The axion shield

Understand

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.