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  • Letter
  • Open Access

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

Mikhail Gorchtein1,2,*, Vaibhav Katyal3,4, B. Ohayon5,†, B. K. Sahoo3,‡, and Chien-Yeah Seng6,7,8,§

  • *Contact author: gorshtey@uni-mainz.de
  • †Contact author: bohayon@technion.ac.il
  • ‡Contact author: bijaya@prl.res.in
  • §Contact author: cseng@utk.edu
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Phys. Rev. Research 7, L042002 – Published 1 October, 2025

DOI: https://doi.org/10.1103/z8g6-9j25

Abstract

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|.

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Phys. Rev. Research 7, L042002– Published 1 October, 2025
  • Received 6 March 2025
  • Revised 9 July 2025
  • Accepted 9 September 2025
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