The axion shield

TL;DR

This paper explores how a relic axion background can break Lorentz invariance and shield cosmic rays by enabling photon emission from high-energy protons, offering a potential indirect detection method for axions.

Contribution

It provides an exact, model-independent analysis of how a time-dependent pseudoscalar background affects charged particle propagation and cosmic ray shielding.

Findings

The axion background enables p-> p gamma processes.

It acts as a shield against ultra-high-energy cosmic rays.

The effect is exact and model independent.

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 the process p-> p gamma is possible and the background acts as a shield against extremely energetic cosmic rays, an effect somewhat similar to the GZK cut-off effect. The effect is model independent and can be computed exactly. The hypothetical detection of the photons radiated via this mechanism would provide an indirect way of verifying the cosmological relevance of axions.