Discontinuities of green functions in field theory at finite temperature and density (II)

Abstract

Extending some previous work on Cutkosky rules at finite temperature and density in a real time formalism, we consider here the case that all external lines of a Feynman graph are physical. We find the corresponding rules simplify considerably, although still do not admit an interpretation in terms of cutting a graph. An analysis of the relationship between the imaginary part of a self-energy graph and decay amplitudes suggests that the graphs which cannot be cut correspond to contributions from the ghost field necessary if real time Feynman rules are used. We also discuss, crudely speaking, where one can “start” and “end” in time in a Feynman graph at finite temperature and density.