Retarded Green's functions and forward scattering amplitudes in thermal field theory - art. no. 065004

In this paper, we give a simple diagrammatic identification of the unique c ombination of the causal n-point vertex functions in the real time formalis m that would coincide with the corresponding functions obtained in the imag inary time formalism. Furthermore, we give a simple calculational method fo r evaluating the temperature dependent parts of the retarded vertex functio ns, to one loop, by identifying them with the forward scattering amplitudes of on-shell thermal particles. As an application of the method, we calcula te and show that the temperature-dependent parts of all the retarded functi ons vanish at one loop order for (1 + 1)-dimensional massless QED. We furth er point out that, in this model, in fact, the temperature-dependent parts of all the retarded vertex functions vanish to all orders in perturbation t heory. [S30556-2821(99)06304-8].