Symmetry breaking of Gamow-Teller and magnetic-dipole transitions and its restoration in calcium isotopes

TL;DR

This paper investigates the relationship and symmetry between M1 and Gamow-Teller transitions in calcium isotopes using a relativistic energy-density functional approach, revealing how residual interactions influence their symmetry and proposing restoration methods.

Contribution

It introduces a unified theoretical framework to analyze M1 and GT transitions in calcium isotopes, highlighting the role of residual interactions and proton-neutron pairing in symmetry restoration.

Findings

The IV-PV residual interaction disrupts M1/GT symmetry in closed-shell nuclei.

Proton-neutron pairing helps restore M1/GT symmetry in open-shell isotopes.

Adjusting pairing strength reproduces experimental GT excitation energies in $^{42}$Ca.

Abstract

Nuclear magnetic-dipole (M1) and Gamow-Teller (GT) transitions provide insight into the spin-isospin properties of atomic nuclei. By considering them as unified spin-isospin transitions, the M1/GT transition strengths and excitation energies are subject to isospin symmetry. The excitation properties associated to the M1/GT symmetry need to be clarified within consistent theoretical approach. In this work, the relationship between the M1 and GT transitions in Ca isotopes is investigated in a unified framework based on the relativistic energy-density functional (REDF) with point-coupling interactions, using the relativistic quasi-particle random-phase approximation (RQRPA). It is shown that the isovector-pseudovector (IV-PV) residual interaction affects both transitions, and the symmetry of M1 and giant-GT transitions is disrupted by this interaction in closed-shell nuclei. In open-shell Ca isotopes, the proton-neutron pairing in the residual RQRPA interaction also plays a role in GT transitions. Due to the interplay between these interactions, the M1/GT symmetry can be restored especially in the $^{42}$Ca nucleus, i.e., the giant-GT strength can become comparable to that of the M1 mode in terms of the unified spin-isospin transitions by adjusting the PN-pairing strength to reproduce the experimental low-lying GT-excitation energies. The mirror symmetry of both M1 and GT transitions is also demonstrated for open-shell mirror partners, $^{42}$Ca and $^{42}$Ti. Further improvements are required to achieve simultaneous reproduction of M1 and GT-transition energies in the REDF framework.