Shear viscosity $\eta$ to electric conductivity $\sigma_{el}$ ratio for the Quark-Gluon Plasma

The transport coefficients of strongly interacting matter are currently subject of intense theoretical and phenomenological studies due to their relevance for the characterization of the quark-gluon plasma produced in ultra relativistic heavy-ion collisions (uRHIC). We discuss the connection between the shear viscosity to entropy density ratio, $\eta/s$, and the electric conductivity, $\sigma_{el}$. Once the relaxation time is tuned to have a minimum value of $\eta/s=1/4\pi$ near the critical temperature $T_c$, one simultaneously predicts $\sigma_{el}/T$ very close to recent lQCD data. More generally, we discuss why the ratio of $(\eta/s)/(\sigma_{el}/T)$ supplies a measure of the quark to gluon scattering rates whose knowledge would allow to significantly advance in the understanding of the QGP phase. We also predict that $(\eta/s)/(\sigma_{el}/T)$, independently on the running coupling $\alpha_s(T)$, should increase up to about $\sim 20$ for $T \rightarrow T_c$, while it goes down to a nearly flat behavior around $\simeq 4$ for $T \geq 4\, T_c$. Therefore we in general predict a stronger T dependence of $\sigma_{el}/T$ with respect to $\eta/s$ that in a quasiparticle approach is constrained by lQCD thermodynamics. A conformal theory, instead, predicts a similar T dependence of $\eta/s$ and $\sigma_{el}/T$.
Comment: 5 pages, 3 figures. Revised for publication