Isoscalar dipole coherence at low energies and forbidden E1 strength

Abstract

In 16O and 40Ca an isoscalar, low-energy dipole transition (IS-LED) exhausting approximately 4% of the isoscalar dipole (ISD) energy-weighted sum rule is experimentally known, but conspicuously absent from recent theoretical investigations of ISD strength. The IS-LED mode coincides with the so-called isospin-forbidden E1 transition. We report that for N=Z nuclei up to 100Sn the fully self-consistent Random-Phase-Approximation with finite-range forces, phenomenological and realistic, yields a collective IS-LED mode, typically overestimating its excitation energy, but correctly describing its IS strength and electroexcitation form factor. The presence of E1 strength is solely due to the Coulomb interaction between the protons and the resulting isospin-symmetry breaking. The smallness of its value is related to the form of the transition density, due to translational invariance. The calculated values of E1 and ISD strength carried by the IS-LED depend on the effective interaction used. Attention is drawn to the possibility that in N-not-equal-Z nuclei this distinct mode of IS surface vibration can develop as such or mix strongly with skin modes and thus influence the pygmy dipole strength as well as the ISD strength function. In general, theoretical models currently in use may be unfit to predict its precise position and strength, if at all its existence.

9 pages, 6 figures, EPJA submitted

References in corpus (10)

• Exotic modes of excitation in atomic nuclei far from stability
• Isospin Character of the Pygmy Dipole Resonance in 124Sn
• Nuclear Structure in the Framework of the Unitary Correlation Operator Method
• Self-consistent description of multipole strength: systematic calculations
• Second RPA and correlated realistic interactions
• Isospin mixing in nuclei within the nuclear density functional theory
• Evolution of the pygmy dipole resonance in nuclei with neutron excess
• Microscopic description of the pygmy and giant electric dipole resonances in stable Ca isotopes
• Relativistic QRPA description of isoscalar compression modes in open-shell nuclei in the A$\approx$60 mass region
• Neutron-Proton Radii in N \approx Z Nuclei

Cited by in corpus (1)

• Quasiparticle Random Phase Approximation with Interactions from the Similarity Renormalization Group