We propose to study the nonequilibrium features of heavy-ion collisions by following the evolution of an initial state with a large number of quanta with a distribution around a momentum $|k_0|$ corresponding to a thin spherical shell in momentum space, a “tsunami.” An $O\left(N\right)({\overrightarrow{\Phi }}^2{)}^2$ model field theory in the large $N$ limit is used as a framework to study the nonperturbative aspects of the nonequilibrium dynamics including a resummation of the effects of the medium (the initial particle distribution). In a theory where the symmetry is spontaneously broken in the absence of the medium, when the initial number of particles per correlation volume is chosen to be larger than a critical value the medium effects can restore the symmetry of the initial state. We show that if one begins with such a symmetry-restored, nonthermal, initial state, nonperturbative effects automatically induce spinodal instabilities leading to a dynamical breaking of the symmetry. As a result there is explosive particle production and a redistribution of the particles towards low momentum due to the nonlinearity of the dynamics. The asymptotic behavior displays the onset of Bose condensation of pions and the equation of state at long times is that of an ultrarelativistic gas although the momentum distribution is nonthermal.

• Received 7 November 1997

DOI:https://doi.org/10.1103/PhysRevD.57.3653