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 GV, the vector coupling constant; this, in turn, yields Vud, 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 GV from the experimental data. As anticipated by the Conserved Vector Current hypothesis, CVC, all fourteen transitions yield consistent values for GV. The value of Vud 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, Vus and Vub, 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.
Highlights
Superallowed beta decay between nuclear analogue states with T = 1 and Jπ = 0+ occurs only via the vector current of the weak interaction: angular momentum conservation completely rules out the axial-vector current, which must carry off a spin of one and cannot connect two states that both have spin zero
Since the parent and daughter states are analogues of one another, the strength of the transition is affected only by the small difference between the parent and daughter configurations resulting from isospin symmetry breaking, not by the dominant nuclear structure common to them both
Superallowed β-decay yields the most precise value for Vud and the most exacting test of CKM unitarity, with a precision of 0.06% on the latter [1, 2]. This precision can be expected to improve further as a result of decay measurements that focus on defining the effects of isospin-symmetry breaking between the analogue parent and daughter states in each superallowed transition
Summary
Superallowed β-decay yields the most precise value for Vud and the most exacting test of CKM unitarity, with a precision of 0.06% on the latter [1, 2]. This precision can be expected to improve further as a result of decay measurements that focus on defining the effects of isospin-symmetry breaking between the analogue parent and daughter states in each superallowed transition. With their likely addition in the near future, even more stringent constraints can be placed on the calculated isospin-symmentry-breaking correction terms, constraints that will in turn result in a reduced uncertainty on Vud
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