The success of ideal Relativistic Fluid Dynamics (RFD) in describing hadron spectra and elliptic flow at RHIC has led to a strong interest in the transport coefficients of QCD, in particular the shear- and bulk-viscosity as well as the shear-viscosity over entropy-density ratio η/s. Of course ideal RFD assumes η to be zero, which is unphysical, but its success has hinted at a very small value for η/s. The purpose of this post is to review what we currently know about η/s of QCD matter and put the different approaches used to study this quantity into context.
The “ridge”, an elongated (in rapidity) structure in the two-particle correlations associated with a hard trigger hadron, keeps attracting the attention of theorists. [Stay tuned for a forthcoming review of the experimental status of the “ridge”.] Two recent preprints explore strikingly different explanations. (more…)