Testing isospin symmetry breaking in ab initio nuclear theory

TL;DR

This paper benchmarks isospin symmetry breaking in ab initio nuclear theory by calculating isobaric multiplet mass equation coefficients and comparing them to experimental data, revealing the current accuracy limits and the role of many-body effects.

Contribution

It introduces a first ab initio approach to assess isospin symmetry breaking in nuclear theory using the valence-space IM-SRG method with chiral Hamiltonians.

Findings

Overall agreement within 250 keV of experimental data for b and c coefficients.

Phenomenological Skyrme interaction yields higher accuracy.

Evolution of valence-space operators does not significantly improve results.

Abstract

In this work we present the first steps towards benchmarking isospin symmetry breaking in ab initio nuclear theory for calculations of superallowed Fermi $\beta$-decay. Using the valence-space in-medium similarity renormalization group, we calculate b and c coefficients of the isobaric multiplet mass equation, starting from two different Hamiltonians constructed from chiral effective field theory. We compare results to experimental measurements for all T=1 isobaric analogue triplets of relevance to superallowed $\beta$-decay for masses A=10 to A=74 and find an overall agreement within approximately 250 keV of experimental data for both b and c coefficients. A greater level of accuracy, however, is obtained by a phenomenological Skyrme interaction or a classical charged-sphere estimate. Finally, we show that evolution of the valence-space operator does not meaningfully improve the quality of the coefficients with respect to experimental data, which indicates that higher-order many-body effects are likely not responsible for the observed discrepancies.