# Quark Flavour Dependence of the Shear Viscosity in a Quasiparticle Model

## TL;DR

This paper investigates how the shear viscosity to entropy density ratio varies with temperature in pure Yang-Mills theory and QCD with quarks, using a quasiparticle kinetic theory approach that accounts for temperature-dependent dispersion relations.

## Contribution

It introduces a temperature-dependent quasiparticle model to analyze shear viscosity in Yang-Mills and QCD, highlighting the impact of quark flavors on the ratio's behavior near phase transitions.

## Key findings

- Shear viscosity ratio shows a non-monotonic behavior with a minimum at the phase transition in pure Yang-Mills.
- In QCD with quarks, the ratio smoothens but still exhibits a minimum near confinement.
- Quark contributions significantly increase the shear viscosity to entropy density ratio at higher temperatures.

## Abstract

We study the temperature-dependence of the shear viscosity to entropy density ratio in pure Yang-Mills theory and in QCD with light and strange quarks within kinetic theory in the relaxation time approximation. As effective degrees of freedom in a deconfined phase we consider quasiparticle excitations with quark and gluon quantum numbers and dispersion relations that depend explicitly on the temperature. The quasiparticle relaxation times are obtained by computing the microscopic two-body scattering amplitudes for the elementary scatterings among the quasiparticles. For pure Yang-Mills theory we show that the shear viscosity to entropy density ratio exhibits a characteristic non-monotonicity with a minimum at the first-order phase transition. In the presence of dynamical quarks the ratio smoothens while still exhibiting a minimum near confinement. Furthermore, there is a significant increase of the shear viscosity to entropy density ratio in QCD resulting from the quark contributions. This observation differs from previously reported estimates based on functional methods but is in line with perturbative QCD expectations at higher temperatures.