Nonequilibrium dynamics in quantum field theory at high density: The "tsunami" - art. no. 045021

The dynamics of a dense relativistic quantum fluid out of thermodynamic equ ilibrium is studied in the framework of the Phi (4) scalar field theory in the large N limit. The time evolution of a particle distribution in momentu m space (the tsunami) is computed. The effective mass felt by the particles in such a high density medium equals the tree level mass plus the expectat ion value of the squared field. The case of negative tree level squared mas s is particularly interesting. In such a case dynamical symmetry restoratio n as well as dynamical symmetry breaking can happen. Furthermore, the symme try may stay broken with a vanishing asymptotic squared mass showing the pr esence of out of equilibrium Goldstone bosons. We study these phenomena and identify the set of initial conditions that lead to each case. We compute the equation of state which turns out to depend on the initial state. Altho ugh the system does not thermalize, the equation of state fur asymptoticall y broken symmetry is of radiation type. We compute the correlation function s at equal times. The two point correlator for late times is the sum of dif ferent terms. One stems from the initial particle distribution. Another ter m accounts for the out of equilibrium Goldstone bosons created by spinodal unstabilities when the symmetry is asymptotically broken. Both terms are of the order of the inverse of the coupling for distances where causal signal s can connect the two points. The contribution of the out of equilibrium Go ldstones exhibits scaling behavior in a generalized sense.