Axion assisted Schwinger effect
  We point out an enhancement of the pair production rate of charged fermions
in a strong electric field in the presence of time dependent classical
axion-like background field, which we call axion assisted Schwinger effect.
While the standard Schwinger production rate is proportional to
$\exp(-\pi(m^2+p_T^2 )/E)$, with $m$ and $p_T$ denoting the fermion mass and
its momentum transverse to the electric field $E$, the axion assisted Schwinger
effect can be enhanced at large momenta to $\exp(-\pi m^2/E)$. The origin of
this enhancement is a coupling between the fermion spin and its momentum,
induced by the axion velocity. As a non-trivial validation of our result, we
show its invariance under field redefinitions associated with a chiral rotation
and successfully reproduce the chiral anomaly equation in the presence of
helical electric and magnetic fields. We comment on implications of this result
for axion cosmology, focussing on axion inflation and axion dark matter
detection.
