Shear viscosity and hierarchical structure of matter

This article deals with the shear viscosity and effects of inter-particle correlations from the viewpoint of the hierarchical structure of matter. Although the change in hierarchical levels, which means that one kind of dominant degrees of freedom transitions into another, does not necessarily accompany the phase transition, and then we cannot expect a clear boundary between them in general, we propose that the shear viscosity is a useful probe to detect it: the shear viscosity takes a minimum around the boundary of the change in hierarchies. The minimum of the shear viscosity itself has attracted much attention and has widely been investigated from various research fields, ranging from low-energy condensed matter physics to high-energy nuclear physics, due to the so-called Kovtun–Son–Starinets (KSS) conjecture (which predicts the existence of a universal lower bound (KSS bound) of this quantity). We discuss a viscosity minimum and the KSS bound in two examples: quark matter in high-energy physics and an ultracold Fermi gas in low-energy physics.