The two-particle problem in a many-particle system: I. Dynamically screened ladder approximation

The two-particle problem within a nonequilibrium many-particle system is investigated in the framework of real-time Green's functions. Starting from the dynamically screened ladder approximation of the nonequlibrium Bethe-Salpeter equation, a nonequilibrium Dyson equation is given for two-time two-particle Green's functions. Thereby the well-known Kadanoff-Baym equations are generalized to the case of two-particle functions. The two-time structure of the equations is achieved in an exact way avoiding the so-called Shindo approximation. For the case of thermodynamic equilibrium, the differences to former results obtained for the effective two-particle hamiltonian are discussed.