Far-off-equilibrium expansion trajectories in the QCD phase diagram

Abstract

We consider the hydrodynamic evolution of a quark-gluon gas with nonzero quark masses and net baryon number in its phase diagram. For far-off-equilibrium initial conditions the expansion trajectories appear to violate simple rules based on the second law of thermodynamics that were previously established for ideal or weakly dissipative fluids. For Bjorken flow we present a detailed analysis within kinetic theory that provides a full microscopic understanding of these macroscopic phenomena and establishes their thermodynamic consistency. We point out that, for certain far-off-equilibrium initial conditions, the well-known phenomenon of ``viscous heating'' turns into ``viscous cooling'' where, driven by dissipative effects, the temperature decreases faster than in adiabatic expansion.