Dynamical Quasi-Stationary States in a system with long-range forces

Abstract

The Hamiltonian Mean Field model describes a system of N fully-coupled particles showing a second-order phase transition as a function of the energy. The dynamics of the model presents interesting features in a small energy region below the critical point. In particular, when the particles are prepared in a ``water bag'' initial state, the relaxation to equilibrium is very slow. In the transient time the system lives in a dynamical quasi-stationary state and exhibits anomalous (enhanced) diffusion and Lévy walks. In this paper we study temperature and velocity distribution of the quasi-stationary state and we show that the lifetime of such a state increases with N. In particular when the $N\to \infty$ limit is taken before the $t\to \infty$ limit, the results obtained are different from the expected canonical predictions. This scenario seems to confirm a recent conjecture proposed by C.Tsallis.

Comments: 7 pages, Latex, 4 eps figures included, talk presented at the Intern. Workshop "Classical and quantum complexity and nonextensive thermodynamics", held in Denton, Texas, April 3-6 2000