Fermion damping rate in a hot medium
  In principle every excitation acquires a finite lifetime in a hot system. This nonzero spectral width is calculated self-consistently for massive fermions coupled to massless scalar, vector and pseudoscalar bosons. It is shown that the self-consistent summation of the corresponding Fock diagram for fermions eliminates all infrared divergences although the bosons are not screened at all. Our solutions for the fermion damping rate are analytical in the coupling constant, but not analytical in the temperature parameter around T=0. 
P.A.Henning, R.Sollacher, H.Weigert