Hydrodynamics from free-streaming to thermalization and back again

Abstract

We study the evolution of the Knudsen and Reynolds numbers in (0+1)-dimensionally expanding fluids with Bjorken symmetry for systems whose microscopic mean free path rises more quickly with time than usually assumed. This allows us to explore within a simple 1-dimensional model the transition from initially thermalizing to ultimately decoupling dynamics. In all cases studied the dynamics is found to be controlled by hydrodynamic attractors for both the Knudsen and Reynolds numbers whose trajectories undergo characteristic changes as the dynamics changes from thermalizing to decoupling.