  Contemporary attempts to explain the existence of ultra-high energy cosmic
rays using plasma-based wakefield acceleration deliberately avoid non-Standard
Model particle physics. However, such proposals exploit some of the most
extreme environments in the Universe and it is conceivable that hypothetical
particles outside the Standard Model have significant implications for the
effectiveness of the acceleration process. Axions solve the strong CP problem
and provide one of the most important candidates for Cold Dark Matter, and
their potential significance in the present context should not be overlooked.
Our analysis of the field equations describing a plasma augmented with axions
uncovers a dramatic axion-induced suppression of the energy gained by a test
particle in the wakefield driven by a particle bunch, or an intense pulse of
electromagnetic radiation, propagating at ultra-relativistic speeds within the
strongest magnetic fields in the Universe.
