Strongly Interacting Low Viscosity Matter Created in Heavy Ion Collisions

TL;DR

This paper discusses the creation of low viscosity, strongly interacting matter in heavy ion collisions, highlighting how experimental data and theoretical models suggest a near-perfect fluid behavior and a possible phase transition in QCD.

Contribution

It links experimental observations of collective flow with theoretical predictions of minimal viscosity, proposing eta/s as a marker for the QCD phase transition.

Findings

Data are well-reproduced by perfect fluid dynamics.

The eta/s ratio near phase transitions resembles that of simple fluids.

Experimental measurements could identify the QCD transition via viscosity ratios.

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 eta to entropy density s by Kovtun, Son and Starinets within AdS/CFT yields eta/s = 1/4pi, 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 behavior similar to helium, nitrogen, and water at and near their phase transitions in the ratio eta/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.