We study the pair-production process in the presence of two counterpropagating linearly polarized short laser pulses. By means of a nonperturbative technique, we take into account the full coordinate dependence of the external field going beyond the dipole and standing-wave approximations. In particular, we analyze the momentum distribution of particles created. It is demonstrated that the spatial variations of the laser pulses may play a crucial role. The more accurate treatment reveals a number of prominent features: the pair-production probabilities become considerably smaller, the quantitative behavior of the momentum spectra changes dramatically, and the pulse shape effects become much less pronounced. The results of our study are expected to be very important for future theoretical and experimental investigations.

We investigate the phenomenon of electron-positron pair production from vacuum in the presence of a strong electric field superimposed by a weak but fast varying pulse which substantially increases the total particle yield. We employ a nonperturbative numerical technique and perform the calculations beyond the spatially-uniform-field approximation, i.e., dipole approximation, taking into account the coordinate dependence of the fast component. The analysis of the main characteristics of the pair-production process (momentum spectra of particles and total amount of pairs) reveals a number of important features which are absent within the previously used approximation. In particular, the structure of the momentum distribution is modified both qualitatively and quantitatively, and the total number of pairs created as well as the enhancement factor due to dynamical assistance become significantly smaller.

In the present investigation we revisit the widely-used locally-constant field approximation (LCFA) in the context of the pair-production phenomenon in strong electromagnetic backgrounds. By means of nonperturbative numerical calculations, we assess the validity of the LCFA considering several spatially homogeneous field configurations and a number of space-time-dependent scenarios. By studying the momentum spectra of particles produced, we identify the criteria for the applicability of the LCFA. It is demonstrated that the Keldysh parameter itself does not allow one to judge if the LCFA should perform accurately. In fact, the external field parameters must obey less trivial relations whose form depends on the field configuration. We reveal several generic properties of these relations which can also be applied to a broader class of other pair-production scenarios.