Tests of isospin mixing corrections in superallowed 0+→0+ β decays

Measurements of superallowed 0+→0+ nuclear β 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 38mK, 46V, 50Mn, and 54Co, and tests of the model predictions for isospin-mixing corrections. The resulting unitarity test of the top row of the Kobayashi-Mashawa matrix is 0.9983±0.0015