LAUSR

The shear viscosity η and the so-called jet quenching parameter q̂ are two important transport coefficients that characterize the quark-gluon plasma. In order to eliminate the trivial temperature dependence of these quantities, it is customary to consider the two dimensionless quantities η/s, the ratio of the shear viscosity and the entropy density, sometimes called the specific shear viscosity, and q̂/T , where T is the temperature. Both dimensionless quantities are sensitive to the effective coupling strength in the plasma. When the coupling is weak, η/s is large and q̂/T 3 is small; when the coupling is strong, η/s is small and q̂/T 3 is large. The fact that the analysis of experimental data from relativistic heavy ion collisions indicates that η/s ≈ 0.1−0.2 [1, 2] and q̂/T 3 ≈ 3−4 [3, 4] is generally taken as an indication that the quark-gluon plasma produced in such collisions is strongly coupled. Some time ago, Majumder, Müller, and Wang (MMW) argued that the two quantities are connected by a general relation of the form [5]: