Strongly Interacting Low-Viscosity Matter Created in Relativistic Nuclear Collisions

TLDR

Theoretical work predicts a universal lower bound for the shear viscosity to entropy density ratio, η/s = ħ/4πk_B, in strongly coupled systems. The authors aim to show that the QCD transition behaves like conventional fluids near phase transitions in η/s and to locate it experimentally. Heavy‑ion collisions exhibit strong collective flow that matches perfect‑fluid dynamics, implying the produced matter has η/s close to the conjectured lower bound and behaves like ordinary fluids near phase transitions.

Abstract

Substantial collective flow is observed in collisions between large nuclei at BNL 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 $\ensuremath{\eta}$ to entropy density $s$ by Kovtun, Son, and Starinets within anti-de Sitter space/conformal field theory yields $\ensuremath{\eta}/s=\ensuremath{\hbar}/4\ensuremath{\pi}{k}_{B}$, 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 $\ensuremath{\eta}/s$. We suggest that experimental measurements can pinpoint the location of this transition or rapid crossover in QCD.