Low-energy dipole strength and the critical case of 48Ca

TL;DR

This study uses QRPA with various interactions to analyze low-energy dipole strength in calcium isotopes, highlighting discrepancies in neutron-skin mode predictions and proposing experiments to clarify the nature of these excitations.

Contribution

It provides a comprehensive QRPA analysis of all even calcium isotopes and suggests experimental approaches to resolve conflicting theoretical predictions.

Findings

QRPA predicts low-energy dipole transitions in all calcium isotopes.

Gogny D1S interaction accounts for (gamma,gamma') data up to N=28.

Neutron-skin oscillation is predicted for N≥30, contradicting some models.

Abstract

Recent theoretical work has not led to a consensus regarding the nature of the low-energy E1 strength in the 40,44,48Ca isotopes, for which high-resolution (gamma,gamma') data exist. Here we revisit this problem using the first-order quasi-particle random-phase approximation (QRPA) and different interactions. First we examine all even Ca isotopes with N=14-40. All isotopes are predicted to undergo dipole transitions at low energy, of large and comparable isoscalar strength but of varying E1 strength. Provided a moderate and uniform energetic shift is introduced to the results, QRPA with the Gogny D1S interaction is able to account for the (gamma,gamma') data, because, up to N=28, it yields a rather pure isoscalar oscillation. A neutron-skin oscillation is anticipated for N larger or equal to 30. This contradicts existing predictions that 44,48Ca develop a neutron-skin mode. Which theoretical result is correct cannot be resolved conclusively using the available data. We propose that alpha-scattering, possibly followed by an electroexcitation experiment, could resolve the situation and thereby help to improve the different models aspiring to describe reliably the low-energy dipole strength of nuclei.