  It is expected that in a hot QCD system, a local parity-odd domain can be
produced due to nonzero chirality, which is induced from the difference of
winding numbers carried by the gluon topological configuration (QCD sphaleron).
This local domain is called the chiral-imbalance medium characterized by
nonzero chiral chemical potential, which can be interpreted as the time
variation of the strong CP phase. We find that the chiral chemical potential
generates the parity breaking term in the electromagnetic form factor of
charged pions. Heavy ion collision experiments could observe the
phenomenological consequence of this parity-odd form factor through the elastic
scattering of a pion and a photon in the medium. Then we quantify the asymmetry
rate of the parity violation by measuring the polarization of the photon
associated with the pion, and discuss how it could be measured in a definite
lab frame. We roughly estimate the typical size of the asymmetry, just by
picking up the pion resonant process, and find that the signal can be
sufficiently larger than possible background events from parity-breaking
electroweak process. Our findings might provide a novel possibility to make a
manifest detection for the remnant of the strong CP violation.
