  The electron-positron pair production in the collision of two intense gamma
lasers is studied beyond the dipole approximation. The complete
electron/positron momentum spectrum obtained by three-dimensional computation
shows directly the multi-photon pair production channels and provides a
sensitive test on the widely-used concept of the effective mass.
Nonperturbative features are identified in the dependence of the total yield on
the field intensity. In particular, various breaks in the momentum rings are
observed and proved to be a manifestation of particle's energy gaps in the
standing light wave induced by the Kapitza-Dirac scattering. A theoretical
model of multi-level systems coupled by the Kapitza-Dirac scattering and pair
production channels is established. Multi-channel interference effects and pair
production suppression are revealed in these systems.
