Viscosity coefficients for hadron and quark–gluon phases

The shear (η) and bulk (ζ) viscosities are calculated in a quasiparticle relaxation time approximation. The hadron phase is described within the relativistic mean-field-based model with scaled hadron masses and couplings. The quark phase is treated in terms of the heavy quark bag model fitted to the lattice data. A two-phase model allowing for the first-order phase transition from the hadron phase to the strongly coupled quark–gluon plasma is constructed by means of the Gibbs conditions. Temperature and baryon density dependence of the calculated viscosity-to-entropy ratios (η/s, ζ/s) are analyzed and compared with those obtained in other models. Special attention is paid to the behavior of viscosity coefficients near the critical temperature, from both hadron and quark–gluon side. Effects of resonance widths on viscosities and viscosity-to-entropy ratios are estimated.