Hard thermal QCD, forward scattering and effective actions

We study the high-temperature behaviour of the n-point function to one-loop order in thermal QCD, in the analytic continuation of the imaginary-time formalism. Using ideas of Barton, we relate these functions to angular integrals of the forward-scattering amplitudes (for the thermal particles) in the high-energy limit. These amplitudes are Lorentz and gauge invariant, and the Ward identities determine all of them uniquely in terms of the two-point one. We are thus able, by gauge invariance, to write down a generating function for all n-point functions. Its relationship to a previous one (with a Lorentz and gauge non-invariant integrand) is explained. (orig.)