P- andCP-odd terms in the photon self-energy within a medium

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Abstract
For the propagation of electromagnetic fields within a medium (e.g., a plasma or a heat bath at finite temperature), we write down the most general expression for the vacuum-polarization tensor that is consistent with gauge and Lorentz invariance in four dimensions. The expression contains a term which signals parity and CP violation either in the Lagrangian, or in the background, or both. The effect of this term is to split the otherwise degenerate transverse degrees of polarization of the photon. The physical implications of this term are discussed.