Simple out-of-equilibrium field theory formalism?

In this paper, we determine a condition of the applicability of a very simple formalism for the calculation of a fast process taking place in a nonequilibrium medium, in which the particle distribution functions are frozen in time. Indeed, a well-known obstacle for the use of such a naive formalism is the appearance of the so-called ''pinch singularities.'' We argue that these potentially dangerous terms can be regularized, and that they are negligible if the characteristic time scale of the process under study is small compared to the relaxation time of the system