Cutting rules in the real-time formalisms at finite temperature Original Research Article

In this paper, we review the set of rules specific to the calculation of the imaginary part of a Greenʹs function at finite temperature in the real-time formalisms. Emphasis is put on the clarification of a recent controversy concerning these rules in the time-ordered version of the real-time formalism, more precisely on the issue related to the interpretation of these rules in terms of cut diagrams, like at T = 0. Also, new results are presented, enabling one to calculate the imaginary part of thermal Greenʹs functions in other formulations of the real-time formalism, like the “retarded/advanced” formalism in which a lot of simplifications occur.