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 fourteen 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 demands that the matrix itself 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 of the top-row elements as determined from experiment is 0.9999 ± 0.0005.
The determination of a transition’s ft-value requires the measurement of three quantities: its Q-value, branching ratio and parent half-life. To achieve the 0.1% precision obtained for the superallowed transitions, each of these quantities had to be measured to substantially better precision, a challenging standard which has led to special techniques being developed. I will describe some current experiments in the field, and overview the up-to-date results from a new 2014 survey of world data.
Argonne Physics Division Seminar Schedule