TIME EVOLUTION OF THE CHIRAL PHASE-TRANSITION DURING A SPHERICAL EXPANSION

We examine the nonequilibrium time evolution of the hadronic plasma pr oduced in a relativistic heavy ion collision, assuming a spherical exp ansion into the vacuum. We study the 0(4) linear sigma model to leadin g order in a large-N expansion. Starting at a temperature above the ph ase transition, the system expands and cools, finally settling into th e broken symmetry vacuum state. We consider the proper time evolution of the effective pion mass, the order parameter [sigma], and the parti cle number distribution. We examine several different initial conditio ns and look for instabilities (exponentially growing long wavelength m odes) which can lead to the formation of disoriented chiral condensate s (DCC's). We find that instabilities exist for proper times which are less than 3 fm/c. We also show that an experimental signature of doma in growth is an increase in the low momentum spectrum of outgoing pion s when compared to an expansion in thermal equilibrium. In comparison to particle production during a longitudinal expansion, we find that i n a spherical expansion the system reaches the ''out'' regime much fas ter and more particles get produced. However the size of the unstable region, which is related to the domain size of DCC's, is not enhanced.