  Short-pulse effects are investigated for the nonlinear Breit-Wheeler process,
i.e. the production of an electron-positron pair induced by a gamma photon
inside an intense plane-wave laser pulse. To obtain the total pair-creation
probability we verify (to leading-order) the cutting rule for the polarization
operator in the realm of strong-field QED by an explicit calculation. Using a
double-integral representation for the leading-order contribution to the
polarization operator, compact expressions for the total pair-creation
probability inside an arbitrary plane-wave background field are derived.
Correspondingly, the photon wave function including leading-order radiative
corrections in the laser field is obtained via the Schwinger-Dyson equation in
the quasistatic approximation. Moreover, the influence of the carrier-envelope
phase and of the laser pulse shape on the total pair-creation probability in a
linearly polarized laser pulse is investigated, and the validity of the (local)
constant-crossed field approximation analyzed. It is shown that with presently
available technology pair-creation probabilities of the order of ten percent
could be reached for a single gamma photon.
