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

We report the first branching-ratio measurement of the superallowed 0(+) -> 0(+)beta transition from Ca-38. The result, 0.7728(16), leads to an ft value of 3062.3(68) s with a relative precision of +/- 0.2%. This makes possible a high-precision comparison of the ft values for the mirror superallowed transitions, Ca-38 -> (38m) K and K-38m -> Ar-38, 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 beta decay. The result supports the corrections currently used and points the way to even tighter constraints on CKM unitarity.