Vibrations versus collisions and the iterative structure of two-body dynamics

We adopt a truncated version of two-body dynamics by neglecting three-body correlations, as is supported by microscopic numerical calculations. Introducing orthogonal channel correlations for the pp- and the ph-channel and integrating the latter in terms of vibrational RPA-states we derive a retarded two-body equation. Its solution is nonperturbative with respect to loops, ladders and mixed contributions. In the stationary limit we obtain an equation for a generalised effective interaction which iterates both the G-matrix and the polarisation matrix. An in-medium scattering approach transparently demonstrates the collisional damping of the vibrations. ((orig.))