Shear viscosity of an ultrarelativistic Boltzmann gas with isotropic   inelastic scattering processes
  We derive an analytic expression for the shear viscosity of an ultra-relativistic gas in presence of both elastic $2\to 2$ and inelastic $2\leftrightarrow 3$ processes with isotropic differential cross sections. The derivation is based on the entropy principle and Grad's approximation for the off-equilibrium distribution function. The obtained formula relates the shear viscosity coefficient $\eta$ to the total cross sections $\sigma_{22}$ and $\sigma_{23}$ of the elastic resp. inelastic processes. The values of shear viscosity extracted using the Green-Kubo formula from kinetic transport calculations are shown to be in excellent agreement with the analytic results which demonstrates the validity of the derived formula. 
Andrej El, Francesco Lauciello, Christian Wesp, Ioannis Bouras, Zhe Xu   and Carsten Greiner
hep-th hep-ph