Bubble Nucleation with Chern-Simons Interaction

We study axion electrodynamics, including the Chern-Simons interaction term, in the presence of parallel background electric and magnetic fields. In this setup, we find a new back-reacted instanton solution which corresponds to the nucleation of an axion domain wall that screens the electromagnetic fields in a process analogous to Schwinger pair production, despite the absence of light charged particles. The full solution includes the effect of the Chern-Simons induced charges and currents on the axion domain wall arising from the Witten and Sikivie (anomalous Hall) effects, respectively. The Euclidean solution has a reduced O(2)×O(2) symmetry which describes the nucleation of a prolate bubble in its rest-frame. A unique feature of this solution is that the region of lower energy density is outside the bubble rather than inside.