The β Decay of ³⁸Ca: Sensitive Test of Isospin Symmetry-Breaking Corrections from Mirror Superallowed 0⁺ → 0⁺ Transitions

Abstract

We report the first branching-ratio measurement of the superallowed 0 + → 0 + β-transition from 38 Ca. The result, 0.7728(16), leads to an f t value of 3062.3(68)s with a relative precision of ±0.2%. This makes possible a high-precision comparison of the f t values for the mirror superallowed transitions, 38 Ca → 38m K and 38m K → 38 Ar, which sensitively tests the isospin symmetry-breaking corrections required to extract V ud , the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, from superallowed β decay. The result supports the corrections currently used, and points the way to even tighter constraints on CKM unitarity.

$|V_{ud}|$ from nuclear $\beta$ decays

“…The second test involves the measurement of mirror pairs of superallowed transitions. It turns out that the ratio of mirror f t values offers a particularly sensitive test of the correction terms, but it is only recently that the first mirror pair, 38 Ca → 38m K and 38m K → 38 Ar, has been precisely measured [12]. This result confirmed the same set of calculated corrections [6] as the first test.…”