Experimental Validation of the Largest Calculated Isospin-Symmetry-Breaking Effect in a Superallowed Fermi Decay

TL;DR

This study experimentally confirms the largest calculated isospin-symmetry-breaking effect in a superallowed Fermi decay, providing critical validation for theoretical models and impacting Standard Model tests.

Contribution

It provides the first experimental validation of a large isospin-symmetry-breaking effect in superallowed Fermi decay, supporting shell-model calculations.

Findings

Measured isospin-symmetry breaking delta_C=5.3(9)%

Confirmed theoretical shell-model predictions

Set a benchmark for symmetry-breaking calculations

Abstract

A precision measurement of the gamma yields following the beta decay of 32Cl has determined its isobaric analogue branch to be (22.47^{+0.21}_{-0.19})%. Since it is an almost pure Fermi decay, we can also determine the amount of isospin-symmetry breaking in this superallowed transition. We find a very large value, delta_C=5.3(9)%, in agreement with a shell-model calculation. This result sets a benchmark for isospin-symmetry-breaking calculations and lends support for similarly-calculated, yet smaller, corrections that are currently applied to 0+ -> 0+ transitions for tests of the Standard Model.