What do elliptic flow measurements tell us about the matter created in the little Bang at RHIC?

Abstract

Elliptic flow measurements are presented and discussed with emphasis on the hydrodynamic character of the hot and dense QCD matter created in heavy ion collisions at RHIC. Predictions from perfect fluid hydrodynamics for the scaling of the elliptic flow coefficient $v_2$ with eccentricity, system size and transverse energy are validated. A universal scaling for the flow of both mesons and baryons is observed for a broad transverse kinetic energy range when quark number scaling is employed. This suggests a new state of nuclear matter at extremely high density and temperature whose primary constituents have the quantum numbers of quarks and anti-quarks in chemical equilibrium. The scaled flow is used to constrain estimates for several transport coefficients including the sound speed $c_s$, shear viscosity to entropy ratio $η/s$, diffusion coefficient ($D_c$) and sound attenuation length ($Γ$). The estimated value $η/s \sim 0.1$, is close to the absolute lower bound ($1/4π$), and may signal thermodynamic trajectories for the decaying matter which lie close to the QCD critical end point.