Charged pions tagged with polarized photons probing strong CP violation in a chiral-imbalance medium

摘要

It is expected that in a hot QCD system, a local parity-odd domain can beproduced due to nonzero chirality, which is induced from the difference ofwinding numbers carried by the gluon topological configuration (QCD sphaleron).This local domain is called the chiral-imbalance medium characterized bynonzero chiral chemical potential, which can be interpreted as the timevariation of the strong CP phase. We find that the chiral chemical potentialgenerates the parity breaking term in the electromagnetic form factor ofcharged pions. Heavy ion collision experiments could observe thephenomenological consequence of this parity-odd form factor through the elasticscattering of a pion and a photon in the medium. Then we quantify the asymmetryrate of the parity violation by measuring the polarization of the photonassociated with the pion, and discuss how it could be measured in a definitelab frame. We roughly estimate the typical size of the asymmetry, just bypicking up the pion resonant process, and find that the signal can besufficiently larger than possible background events from parity-breakingelectroweak process. Our findings might provide a novel possibility to make amanifest detection for the remnant of the strong CP violation.