Coupling relativistic viscous hydrodynamics to Boltzmann descriptions

Models of relativistic heavy-ion collisions typically involve both a hydrodynamic module to describe the high-density liquidlike phase and a Boltzmann module to simulate the low-density breakup phase, which is gaslike. Coupling the prescriptions is more complicated for viscous prescriptions if one wants to maintain continuity of the entire stress-energy tensor and currents. Derivations for the viscosity for a gas are reviewed, which then lead to expressions for changes in the phase-space occupation based on simple relaxation-time pictures of viscosity. These expressions are shown to consistently reproduce the nonequilibrium components of the stress-energy tensor. An algorithm for generating a Monte Carlo sampling of particles with which to initiate the Boltzmann calculations is also presented.