  We present a general construction of KMS states in the framework of
perturbative algebraic quantum field theory (pAQFT). Our approach may be
understood as an extension of the Schwinger-Keldysh formalism. We obtain in
particular the Wightman functions at positive temperature, thus solving a
problem posed some time ago by Steinmann. The notorious infrared divergences
observed in a diagrammatic expansion are shown to be absent due to a consequent
exploitation of the locality properties of pAQFT. To this avail, we introduce a
novel, Hamiltonian description of the interacting dynamics and find, in
particular, a precise relation between relativistic QFT and rigorous quantum
statistical mechanics.
