Causal amplitudes in the Schwinger model at finite temperature
  We show, in the imaginary time formalism, that the temperature dependent parts of all the retarded (advanced) amplitudes vanish in the Schwinger model. We trace this behavior to the CPT invariance of the theory and give a physical interpretation of this result in terms of forward scattering amplitudes of on-shell thermal particles. 
Ashok Das, R. R. Francisco and J. Frenkel
hep-th