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

TL;DR

Relativistic viscous hydrodynamic fits to Brookhaven Rel ativistic Heavy Ion Collider data on the centrality dependence of multiplicity, transverse, and elliptic flow for square root s = 200 GeV Au+Au collisions seem to favor a much smaller viscosity over entropy ratio, below the bound from the anti-de Sitter conformal field theory conjecture.

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.

Figures (3)

• Figure 1: Correlation function $f(k,\tau,\tau_0=1 {\rm fm/c})$ as a function of momentum $k$, measured for our hydrodynamics code on a $64^2$ lattice with a lattice spacing of $1 {\rm GeV}^{-1}$ (symbols), compared to the "analytic" result from the linearized Eq. (\ref{['baseq']}) (full lines). The good overall agreement indicates the code is solving Eq. (\ref{['baseq']}) correctly in the linear regime (see Baier:2006gy for details).
• Figure 2: Total multiplicity $dN/dy$ and mean momentum for $\pi^+,\pi^-,K^+,K^-,p$ and $\bar{p}$ from PHENIX Adler:2003cb for Au+Au collisions at $\sqrt{s}=200$ GeV, compared to our hydrodynamic model for various viscosity ratios $\eta/s$.
• Figure 3: PHOBOS Alver:2007qw data on $p_T$ integrated $v_2$ and STAR Adams:2003zg data on minimum bias $v_2$, for charged particles in Au+Au collisions at $\sqrt{s}=200$ GeV, compared to our hydrodynamic model for various viscosity ratios $\eta/s$. Error bars for PHOBOS data show 90% confidence level systematic errors while for STAR only statistical errors are shown.