Isoscalar dipole coherence at low energies and forbidden E1 strength

TL;DR

This paper investigates the isoscalar low-energy dipole (IS-LED) mode in N=Z nuclei, revealing its collective nature, the role of Coulomb interaction in E1 strength, and implications for nuclear structure models.

Contribution

It demonstrates the existence and characteristics of the IS-LED mode across N=Z nuclei using self-consistent RPA calculations, highlighting the influence of Coulomb interaction and model dependencies.

Findings

IS-LED mode exists in N=Z nuclei and is collective.

E1 strength arises solely from Coulomb-induced isospin-symmetry breaking.

Model predictions vary, affecting the accuracy of IS-LED and E1 strength estimates.

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.