Transient phenomena in the axion assisted Schwinger effect

A bstract Particle production induced by a time-dependent background is well understood as the projection of time-evolved initial state onto set final states. While asymptotic and states are defined in usual way, definition particles antiparticles at intermediate times presence external fields ambiguous. These moreover induce divergences which require regularization. In this paper we clarify some subtleties computation transient effects physical quantities for fermions homogeneous axion background, including Schwinger electromagnetic fields. The requires particular care knowledge UV theory when regulating computing vacuum contribution to fermion energy.