Laser assisted Breit-Wheeler and Schwinger processes
  The assistance of an intense optical laser pulse on electron-positron pair production by the Breit-Wheeler and Schwinger processes in XFEL fields is analyzed. The impact of a laser beam on high-energy photon collisions with XFEL photons consists in a phase space redistribution of the pairs emerging in the Breit-Wheeler sub-process. We provide numerical examples of the differential cross section for parameters related to the European XFEL. Analogously, the Schwinger type pair production in pulsed fields with oscillating components referring to a superposition of optical laser and XFEL frequencies is evaluated. The residual phase space distribution of created pairs is sensitive to the pulse shape and may differ significantly from transiently achieved mode occupations. 
T. Nousch, A. Otto, D. Seipt, B. K\"ampfer, A. I. Titov, D. Blaschke,   A. D. Panferov, and S. A. Smolyansky
hep-ph physics.atom-ph