Interviewing the Weak with Strong Coupling Regimes via the Bulk to Shear Viscosity Ratio in QCD

We analyze the thermal profiles of the bulk to shear viscosity ratio in a quasiparticle framework which describes the interaction in a deconfined medium through dynamical masses of its constituents. The temperature dependence of the effective masses is specified by a running coupling deduced from the lattice QCD thermodynamics. To study the impact of dynamical quarks on the transport properties of the hot medium, we confront the results in Nf = 2 + 1 QCD with the observations in pure Yang-Mills theory. We show that dynamical quarks modify the behavior of the bulk to shear viscosity ratio and delay the restoration of conformal invariance. Around the (pseudo)critical temperature in both theories, the bulk to shear viscosity ratio behaves linearly in the squared speed of sound, as found in the AdS/CFT approach. At high temperature, the behavior of the ratio becomes quadratic, which corresponds to the perturbative QCD scaling. Thus, we find that the quasiparticle model is capable of describing the transport properties of the QCD in the weak and strong coupling regimes.