Collision term dependence of the hadronic shear viscosity and diffusion coefficients

Abstract

In this work the shear viscosity $η$ and the diffusion coefficients of conserved charges $κ_{ij}$ with $i,j\in\{B,Q,S\}$ of hadronic matter are investigated within the hadronic transport approach SMASH. We systematically study the effect of multi-particle reactions, angular distributions and additional elastic cross sections via the additive quark model description, the numbers of degrees of freedom and the baryon density on the transport coefficients using the Green-Kubo formalism. We find that multi-particle reactions decrease the shear viscosity in a simplified hadron gas whereas the electric charge diffusion coefficient is not modified. Furthermore, additional elastic cross sections have a strong impact on both $η$ and $κ_{ij}$ whereas anisotropic scatterings enhance the shear viscosity in the full hadron gas. When increasing the number of degrees of freedom the shear viscosity is only slightly modified in comparison to the diffusion coefficients. Finally, the calculation within a finite baryon chemical potential reveals that the shear viscosity itself does not depend on $μ_B$ but on the ratio $η/ s$. The diffusion coefficients show a strong dependency which we compare to Chapman-Enskog calculations.