Transport properties of the fluid produced at RHIC

TL;DR

This paper reviews recent progress in understanding the transport properties of the quark-gluon fluid produced at RHIC, including shear and bulk viscosity, diffusion, and the implications for future LHC experiments.

Contribution

It summarizes recent theoretical and experimental findings on transport coefficients of the quark-gluon plasma and discusses future research opportunities with LHC data.

Findings

Shear viscosity to entropy density ratio is extremely low.

The lower bound conjecture from AdS/CFT is violated.

Lattice QCD and perturbative methods provide complementary insights.

Abstract

It is by now well known that the relativistic heavy-ion collisions at RHIC, BNL have produced a strongly interacting fluid with remarkable properties, among them the lowest ever observed ratio of the coefficient of shear viscosity to entropy density. Arguments based on ideas from the String Theory, in particular the AdS/CFT correspondence, led to the conjecture --- now known to be violated --- that there is an absolute lower limit $1/4 \pi$ on the value of this ratio. Causal viscous hydrodynamics calculations together with the RHIC data have put an upper limit on this ratio, a small multiple of $1/4 \pi$, in the relevant temperature regime. Less well-determined is the ratio of the coefficient of bulk viscosity to entropy density. These transport coefficients have also been studied nonperturbatively in the lattice QCD framework, and perturbatively in the limit of high-temperature QCD. Another interesting transport coefficient is the coefficient of diffusion which is also being studied in this context. I review some of these recent developments and then discuss the opportunities presented by the anticipated LHC data, for the general nuclear physics audience.