Abstract
We discuss spectral distortions, time delays and refraction of light in an axion or axion-plasma background. This involves solving the full set of geodesic equations associated to the system of Hamiltonian optics, allowing us to self-consistently take into account the evolution of the momentum, frequency and position of photons. We support our arguments with analytic approximations and full numerical solutions. Remarkably, the introduction of a plasma enhances the sensitivity to axion-induced birefringence, allowing these effects to occur at linear order in the axion-photon coupling even when the axion background is not present at either the emission or detection points. This suggests a general enhancement of axion-induced birefringence when the background refractive index is different from one.
Highlights
Axions remain promising candidates for beyond the Standard Model physics
We have examined the dispersive properties of axion backgrounds which lead to spectral distortions, time delays, and refraction of crossing light
We studied these effects by solving the geodesic equations of Hamiltonian optics, deriving both numerical and analytic results
Summary
Axions remain promising candidates for beyond the Standard Model physics. The QCD axion emerges as a solution to the strong-CP problem [1,2,3], while string theory extensions of the Standard Model predicate a plethora of axionlike particles [4,5]. As the axion background is both time and space dependent, frequency and momentum are no longer conserved quantities, and a proper discussion of dispersion must self-consistently take into account variations of these and the position along the ray path This can be achieved by deriving a system of Hamiltonian optics equations [32] whose solutions give the photon geodesics and capture all. The necessary details about group dispersion, frequency and momentum shifts, time delays, and refraction These equations allow one to study general axion backgrounds without the need to assume any hierarchy between temporal and spatial gradients as would happen for nonrelativistic backgrounds. We demonstrate polarization-dependent group dispersion at first order in gaγγ for background refractive indices different from 1, n0 ≠ 1, provided here by plasma We show that this leads to polarization-dependent time delays at OðgaγγÞ. We have used natural Lorentz-Heaviside units c; ε0; μ0 1⁄4 1 in this work
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