Isospin-symmetry breaking in superallowed Fermi beta-decay due to isospin-nonconserving forces

TL;DR

This study uses large-scale shell-model calculations to analyze isospin-symmetry breaking in sd-shell nuclei, accurately describing displacement energies and predicting separation energies, but finds that these interactions do not explain anomalies in superallowed Fermi beta-decay.

Contribution

It introduces additional T=1,J=2 isospin nonconserving interactions to explain observed isospin-breaking effects in specific beta-decays.

Findings

Coulomb and triplet displacement energies are accurately described.

Predicted proton separation energies and drip-line locations.

Additional T=1,J=2 interactions explain observed isospin-breaking effects.

Abstract

We investigate isospin-symmetry breaking effects in the sd-shell region with large-scale shell-model calculations, aiming to understand the recent anomalies observed in superallowed Fermi beta-decay. We begin with calculations of Coulomb displacement energies (CDE's) and triplet displacement energies (TDE's) by adding the T=1,J=0 isospin nonconserving (INC) interaction into the usual isospin-invariant Hamiltonian. It is found that CDE's and TDE's can be systematically described with high accuracy. A total number of 122 one- and two-proton separation energies are predicted accordingly, and locations of the proton drip-line and candidates for proton-emitters are thereby suggested. However, attempt to explain the anomalies in the superallowed Fermi beta-decay fails because these well-fitted T=1,J=0 INC interactions are found no effects on the nuclear matrix elements. It is demonstrated that the observed large isospin-breaking correction in the 32Cl beta-decay, the large isospin-mixing in the 31Cl beta-decay, and the small isospin-mixing in the 23Al beta-decay can be consistently understood by introducing additional T=1,J=2 INC interactions related to the s1/2 orbit.