Extending the Euler-Heisenberg action to include effects of local Lorentz-symmetry violating backgrounds

TL;DR

This paper extends the Euler-Heisenberg effective action to include Lorentz-symmetry violating backgrounds, revealing new effects on electromagnetic dynamics, energy-momentum conservation, and wave propagation in such modified spacetime scenarios.

Contribution

It provides a second-order correction to the Euler-Heisenberg action incorporating spacetime-dependent Lorentz-violating parameters within the SME framework, including novel axion-like terms and modified Maxwell equations.

Findings

Violation of Furry's theorem at second-order for certain parameters

Emergence of axion-like terms in the effective action

Spacetime-dependent backgrounds cause wave amplification or attenuation

Abstract

This work sets out to compute the corrections to the Euler-Heisenberg effective action that arise from spacetime-dependent background anisotropies that violate Lorentz symmetry. To accomplish our task, we evaluate the functional determinant of the modified Dirac operator using the spectral regularization method. Within the framework of the Standard Model Extension (SME), these Lorentz-violating parameters correspond to the coefficients $m_5(x)$, $a_\mu(x)$, and $b_\mu(x)$. The corrected effective action is attained up to the second-order in the background parameters. Our results indicate a violation of Furry's theorem at second-order for specific CPT-even combinations of these parameters, in agreement with previous analyses based on explicit Feynman diagram calculations in scenarios with Lorentz-symmetry violation. In the kinetic (bilinear) piece of the effective action, non-dynamical axion-like terms emerge, resembling structures commonly encountered in condensed-matter systems, such as Weyl semimetals. We show how this procedure modifies the Maxwell equations, from which we present both the corresponding (non-)conservation of the energy-momentum tensor and the wave equation. We also notice that the vacuum behaves as an inhomogeneous medium, and compute the local dispersion relation by working in the eikonal approximation. As a result of the spacetime-dependence of the background, there appear imaginary contributions in the dispersion relation and, consequently, the wave amplitudes may be amplified or attenuated, which expresses the energy-momentum exchange between the waves and the background.