Shear viscosity of pion gas due to $\rho\pi\pi$ and $\sigma\pi\pi$   interactions
  We have evaluated the shear viscosity of pion gas taking into account its scattering with the low mass resonances, $\sigma$ and $\rho$ during propagation in the medium. The thermal width (or collisional rate) of the pions is calculated from $\pi\sigma$ and $\pi\rho$ loop diagrams using effective interactions in the real time formulation of finite temperature field theory. A very small value of shear viscosity by entropy density ratio ($\eta/s$), close to the KSS bound, is obtained which approximately matches the range of values of $\eta/s$ used by Niemi et al. \cite{Niemi} in order to fit the RHIC data of elliptic flow. 
Sabyasachi Ghosh, Gast\~ao Krein, Sourav Sarkar
nucl-th hep-ph