Exact hard-sphere moments of the nonlinear collision integral for anisotropic distributions show the relaxation-time approximation relaxes roughly twice as fast as true binary collisions, and a two-moment closure resolves the hierarchy-closure ambiguity.
Matching pre-equilibrium dynamics and viscous hydrodynamics
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abstract
We demonstrate how to match pre-equilibrium dynamics of a 0+1 dimensional quark gluon plasma to 2nd-order viscous hydrodynamical evolution. The matching allows us to specify the initial values of the energy density and shear tensor at the initial time of hydrodynamical evolution as a function of the lifetime of the pre-equilibrium period. We compare two models for the pre-equilibrium quark-gluon plasma, longitudinal free streaming and collisionally-broadened longitudinal expansion, and present analytic formulas which can be used to fix the necessary components of the energy-momentum tensor. The resulting dynamical models can be used to assess the effect of pre-equilibrium dynamics on quark-gluon plasma observables. Additionally, we investigate the dependence of entropy production on pre-equilibrium dynamics and discuss the limitations of the standard definitions of the non-equilibrium entropy.
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Exact solutions for the moments of the binary collision integral and its relation to the relaxation-time approximation in leading-order anisotropic fluid dynamics
Exact hard-sphere moments of the nonlinear collision integral for anisotropic distributions show the relaxation-time approximation relaxes roughly twice as fast as true binary collisions, and a two-moment closure resolves the hierarchy-closure ambiguity.