Effective viscosities in a hydrodynamically expanding boost-invariant QCD plasma

Understand

Background: The near-equilibrium properties of a QCD plasma can be encoded into transport coefficients such as bulk and shear viscosity.

• In QCD, the ratio of these transport coefficients to entropy density, $\zeta/s$ and $\eta/s$, depends non-trivially on the plasma's temperature.
• Purpose: We show that in a 0+1D boost-invariant fluid, a temperature-dependent $\zeta/s(T)$ or $\eta/s(T)$ can be described by an equivalent effective viscosity $\left\langle \zeta/s \right\rangle_{\textrm{eff}}$ or $\left\langle \eta/s \right\rangle_{\textrm{eff}}$.
• We extend the concept of effective viscosity in systems with transverse expansion, and discuss how effective viscosities can be used to identify families of $\zeta/s(T)$ and $\eta/s(T)$ that lead to similar hydrodynamic evolution.