Pygmy dipole resonances and

Collectivity of the low-lying dipole strength in relativistic random phase approximation

Giant isovector dipole resonances and the corresponding low-lying response are described in the framework of self-consistent relativistic RPA over the entire range of periodic table. Collectivity of excitations in the low energy region is investigated, in order to disclose its relation with collective soft dipole (Pygmy) resonances. For the light neutron rich nuclei, the low-lying peaks show no evidence of collectivity. When moving toward heavier nuclei, many neutron particle-hole configurations participate in the low lying energy region, and degree of collectivity increases. The corresponding relativistic RPA transition densities and velocity distributions display collective vibrations of the excess neutrons against the proton-neutron core.

• Introduction
• Isovector dipole excitations in the relativistic RPA
• Low-lying dipole strength in oxygen and calcium isotopes
• Onset of collective low-energy dipole resonances
• Conclusions
• Bibliography

Transparencies from the talk in the NATO Advanced Research Workshop "The Nuclear Many-Body Problem 2001", (Collective properties of the low energy dipole response in the relativistic RPA)
References:

1. D. Vretenar, N. Paar, P. Ring and G. A. Lalazissis, "Pygmy dipole resonances in relativistic random phase approximation", Phys. Rev. C 63, 047301 (2001). nucl-th/0009057

2. D. Vretenar, N. Paar, P. Ring and G. A. Lalazissis, "Collectivity of the low-lying dipole strength in relativistic random phase approximation", Nucl. Phys. A, 692 (3-4) 496-517 (2001). nucl-th/0101063

3. N. Paar, T. Niksic, D. Vretenar and P. Ring "Quasiparticle random phase approximation based on the relativistic Hartree-Bogoliubov model" , Phys. Rev. C 67, 034312 (2003). nucl-th/0212011
4. D. Vretenar, T. Niksic, N. Paar, and P. Ring, "Relativistic QRPA description of low-lying dipole strength in neutron-rich nuclei", Nucl. Phys. A 731, 281 (2004).
5. D. Vretenar, T. Niksic, P. Ring, N. Paar, G. A. Lalazissis, and P. Finelli, "Relativistic Hartree-Bogoliubov and QRPA description of exotic nuclear structure", Eur. Phys. J. A 20, 75 (2004).
6. N. Paar, T. Niksic, D. Vretenar and P. Ring, "Quasiparticle random phase approximation based on the relativistic Hartree-Bogoliubov model II: Nuclear spin and isospin excitations", Phys. Rev. C 69, 054303 (2004).nucl-th/040209 4
7. N. Paar, T. Niksic, D. Vretenar and P. Ring, "Isotopic dependence of the pygmy dipole resonance", Phys. Lett. B 606, 288 (2005). nucl-th/0404055
8. N. Paar, T. Niksic, D. Vretenar and P. Ring, "Relativistic description of exotic collective excitation phenomena in atomic nuclei", Int. J. Mod. Phys. E 14, 1 (2005). nucl-th/0407064
9. N. Paar, D. Vretenar, and P. Ring, "Proton electric pygmy dipole resonance", Phys. Rev. Lett. 94, 182501 (2005). nucl-th/0504035
10. N. Paar, P. Papakonstantinou, V. Yu. Ponomarev, and J. Wambach, "Low-energy dipole excitations towards the proton drip-line: doubly magic 48Ni", submitted to Phys. Lett. B (2005). nucl-th/0506010

Nils Paar, 2003.