Photon quantum kinetic equations and collective modes in an axion background

We develop a quantum kinetic theory for photons in the presence of an axion background and in the collisioness limit. In deriving the classical regime of our quantum kinetic equations, we observe that they capture well-known features of axion electrodynamics. By projecting the Wigner function onto a polarization basis, relating the Wigner matrix function with the Stokes parameters, we establish the dispersion relations and transport equations for each polarization space component. Additionally, we investigate how the axion background affects the dispersion relations of photon collective modes within an electron-positron plasma at equilibrium temperature <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>T</a:mi></a:math>. While the plasmon remains unaffected, we find that the axion background breaks the degeneracy of transverse collective modes at order <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>e</c:mi><c:msub><c:mi>g</c:mi><c:mrow><c:mi>a</c:mi><c:mi>γ</c:mi></c:mrow></c:msub><c:mi>T</c:mi><c:mo stretchy="false">(</c:mo><c:mo>∂</c:mo><c:mi>a</c:mi><c:mo stretchy="false">)</c:mo></c:math>, where <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>e</g:mi></g:math> represents the electron charge, <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:msub><i:mi>g</i:mi><i:mrow><i:mi>a</i:mi><i:mi>γ</i:mi></i:mrow></i:msub></i:math> denotes the photon-axion coupling, and <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mo>∂</k:mo><k:mi>a</k:mi></k:math> represents the scale associated with variations in the axion field. Published by the American Physical Society 2024