TESTS OF ISOSPIN MIXING CORRECTIONS IN SUPERALLOWED 0+-]0+ BETA-DECAYS

Measurements of superallowed 0+ --> 0+ nuclear beta decays, corrected for radiative and charge-dependent effects, yield fundamental properties of the weak interaction. Model predictions of the necessary corrections exist, but they have never been tested directly by experiment. The present work reports, for the first time, observations of (or limits on) nonanalog 0+ --> 0+ branches in K-38m, V-46, Mn-50, and Co-54, and tests of the model predictions for isospin-mixing corrections. The resulting unitarity test of the top row of the Kobayashi-Maskawa matrix is 0.9983 +/- 0.0015.