Analog-antianalog isospin mixing in $^{47}$K $\beta^-$ decay

TL;DR

This study measures isospin mixing in the decay of $^{47}$K, revealing significant Coulomb matrix elements that support models of analog-antianalog isospin mixing and suggest new avenues for exploring time-reversal symmetry violations.

Contribution

First measurement of isospin mixing in $^{47}$K decay using atom trap techniques, providing evidence for large Coulomb matrix elements supporting existing theoretical models.

Findings

Measured nonzero asymmetry indicating isospin mixing.

Estimated Coulomb matrix element magnitude of 101 ± 37 keV.

Supports models predicting large isospin mixing in nuclei with excess neutrons.

Abstract

We have measured the isospin mixing of the I$^\pi$= 1/2$^+$ E$_x$=2.599 MeV state in nearly-doubly-magic $^{47}$Ca with the isobaric analog 1/2$^+$ state of $^{47}$K. Using the TRIUMF atom trap for $\beta$ decay, we have measured a nonzero asymmetry of the progeny $^{47}$Ca with respect to the initial $^{47}$K spin polarization, which together with the $\beta$ asymmetry implies a nonzero ratio of Fermi to Gamow-Teller matrix elements $y$= 0.098$\pm$0.037 for the $1/2^+ \rightarrow 1/2^+$ transition. Interpreting $y$ as mixing between this state and the isobaric analog state implies a Coulomb matrix element magnitude 101 $\pm$ 37 keV. This relatively large matrix element supports a model from the literature of analog-antianalog isospin mixing, which predicts large matrix elements in cases involving excess neutrons over protons occupying more than one major shell. The result supports pursuing a search for time-reversal odd, parity-even, isovector interactions using a correlation in $^{47}$K $\beta$ decay.