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Determination of fundamental couplings by \beta decay


\beta-decay: The searches for deviations from the unitarity condition of the CKM quark-mixing matrix provide very stringent tests of the Standard Model of electroweak interactions, which could point to the presence of new physics. So far, high-precision measurements of super-allowed (Fermi) \beta-decay properties provide the most precise determination of the Vud matrix element leading to the most stringent unitarity test of the CKM matrix. Pure Fermi transitions are intrinsically simple and can therefore be precisely described by theory. However, as these decays take place in the nuclear medium, corrections are necessary for a comparison between the theoretical predictions and experimental results. The relative precision needed for the ft values of the \beta decays to permit a meaningful comparison between theory and experiment is about 10-3 - 10-4. This requires the \beta-decay half-life and the super-allowed branching ratio to be measured with this precision, whereas the \beta-decay Q value requires 10-8. Such a goal has been achieved to date for 13 nuclei and needs to be extended to other nuclei for an improved understanding of the theoretical corrections. New data for lighter nuclei is also of great importance for improved tests of the electro-weak interaction. In order to perform high-precision measurements on these nuclei, one needs to produce about 1000 particles per second in rather pure conditions, which should be within reach for the DESIR facility.