Quasiparticles in Finite-Temperature Field Theory
  Conventional finite-temperature perturbation theory in which propagators have poles at $k^{2}=m^{2}$ is shown to break down at the two-loop level for self-interacting scalar fields. The breakdown is avoided by using free thermal propagators that have poles at the same energy as the exact thermal propagator. This quasiparticle energy ${\cal E}(\vec{k})$ is temperature-dependent, complex, and gauge invariant. An operator theory containing two self-adjoint scalar fields is presented in which all temperature dependence is incorporated into the Hamiltonian. No thermal traces are required to compute thermal Green functions. Choosing the spectrum of the unperturbed part of the Hamiltonian to contain the exact quasiparticle energy ${\cal E}(\vec{k})$ produces a resummed perturbation theory that has the correct poles and branch cuts. The location of the poles and cuts is explained directly in terms of the spectrum of the Hamiltonian. 
H. Arthur Weldon