Probing the gluon tomography in photoproduction of di-pions

A sizable $\cos 4\varphi$ 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.