The two-nucleon system in the Delta region including full meson retardation

A model is developed for the hadronic and electromagnetic interaction in the two-nucleon system above pion threshold in the framework of meson, nucleon and $\Delta$ degrees of freedom. It is based on time-ordered perturbation theory and includes full meson retardation in potentials and exchange currents as well as loop contributions to the nucleonic one-body current. Results for $NN$ scattering and deuteron photodisintegration are presented.