Abstract
Substantial collective flow is observed in collisions between large nuclei at high energy, as evidenced by single-particle transverse momentum distributions and by azimuthal correlations among the produced particles. The data are well reproduced by perfect fluid dynamics. In a separate development, calculation of the dimensionless ratio of shear viscosity η to entropy density s by Kovtun, Son and Starinets within AdS/CFT yields η/s = 1/4π, and they conjectured that this is a lower bound for any physical system. It is shown that the transition from hadrons to quarks and gluons has behaviour similar to helium, nitrogen and water at and near their phase transitions in the ratio η/s. Therefore, it is possible that experimental measurements can pinpoint the location of this transition or rapid crossover in QCD via the viscosity to entropy ratio in addition to and independently of the equation of state.
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