Early Stages of Relativistic Heavy Ion Collisions

We investigated isotropization and thermalization of the quark-gluon plasma produced by de-caying color-electric flux tubes created at the very early stages of ultra-relativistic heavy ion collisions. The dynamical evolution of the initial classical field, which decays to a plasma by the Schwinger mechanism, is coupled to the dynamics of the many particles system produced. The evolution of such a system is described by relativistic transport theory at fixed values of the viscosity over entropy density ratio. With a single self-consistent computation we obtained quantities which serve as indicators of the equilibration of the plasma for a 1+1 dimensional expanding geometry and for the more realistic 3+1 dimensional case. We find that the initial color-electric field decays within 1 fm/c and particles production occurs in less than 1 fm/c; however, in the case of large viscosity oscillations of the field appear along the whole time evolution of the system, affecting also the behaviour of the ratio between longitudinal and transverse pressure. In case of small viscosity we find that the isotropization time is about 0.8 fm/c and the thermalization time is about 1 fm/c, in agreement with the common lore of hydrodynamics.