Precision $Q_{EC}$-value measurement of $^{23}$Mg for testing the CKM matrix unitarity*

We report a new direct measurement of the $^{23}$Mg $\beta^+$-decay transition energy $Q_{EC}$ using the TITAN Penning trap mass spectrometer. This value agrees with the latest atomic mass evaluation while being four times more precise. The increase in precision changes the uncertainty contribution of the $Q_{EC}$-value on the statistical rate function $f_v$ from 11$\%$ to 0.6$\%$, an improvement by a factor of 18. This enables a more robust determination of the corrected $Ft$-value of this mirror transition to the required precision, making possible further test of the CKM matrix unitarity.