Pair production: the view from the lightfront
  We give an exact, analytic, and manifestly gauge invariant account of pair production in combined longitudinal and transverse electromagnetic fields, both depending arbitrarily on lightfront time. The instantaneous, nonperturbative probability of pair creation is given explicitly along with the spectra of the final particle yield. Our results are relevant to high-intensity QED experiments now being planned for future optical and x-ray free electron lasers. 
Florian Hebenstreit, Anton Ilderton, Mattias Marklund
hep-ph hep-th