Shape of the E1 Strength Function in the Region of Pygmy Anomaly Due to Non Statistical Background Coupling

Systematic deviations from the Lorentzian approximation to the E1-giant dipole resonance (GDR) shape has been observed at the low energy side of the GDR /1,2/. The strength involved amounts to a few percent of the E1 sum rule,so this phenomenon is called pygmy resonance. A remarkable regularity has been mentioned by Lane /3/: The anomaly appears in mass number regions where empty single-particle states with l=0,1 are near the threshold. Applying the 1p-1h description the 1p-1h states containing the mentioned s or p-neutron particle states (threshold states) have unperturbed energies close to the energy of the anomaly. So these states may be saved from being coupled to the GDR. Such calculations /3,4,5/ performed in the pure 1p-1h shell model provide different amounts of the strength of the pygmy resonance as a function of the used residual interaction and basis.