Pair Production Beyond the Schwinger Formula in Time-Dependent Electric   Fields
  We investigate electron-positron pair production in pulse-shaped electric background fields using a non-Markovian quantum kinetic equation. We identify a pulse-length range for subcritical fields still in the nonperturbative regime where the number of produced pairs significantly exceeds that of a naive expectation based on the Schwinger formula. From a conceptual viewpoint, we find a remarkable quantitative agreement between the (real-time) quantum kinetic approach and the (imaginary-time) effective action approach. 
Florian Hebenstreit, Reinhard Alkofer, Holger Gies