Wigner functions of essentially nonequilibrium systems

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