Pygmy dipole resonance: collective features and symmetry energy effects

Abstract

A very important open question related to the pygmy dipole resonance is about its quite elusive collective nature. In this paper, within a harmonic oscillator shell model, generalizing an approach introduced by Brink, we first identify the dipole normal modes in neutron rich nuclei and derive the energy weighted sum rule exhausted by the pygmy dipole resonance. Then solving numerically the self-consistent Landau-Vlasov kinetic equations for neutrons and protons with specific initial conditions, we explore the structure of the different dipole vibrations in the ${}^{132}Sn$ system and investigate their dependence on the symmetry energy. We evidence the existence of a distinctive collective isoscalar-like mode with an energy well below the Giant Dipole Resonance (GDR), very weakly dependent on the isovector part of the nuclear effective interaction. At variance the corresponding strength is rather sensitive to the behavior of the symmetry energy below saturation, which rules the number of excess neutrons in the nuclear surface.

5 pages, 4 figures; an additional section on the role of the symmetry energy upon EWSR exhausted by the pygmy mode is included

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Cited by in corpus (16)

• Nuclear Equation of State from ground and collective excited state properties of nuclei
• Information content of the low-energy electric dipole strength: correlation analysis
• Constraining simultaneously nuclear symmetry energy and neutron-proton effective mass splitting with nucleus giant resonances from a dynamical approach
• The electric dipole response of ${}^{76}$Se above 4 MeV
• Connecting the Pygmy Dipole Resonance to the neutron skin
• Pygmy and Giant Dipole Resonances by Coulomb Excitation using a Quantum Molecular Dynamics model
• Low-energy electric dipole response of Sn isotopes
• Dipole response in neutron-rich nuclei with new Skyrme interactions
• Interplay between low-lying isoscalar and isovector dipole modes: a comparative analysis between semi-classical and quantum approaches
• Nucleus giant resonances from an improved isospin-dependent Boltzmann-Uehling-Uhlenbeck transport approach
• Theoretical predictions of experimental observables sensitive to the symmetry energy: Results of the SMF transport model
• Nuclear excitations within microscopic EDF approaches : pairing and temperature effects on the dipole response
• Nuclear Symmetry Energy and Neutron Skin Thickness of ${}^{208}Pb$ using a finite range effective interaction
• Nuclear collective dynamics within Vlasov approach
• Improvements and Testing Practical Expressions for Photon Strength Functions of E1 Gamma-Transitions
• Reaction mechanisms in transport theories: a test of the nuclear effective interaction