CONSTRAINED EFFECTIVE POTENTIAL IN HOT QCD

Constrained effective potentials in hot gauge theory give the probability that a configuration p of the order parameter (Polyakov loop) occurs. They are important in the analysis of surface effects and bubble formation in the plasma. The vector potential appears non-linearly in the loop; in weak coupling the linear term gives rise to the traditional free-energy graphs. But the non-linear terms generate insertions of the constrained modes into the free-energy graphs, through renormalisations of the Polyakov loop. These insertions are gauge dependent and are necessary to cancel the gauge dependence of the free-energy graphs. The latter is shown, through the BRST identities, to have again the form of constrained mode insertions. We stress that the effective action, without this insertion, is gauge variant and gives quite deceptive information. Amongst more it follows, that absolute minima of the potential are at the center group values of the loop, once the insertion is done. We evaluate the two-loop contributions for SU(N) gauge theories, with and without quarks, for the full domain of the N - 1 variables.