  The only available analytical framework for investigating QED processes in a
strong laser field systematically relies on approximating the latter as a plane
wave. However, realistic high-intensity laser beams feature much more complex
space-time structures than plane waves. Here, we show the feasibility of an
analytical framework for investigating strong-field QED processes in laser
beams of arbitrary space-time structure by determining the energy spectrum of
positrons produced via nonlinear Breit-Wheeler pair production as a function of
the background field in the realistic assumption that the energy of the
incoming photon is the largest dynamical energy in the problem. A numerical
evaluation of the angular resolved positron spectrum shows significant
quantitative differences with respect to the analogous result in a plane wave,
such that the present results will be also important for the design of upcoming
strong laser facilities aiming at measuring this process.
