Standard-model tests with superallowed β-decay:: An important application of very precise mass measurements

Superallowed beta -decay provides a sensitive means for probing the limitations of the Electroweak Standard Model. To date, the strengths (ft-values) of superallowed 0+-->0+ beta -decay transitions have been determined with high precision from nine different short-lived nuclei, ranging from C-10 to Co-54. Each result leads to an independent measure for the vector coupling constant G(V) and collectively the nine values can be used to test the conservation of them weak vector current (CVC). Within current uncertainties, the results support CVC to better than a few parts in 10,000-a clear success for the Standard Model! However, when the average value of G(V), as. determined in this way, is combined with data from decays of the muon and kaon to test another prediction of the Standard Model, the result is much more provocative. A test of the unitarity of the Cabibbo-Kobayashi-Maskawa matrix fails by more than two standard deviations. This result can be made more definitive by experiments that require extremely precise mass measurements, in some cases on very short-lived (less than or equal to 100 ms) nuclei. This talk presents the current status and future prospects for these Standard-Model tests, emphasizing the role of precise mass, or mass-difference measurements. There remains a real challenge to mass-measurement technique with the opportunity for significant new results.