Extraction of Vud from superallowed Fermi β decay by the Wilkinson techniques

The up-down element Vud of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix is derived from superallowed Fermi β decays between isospin T = 1 states using the semi-empirical techniques pioneered by D.H. Wilkinson. Rather than relying directly on the absolute magnitude of theoretical nuclear structure calculations to correct each decay for isospin symmetry breaking effects, these methods extrapolate the experimental ft values to the charge independent limit at Z≈0 where isospin-dependent effects are negligible. The value Vud = 0.97430(29) derived in this work agrees with the value Vud = 0.97425(22) obtained from the most recent shell-model calculations of isospin symmetry breaking that have expanded the shell-model spaces by including select core orbitals. These values are not in agreement with previous shell-model calculations in more restrictive model spaces or the values obtained from recent self-consistent relativistic Hartree and Hartree-Fock calculations based on the random-phase approximation that all underestimate the magnitude of isospin symmetry breaking.