200 MeV, with cuts $1.6 < |y_{1,2}| < 2.4$ and $P_\perp > 200$ MeV.
• Figure 4: The azimuthal asymmetry observables $\langle \cos(2\phi) \rangle$ and $\langle \cos(4\phi) \rangle$, as a function of the invariant mass $Q$ for $J/\psi + \gamma$ and $J/\psi + J/\psi$ production in Pb+Pb UPCs at $\sqrt{s_{\rm NN}} = 5.02$ TeV, under the kinematic constraints $1.6 < |y_{1,2}| < 2.4$, $Q < 20$ GeV and $P_\perp > 200$ MeV.