Axions and Cosmic Rays

TL;DR

This paper explores how a relic axion background could induce Lorentz-violating effects in charged particle propagation, leading to potential photon emission that might serve as an indirect detection method for axions in cosmology.

Contribution

It models the impact of a time-dependent pseudoscalar axion background on charged particles using an extended QED framework with a Chern-Simons term, highlighting potential observable effects.

Findings

Radiation effects are too small to affect cosmic ray propagation significantly.

Photon emission could serve as an indirect axion detection method.

Order of magnitude estimates suggest possible detectability under certain conditions.

Abstract

We investigate the propagation of a charged particle in a spatially constant but time dependent pseudoscalar background. Physically this pseudoscalar background could be provided by a relic axion density. The background leads to an explicit breaking of Lorentz invariance; as a consequence processes such as $p\to p \gamma$ or $e\to e \gamma$ are possible within some kinematical constraints. The phenomenon is described by the QED lagrangian extended with a Chern-Simons term that contains a 4-vector which characterizes the breaking of Lorentz invariance induced by the time-dependent background. While the radiation induced (similar to the Cherenkov effect) is too small to influence the propagation of cosmic rays in a significant way, the hypothetical detection of the photons radiated by high energy cosmic rays via this mechanism would provide an indirect way of verifying the cosmological relevance of axions. We discuss on the order of magnitude of the effect.