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

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• Isoscalar monopole and dipole excitations of cluster states and giant resonances in ${}^{12}$C
• Low-energy dipole strength and the critical case of 48Ca
• Systematics of toroidal dipole modes in Ca, Ni, Zr, and Sn isotopes
• Toroidal, compressive, and $E1$ properties of low-energy dipole modes in ${}^{10}$Be
• Isoscalar and neutron modes in the E1 spectra of Ni isotopes and the relevance of shell effects and the continuum
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• Individual low-energy E1 toroidal and compression states in light nuclei: deformation effect, spectroscopy and interpretation
• Cluster correlation and nuclear vorticity in low-lying $1^{-}$ states of ${}^{24}$Mg
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• Excitation of the electric pygmy dipole resonance by inelastic electron scattering
• Proton pygmy resonances: predictions for N=20 isotones
• Low-energy dipole excitations in ${}^{20}$O with antisymmetrized molecular dynamics