Prethermalization and relaxation rates of observables in isolated quantum systems*

We study the phenomenon of prethermalization following quantum quenches in generic isolated many body systems wherein the relaxation dynamics of observables involve two regimes: a fast prethermalization and a slow thermalization. We show that in order to observe prethermalization, perturbations should break at least one local conserved quantity of the unperturbed system. We calculate the relaxation rates of observables to thermal equilibrium and show that they are given by Fermi's golden rule in the thermodynamic limit. We use numerical linked cluster expansions and exact diagonalization of finite systems for our calculations.