Transport coefficients in the Quark-Gluon Plasma

A derivation of transport coefficients for a hot gas of quarks, antiquarks and gluons is presented. By choosing different collision terms the kinetic coefficients in the relaxation time approximation are calculated with more realistic (though not exactly QCD-like) Nordheim collision terms. In addition transport phenomena for a gas of scalar particles are investigated. The results for a hot gluon gas show that in general one cannot expect to be within the Navier-Stokes domain. Only for very strong couplings there is a chance to reach this domain. In a scalar field theory the situation is even worse: even larger values for the viscosity coefficients result. There it is practically impossible to reach the Navier-Stokes regime. It has become clear that the first order gradient expansion is certainly not sufficient to treat transport phenomena in a quark-gluon plasma. One possible way out of this dilemma would be to go to higher orders in the gradient expansion. (G.P.) 25 refs