From the N-body Schrödinger Equation to the Quantum Boltzmann Equation: a Term-by-Term Convergence Result in the Weak Coupling Regime

In this paper we analyze the asymptotic dynamics of a system of N quantum particles, in a weak coupling regime. Particles are assumed statistically independent at the initial time. Our approach follows the strategy introduced by the authors in a previous work: we compute the time evolution of the Wigner transform of the one-particle reduced density matrix; it is represented by means of a perturbation series, whose expansion is obtained upon iterating the Duhamel formula; this approach allows us to follow the arguments developed by Lanford for classical interacting particles evolving in a low density regime. We prove, under suitable assumptions on the interaction potential, that the complete perturbation series converges term-by-term, for all times, towards the solution of a Boltzmann equation. The present paper completes the previous work [BCEP1]: it is proved there that a subseries of the complete perturbation expansion converges uniformly, for short times, towards the solution to the nonlinear quantum Boltzmann equation. This previous result holds for (smooth) potentials having possibly non-zero mean value. The present text establishes that the terms neglected at once in [BCEP1], on a purely heuristic basis, indeed go term-by-term to zero along the weak coupling limit, at least for potentials having zero mean. Our analysis combines stationary phase arguments with considerations on the nature of the various Feynman graphs entering the expansion.