  After a sudden disruption, weakly interacting quantum systems first relax to
a prethermalized state that can be described by perturbation theory and a
generalized Gibbs ensemble. Using these properties of the prethermalized state
we perturbatively derive a kinetic equation which becomes a quantum Boltzmann
equation in the scaling limit of vanishing interaction. Applying this to
interaction quenches in the fermionic Hubbard model we find that the momentum
distribution relaxes to the thermal prediction of statistical mechanics. For
not too large interaction, this two-stage scenario provides a quantitative
understanding of the time evolution leading from the initial pure via a
metastable prethermal to the final thermal state.
