# Hydro regime of strongly coupled field theory, low viscosity

I am trying to get an intuition for the following argument: In heavy ion collisions the central collision area can be described by almost ideal hydrodynamics at very early times after the impact. This leads to an asymmetric distribution of the final state hadrons in the angle $\phi$ around the beam axis rather than a uniform distribution. This was measured at RHIC. The fact that ideal hydrodynamics is a good description in particular means that the shear viscosity $\eta$ is small.

So far so good. Now, the crucial step is that this means that the underlying field theory is strongly coupled. The argument I read is, that strong coupling prevents a quasiparticle description. Quasiparticles would transport momentum and therefore damp shear flows. The absence of quasiparticles leads consequently leads to low shear viscosity.

My intuition would by somewhat opposite. Strong coupling suggests that the constituents of matter "stick together" strongly and therefore the viscosity is high. This intuition is obviously wrong.

Is the idea that some new degrees of freedom arise in the strong coupling limit whose effective coupling is small? Or is there a different explanation why strong coupling leads to low shear viscosity?

• I also wanted to know this for quite some time now – Hydro Guy Oct 6 '14 at 20:03

In a weakly coupled fluid the viscosity scales as $\eta\sim 1/g^a$, where $a$ is a positive power and $g$ is the coupling. In QCD, up to logs, $a=4$. This means that in perturbation theory $\eta$ is parametrically large, and strong coupling is required to describe low viscosity fluids.