Substantial collective flow is observed in collisions between large nuclei at RHIC (Relativistic Heavy Ion Collider) 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. A calculation of the dimensionless ratio of shear viscosity η to entropy density s by Kovtun, Son and Starinets within anti-de Sitter space/conformal field theory yields η/s =h/4πk B which has been conjectured to be a lower bound for any physical system. Motivated by these results, we show that the transition from hadrons to quarks and gluons has behavior similar to helium, nitrogen, and water at and near their phase transitions in the ratio η/s. We suggest that experimental measurements can pinpoint the location of this transition or rapid crossover in QCD.

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A review of earlier fluid dynamical calculations with QGP show a softening of the directed flow while with hadronic matter this effect is absent. The effect shows up in the reaction plane as enhanced emission which is orthogonal to the directed flow. Thus, it is not shadowed by the deflected projectile and target. As both of these flow components are in the reaction plane these form an enhanced 'elliptic flow' pattern. Recent experimental data at 11 AGeV and above show the same softening, hinting at QGP formation.Comment: 12 pages, Latex, and 3 figures (.eps), 2 style files (.sty