Monodromic transparency of axion domain walls

Abstract: We revisit the study of light interacting with QCD axion domain walls from the perspective of the non-linear axion coupling to photons, $g(a) F \tilde F$, which encodes the effects related to the breaking of the axion shift symmetry including the well-known mixing with meson states. As the axion makes an $\mathcal{O}(1)$ excursion of its fundamental period around strings and domain walls, the standard linear coupling to photons is generally insufficient to accurately describe the interaction of light with the defects, and one needs to consider the full structure of $g(a)$. We take this into account in evaluating the friction experienced by axion domain walls moving in a thermal bath of photons, as well as in deriving the birefringent properties of the walls. This clarifies some results in the literature dealing with a special cancellation that takes place for the QCD axion with the electromagnetic and color anomaly as predicted by minimal Grand Unified Theories.