Cabibbo-Kobayashi-Maskawa unitarity deficit reduction via finite nuclear size

We revisit the extraction of the |Vud| Cabibbo-Kobayashi-Maskawa (CKM) matrix element from the superallowed transition decay rate of Al26m→Mg26, focusing on finite nuclear size effects. The decay rate dependence on the Al26m charge radius is found to be four times higher than previously believed, necessitating precise determination. However, for a short-lived isotope of an odd Z element such as Al26m, radius extraction relies on challenging many-body atomic calculations. We performed the needed calculations, finding an excellent agreement with previous ones, which used a different methodology. This sets a new standard for the reliability of isotope shift factor calculations in many-electron systems. The Ft value obtained from our analysis is lower by 2.2σ than the corresponding value in the previous critical survey, resulting in an increase in |Vud|2 by 0.9σ. Adopting |Vud| from this decay alone reduces the CKM unitarity deficit by one standard deviation, irrespective of the choice of |Vus|.