  The aim of the article is to discuss the S-matrix interpretation of
perturbation theory for the Wigner functions generating functional at a finite
temperature. For sake of definiteness, fruitful from pedagogical point of view,
the concrete problem from particle physics of high-temperature initial states
dissipation into cold one is considered from experimental and theoretical
points of view. The temperature is introduced in the theory by typical for the
microcanonical description way. The perturbation theory contains two-
temperature (of initial and final states) Green functions. Two possible
boundary conditions are considered. One of them is usual in a field theory
vacuum boundary condition. Corresponding generating functional of Wigner
functions can be used in the particle physics. Another type of the boundary
condition assumes that the system under consideration is in environment of the
black-body radiation. This leads to the usual in statistics
Kubo-Martin-Schwinger boundary condition at the equilibrium (one-temperature)
limit. The comparison of the S-matrix approach with Schwinger-Keldysh real-time
finite-temperature field theory and with nonstationary statistical operator
approach of Zubarev are considered. The range of applicability of the
finite-temperature description of dissipation processes is shown.
