Argonne Physics Division Seminar - 17 Nov 2014

3:30 PM, Building 203, Auditorium

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 V_{ud}, the
leading diagonal element of the quark mixing matrix, the
Cabibbo-Kobayashi-Maskawa (CKM) matrix. Not only is this the most
precise determination of V_{ud}, 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 V_{ud} 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