VISCOSITIES OF QUARK-GLUON PLASMAS

The quark and gluon viscosities are calculated in quark-gluon plasmas to leading orders in the coupling constant by including screening. For weakly interacting QCD and QED plasmas, dynamical screening of transverse interactions and Debye screening of longitudinal interactions controls the infrared divergences. For strongly interacting plasmas other screening mechanisms taken from lattice calculations are employed. By solving the Boltzmann equation for quarks and gluons including screening, the viscosity is calculated to leading orders in the coupling constant. The leading logarithmic order is calculated exactly by a full variational treatment. The next to leading orders are found to be important for the transport properties of quark-gluon plasmas created in relativistic heavy-ion collisions and the early universe, where the coupling constant is large.