Relaxation time ansatz and shear and bulk viscosities of gluon matter
  Shear and bulk viscosity-to-entropy density ratios are calculated for the pure gluon matter in a non-equilibrium mean-field quasiparticle approach within the relaxation time approximation. We study how different approximations used in the literature affect the results for the shear and bulk viscosities. Though the results for the shear viscosity turned out to be quite robust, all evaluations of the shear and bulk viscosities obtained in the framework of the relaxation time approximation can be considered only as rough estimations. 
A. S. Khvorostukhin, V. D. Toneev, D. N. Voskresensky
nucl-th hep-ph