Low Lying Electric Dipole Excitations and the Interacting Boson Model

Abstract

The idea of the Interacting Boson Approximation (IBA) by Arima and Iachello simplified the description of low lying collective states enormously [1, 2]. Now it became relatively easy to reproduce the experimental observables in a wide variety of nuclei ranging from spherical to well deformed [3]. Further improvement was achieved when proton- and neutron-bosons were distinguished explicitly in the 1BA-2 [4]. In the early eighties Iachello used the IBA-2 to predict a new class of states which are not fully symmetric under the exchange of protons and neutrons. In axially deformed nuclei the lowest lying member of these states should be a collective 1⁺-state around 2 – 4 MeV excitation energy [5]. The experimental finding of this so called “scissors mode” in electron scattering experiments by a group around A. Richter from Darmstadt [6] was an impressive proof for the power of the IBA-2. Whereas the original IBA-models with s- and d-bosons (representing bosons with J^π=0⁺ and J^π=2⁺) were only capable of describing positive parity states, the model can be extended in a natural way to allow the description of negative parity states by including a p-boson with J^π=1^- and a f-boson with J^π=3^- [7–10] The investigation of negative parity states will be the subject of this contribution.