200<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>A</mml:mi></mml:math>GeV<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Au</mml:mi><mml:mo>+</mml:mo><mml:mi>Au</mml:mi></mml:math>Collisions Serve a Nearly Perfect Quark-Gluon Liquid

A new robust method to extract the specific shear viscosity $(\ensuremath{\eta}/s{)}_{\mathrm{QGP}}$ of a quark-gluon plasma (QGP) at temperatures ${T}_{c}&lt;T\ensuremath{\lesssim}2{T}_{c}$ from the centrality dependence of the eccentricity-scaled elliptic flow ${v}_{2}/\ensuremath{\epsilon}$ measured in ultrarelativistic heavy-ion collisions is presented. Coupling viscous fluid dynamics for the QGP with a microscopic transport model for hadronic freeze-out we find for 200 $A$ GeV $\mathrm{Au}+\mathrm{Au}$ collisions that ${v}_{2}/\ensuremath{\epsilon}$ is a universal function of multiplicity density $(1/S)(d{N}_{\mathrm{ch}}/dy)$ that depends only on the viscosity but not on the model used for computing the initial fireball eccentricity $\ensuremath{\epsilon}$. Comparing with measurements we find $1&lt;4\ensuremath{\pi}(\ensuremath{\eta}/s{)}_{\mathrm{QGP}}&lt;2.5$ where the uncertainty range is dominated by model uncertainties for the values of $\ensuremath{\epsilon}$ used to normalize the measured ${v}_{2}$.

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