Testing CVC and CKM Unitarity via superallowed nuclear beta decay

Abstract

Currently, the most restrictive test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix is anchored by nuclear beta decay. Precise measurements of the ft-values for superallowed beta transitions between analog 0⁺ states are used to determine G_V, the vector coupling constant; this, in turn, yields V_ud, the up-down quark-mixing element of the CKM matrix. 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 0.1% precision on the final result, each of these quantities must be measured to substantially better precision, for which special techniques have had to be developed. A new survey and analysis of world data reveals that there are now fourteen such transitions with ft-values known to ∼ 0.1% precision or better, and that they span a wide range of nuclear masses, from ¹⁰C, the lightest parent, to ⁷⁴Rb, the heaviest. Of particular interest is the recent completion of the first mirror pair of 0⁺ → 0⁺ transitions, ³⁸Ca → ^38mK and ^38mK → ³⁸Ar, which provides a valuable constraint on the calculated isospin-symmetry-breaking corrections needed to derive G_V from the experimental data. As anticipated by the Conserved Vector Current hypothesis, CVC, all fourteen transitions yield consistent values for G_V. The value of V_ud derived from their average makes it by far the most precisely known element of the CKM matrix, which, when combined with the other top-row elements, V_us and V_ub, leads to the most demanding test available of the unitarity of that matrix. Since CKM unitarity is a key pillar of the Electroweak Standard Model, this test is of fundamental significance.