Effective shear and bulk viscosities of the quark-gluon plasma: QCD versus heavy-ion data

TL;DR

This paper estimates the effective shear and bulk viscosities of the quark-gluon plasma using first-principles calculations and compares them with values extracted from heavy-ion collision data, finding good agreement and emphasizing shear viscosity's dominant role.

Contribution

The study provides a first-principles estimate of the effective viscosities of the quark-gluon plasma and compares these with experimental data, clarifying the dominant role of shear viscosity in flow damping.

Findings

Effective shear viscosity estimated between 0.17 and 0.21.

Effective bulk viscosity less than 0.08.

Experimental data compatible with theoretical predictions.

Abstract

In recent years, there has been a significant effort to extract the temperature-dependent shear ($\eta/s$) and bulk ($\zeta/s$) viscosity over entropy ratios of the quark-gluon plasma from a global comparison of heavy-ion data with results of hydrodynamic simulations. However, anisotropic flow, which is arguably the most sensitive probe of viscosity, is only sensitive to an {\it effective\/} viscosity over entropy ratio, which is obtained by taking a weighted average over the temperature, and summing the contributions of shear and bulk. We estimate this effective viscosity using existing first-principles calculations, which give $0.17<(\eta/s)_{\rm eff}<0.21$, and $(\zeta/s)_{\rm eff}<0.08$, implying that the damping of anisotropic flow at the LHC is mostly due to shear viscosity. The values extracted from global data analyses are compatible with these theory predictions.