Particle production from axial fields
  We study the production of massive fermions in arbitrary vector and axial-vector classical backgrounds using effective action techniques. A perturbative calculation shows the different features of each field and in particular it is seen that pure temporal axial fields can produce particles whereas it is not possible for a pure vector background. We also analyze from a non-perturbative point of view a particular configuration with constant electric and axial fields and show that the presence of the axial background inhibits the production from the electric field. 
Antonio L. Maroto