Cutting rules at finite temperature
  We discuss the cutting rules in the real time approach to finite temperature field theory and show the existence of cancellations among classes of cut graphs which allows a physical interpretation of the imaginary part of the relevant amplitude in terms of underlying microscopic processes. Furthermore, with these cancellations, any calculation of the imaginary part of an amplitude becomes much easier and completely parallel to the zero temperature case. 
P.F.Bedaque (MIT), A.Das (U. of Rochester) and S.Naik (Mehta R.I.)