Abstract
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 left(-pi left({m}^2+{p}_T^2right)/Eright) , with m and pT 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(−πm2/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.
Highlights
While the standard Schwinger production rate is proportional to exp(−π(m2 + p2T )/E), with m and pT 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(−πm2/E)
The fermion production and the resulting induced current lead to the formation of electric and magnetic fields anti-aligned to the background fields, and to a reduction of the net gauge field background generated in axion inflation by several orders of magnitude
We find that the Schwinger production rate is exponentially enhanced when the axion velocity is sufficiently large (1.3) and the transverse momentum of the produced particle is non-zero, which we dub axion assisted Schwinger effect
Summary
Where the magnetic field B is constant and as before the electric field is given by −Az. Here, our starting point will be the most general action coupling axions, fermions and gauge bosons through dimension five operators, while preserving the shift-symmetry of the axion and CP -invariance, S=. For the higher Landau levels, with u0, v0 and un,λ, vn,λ denoting the eigenfunctions of the fermionic part of the Hamiltonian, see appendix C for details. As in the case without a magnetic field, one can define the fermionic annihilation and creation operators at any given time t as. The former trivially follows from eqs. (3.8) and (3.9), while the latter is shown to be satisfied in appendix D
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