Action diffusion and lifetimes of quasistationary states in the Hamiltonian mean-field model

Out-of-equilibrium quasistationary states (QSSs) are one of the signatures of a broken ergodicity in long-range interacting systems. For the widely studied Hamiltonian mean-field model, the lifetime of some QSSs has been shown to diverge with the number N of degrees of freedom with a puzzling N1.7 scaling law, contradicting the otherwise widespread N scaling law. It is shown here that this peculiar scaling arises from the locality properties of the dynamics captured through the computation of the diffusion coefficient in terms of the action variable. The use of a mean first passage time approach proves to be successful in explaining the nontrivial scaling at stake here and sheds some light on another case, where lifetimes diverging as eN above some critical energy have been reported.