The Role of Meson Retardation in the NN Interaction above Pion Threshold

A model is developed for the hadronic interaction in the two-nucleon system above pion threshold which is based on meson, nucleon and $\Delta$ degrees of freedom and which includes full meson retardation in the exchange operators. For technical reasons, the model allows maximal one meson to be present explicitly. Thus the Hilbert space contains besides $NN$ and $N\Delta$ also configurations consisting of two nucleons and one meson. For this reason, only two- and three-body unitarity is obeyed, and the model is suited for reactions in the two nucleon sector only, where one pion is produced or absorbed. Starting from a realistic pure nucleonic retarded potential, which had to be renormalized because of the additional $\pi$ and $\Delta$ degrees of freedom, a reasonable fit to experimental $NN$-scattering data could be achieved.