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

TL;DR

This paper proposes a method to detect strong CP violation in a hot QCD medium by observing polarization asymmetries in charged pion and photon scattering, offering a potential experimental signature.

Contribution

It introduces a novel parity-odd electromagnetic form factor for charged pions induced by chiral chemical potential, suggesting a new way to observe strong CP violation.

Findings

Parity violation asymmetry could be detectable in pion-photon scattering.

The estimated signal size exceeds background from electroweak processes.

The approach provides a new experimental avenue to identify strong CP violation.

Abstract

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.