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The goal of heavy-ion collision experiments at RHIC and the LHC is to create a hot and dense plasma of quarks and gluons and study its properties. It has become popular to model the evolution of the plasma using relativistic viscous hydrodynamics. The primary appeal of hydrodynamics is that it directly involves thermal and transport properties of the medium, providing experimental access to the equation of state and the also shear and bulk viscosities of the plasma. It is less known, however, that hydrodynamics alone cannot be compared to experiments because the latter can only detect the energy and momentum of particles that reach the detectors. Hydrodynamic fields must be first converted to particles, which requires additional theory ingredients. The main challenge is to describe how local phase space distributions are modified compared to their local equilibrium form in the presence of gradiens and dissipation in the system. In fact, an infinite class of phase space distributions can describe the same hydrodynamic fields (even in one-component systems!).
There are several "particlization" models in use, most of which completely ignore the microscopic dynamics that keeps the system near local equilibrium. We instead advocate a self-consistent framework that obtains phase space corrections from relativistic kinetic theory. The technical aspects of the approach share a lot of similarity with the calculation of transport coefficients in kinetic theory (see [1] for a detailed discussion of shear viscous corrections). I will present self-consistent shear viscous and bulk viscous phase space corrections calculated for a variety of systems (one-, few-, and many-component hadron mixtures), and will discuss how the corrections affect harmonic flow coefficients v_n = <cos(n*phi)> calculated from hydrodynamics. Results for the hadron gas viscosity, both shear and bulk, will also be presented.
[1] D. Molnar & Z. Wolff, PRC 95 (2017), 024903