Abstract
Electromagnetic waves in a dynamical axion background exhibit superluminal group velocities at high frequencies and instabilities at low frequencies, altering how photons propagate through space. Local disturbances propagate causally, but unlike in ordinary Maxwell theory, propagation occurs inside as well as on the lightcone. For the unstable modes, the energy density in the electromagnetic field grows exponentially along timelike displacements. In this paper we derive retarded Green functions in axion electrodynamics in various limits and study the time-domain properties of propagating signals.
Highlights
Axion models provide some of the most well-motivated extensions to the Standard Model, providing a mechanism to resolve the strong CP problem and a class of dark matter candidates
Through its coupling to gluons, the vacuum expectation value of the axion field cancels the θparameter of quantum chromodynamics (QCD) and restores CP symmetry [1,2], explaining the surprisingly small experimentally measured value, jθj < 6 × 10−11 [3,4]
Rather than focusing only on those axion models that provide a natural solution to the strong CP problem, we consider the broader realm of axionlike particles (ALPs), where the axion mass ma and decay constant fa are not required to satisfy mafa ∼ mπfπ, and the value of gaγγ is not determined by ma
Summary
Axion models provide some of the most well-motivated extensions to the Standard Model, providing a mechanism to resolve the strong CP problem and a class of dark matter candidates. Despite the presence of plane wave solutions with superluminal group velocities, axion electrodynamics is a causal theory: local disturbances do not propagate outside the light cone This was first shown long ago in the case of tachyonic scalar field theory by Aharonov, Komar, and Susskind [34]. Rather than focusing only on those axion models that provide a natural solution to the strong CP problem, we consider the broader realm of axionlike particles (ALPs), where the axion mass ma and decay constant fa are not required to satisfy mafa ∼ mπfπ, and the value of gaγγ is not determined by ma These axions can still provide a wide range of dark matter candidates (see, e.g., [35,36,37]). II and III both exhibit the novel inside-the-light-cone propagation and exponential growth in certain modes
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