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

TL;DR

This paper investigates the hydrodynamic evolution of a quark-gluon plasma with finite quark masses and baryon number, revealing that far-from-equilibrium conditions can lead to counterintuitive temperature behaviors like viscous cooling, supported by kinetic theory analysis.

Contribution

It provides a detailed kinetic theory analysis of far-off-equilibrium expansion trajectories in the QCD phase diagram, demonstrating thermodynamic consistency and novel viscous cooling phenomena.

Findings

Far-off-equilibrium initial conditions can violate traditional thermodynamic rules.

Kinetic theory explains the microscopic origin of these phenomena.

Viscous cooling can occur instead of viscous heating in certain conditions.

Abstract

We consider the hydrodynamic evolution of a quark-gluon gas with non-zero 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.