Texas A&M University
Testing CVC and CKM unitarity via superallowed nuclear beta decay
Cyclotron Institute, Texas A&M University, College Station, Texas 77843, USA
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 → 38Km and 38Km → 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.
Priorities for the future are: 1) Reduce the uncertainty of the calculated ΔR radiative correction; 2) test and refine the calculated nuclear-structure-dependent corrections by targeted experiments; 3) if theoretical improvements are realized, improve the experimental precision on the most prolifically produced superallowed transitions; and 4) examine isospin mixing in heavier N~Z nuclei via superallowed beta decay.