The Role of Meson Retardation in Deuteron Photodisintegration above Pion   Threshold
  Photodisintegration of the deuteron above $\pi$ threshold is studied in a coupled channel approach including $N \Delta$ and $\pi d$ channels with consideration of pion retardation in potentials and exchange currents. A much improved description of total and differential cross sections in the energy region between $\pi$ threshold and 400-450 MeV is achieved. With respect to polarization observables, the description of the linear photon asymmetry and the proton polarization remains problematic. 
Michael Schwamb and Hartmuth Arenhoevel