Superallowed 0(+) -> 0(+) beta-decay from T-z =-1 sd-shell nuclei
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Superallowed nuclear beta-decay between 0+ analogue states probes the vector part of the weak interaction, with the measured ft-value of each such transition leading to a value for the vector coupling constant, Gv. To date, the ft-values for thirteen 0+ 0+ transitions have been measured with 0.1% precision or better. The results yield fully consistent values for Gv and an experimental value for V ud, which is the leading diagonal element of the quark mixing matrix, the Cabibbo-Kobayashi-Maskawa (CKM) matrix. With a precise value for V ud established, the unitarity of the CKM matrix can be tested and limits set on the possibility of new physics lying beyond the Standard Model. This work demands high experimental precision and a high degree of reliability in the small theoretical correction terms required to extract Gv and Vud from the experimental data. One of the correction terms must account for isospin symmetry-breaking between the parent and daughter nuclei, and its associated uncertainty contributes significantly to the uncertainty in Vud. The superallowed decays of Tz = -1 sd-shell nuclei, such as 22Mg, 26Si, 34Ar and 38Ca, have so far not played a significant role in the determination of Vud because experimental challenges have made high precision unattainable for these transitions. However, if they were to be measured precisely, they would enable important tests of the calculated isospin symmetry-breaking corrections and potentially reduce the latter's uncertainties. Experiments aimed at characterizing these transitions are described. Published under licence by IOP Publishing Ltd.
