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Nuclear Tests of the Standard Model : Vector Current Conservation and CKM Unitarity

J.C. Hardy, Cyclotron Institute, Texas A&M University, College Station, Texas 77843, USA
Lundi 2 Mai 2011 à 11h00 - Salle des Séminaires

Very precise measurements in nuclei can offer demanding tests of the Standard Model of particle physics. In particular, “superallowed” nuclear beta-decay between 0+ analogue states is a sensitive probe of the vector part of the weak interaction, and the measured strength (i.e. ft-value) of each such transition yields a direct measure of the vector coupling constant, GV. To date, the ft-values for thirteen 0+ —> 0+ transitions have been measured with 0.1% precision or better, and these results yield fully consistent values for GV, thus confirming the conservation of the vector current to a part in ten thousand. The resultant GV in turn yields an experimental value for Vud, the leading diagonal element of the quark mixing matrix, the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Not only is this the most precise determination of Vud, it is the most precise result for any element in the CKM matrix. The CKM matrix is a central pillar of the Standard Model and, although the model does not predict values for the matrix elements, it absolutely requires that the matrix be unitary. The experimental value for Vud obtained from superallowed beta-decay leads to the most demanding test available of CKM unitarity, a test which it passes with flying colors : the unitarity sum as determined from experiment is 0.9999 ± 0.0006. I will describe some current experiments in the field and overview the up-to-date results from world data.