Fermionic dynamical Casimir effect: Magnus expansion

We study pair creation out of the vacuum, for a system consisting of a massive Dirac field in $1+1$ dimensions, contained between a pair of perfectly reflecting boundaries, one of them oscillating. After analyzing some general properties of the vacuum-decay process, we evaluate the corresponding transition amplitude in a Magnus expansion of the S-matrix. We show how this yields, besides the single-pair creation amplitude, multipair ones, as well as corrections to the single pair amplitude. We also apply it to obtain an approximate, yet explicitly unitary expression for the Bogoliubov transformation between the in and out Fock spaces.