Probing the gluon tomography in photoproduction of di-pions

TL;DR

This paper investigates the origin of a significant azimuthal asymmetry in di-pion production during ultraperipheral heavy ion collisions, suggesting it arises from gluon distributions and photon radiation, indicating complex gluon phase correlations.

Contribution

It demonstrates that both gluon Wigner distributions and soft photon radiation contribute to the observed asymmetry, highlighting potential nontrivial gluon phase correlations.

Findings

Gluon Wigner distribution influences azimuthal asymmetry

Soft photon radiation contributes to the effect

QED effects alone underestimate the asymmetry

Abstract

A sizable $\cos 4\phi$ azimuthal asymmetry in exclusive di-pion production near $\rho^0$ resonance peak in ultraperipheral heavy ion collisions recently has been reported by STAR collaboration. We show that both elliptic gluon Wigner distribution and final state soft photon radiation can give rise to this azimuthal asymmetry. The fact that the QED effect alone severely underestimates the observed asymmetry might signal the existence of the nontrivial correlation in quantum phase distribution of gluons.