  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.
