Viscous Hydrodynamics and the Quark Gluon Plasma

Understand

One of the most striking results from the Relativistic Heavy Ion Collider is the strong elliptic flow.

• This review summarizes what is observed and how these results are combined with reasonable theoretical assumptions to estimate the shear viscosity of QCD near the phase transition.
• A data comparison with viscous hydrodynamics and kinetic theory calculations indicates that the shear viscosity to entropy ratio is surprisingly small, $\eta/s < 0.4$.
• The preferred range is $\eta/s \simeq (1\leftrightarrow 3) \times 1/4\pi$.