Viscosity Information from Relativistic Nuclear Collisions: How Perfect is the Fluid Observed at RHIC?

TL;DR

This paper uses relativistic viscous hydrodynamics to analyze RHIC collision data, suggesting the quark-gluon plasma behaves as an almost perfect fluid with very low viscosity, possibly below the theoretical lower bound.

Contribution

It provides new constraints on the shear viscosity to entropy density ratio of the quark-gluon plasma using detailed hydrodynamic fits to experimental data.

Findings

Integrated v_2 data compatible with η/s ≤ 0.16

Minimum bias v_2 data suggests η/s below the AdS/CFT bound

Discussion of uncertainties and caveats in viscosity extraction

Abstract

Relativistic viscous hydrodynamic fits to RHIC data on the centrality dependence of multiplicity, transverse and elliptic flow for sqrt{s}=200 GeV Au+Au collisions are presented. For Glauber-type initial conditions, while data on integrated v_2 is consistent with a ratio of viscosity over entropy density up to eta/s=0.16, data on minimum bias v_2 seems to favor a much smaller viscosity over entropy ratio, below the bound from the AdS/CFT conjecture. Some caveats on this result are discussed.