Seminar in Hadronic Physics

Converting a viscous fluid to particles

Denes Molnar

Purdue University

Comparing hydrodynamic calculations to heavy-ion data inevitably requires the conversion of the fluid to particles. The conversion is ambiguous for viscous fluids because an infinite class of phase space densities matches the same hydrodynamic variables. It is common to postulate that dissipative phase space density corrections due to shear stress are quadratic in momentum and have the same prefactor for each particle species (a prescription we termed “democratic Grad approach”). Though appealingly simple, this choice ignores microscopic dynamics that governs how close the various particle species are to local equilibrium in the hadron gas. We obtain instead dissipative phase space corrections from the linearized Boltzmann equation, and use those in the Cooper-Frye formalism to convert the fluid to particles. In general we find that viscous corrections depend on the hadron species. For example, proton are better equilibrated than pions and, therefore, have smaller viscous corrections.