Nuclear beta decays and CKM unitarity

TL;DR

Precise measurements of nuclear beta decays provide a highly accurate value for the Vud element of the CKM matrix, confirming the consistency of theoretical corrections and supporting the unitarity of the matrix.

Contribution

This paper presents the most precise determination of Vud from superallowed nuclear beta decays, validating theoretical correction terms and confirming CKM unitarity.

Findings

Vud value is 0.97420(21) with 0.1% precision

Consistency among 14 superallowed transitions supports correction validity

Results align with less precise neutron and pion decay measurements

Abstract

Nuclear beta decays between (J^pi,T) = (0^+,1) analog states yield the best value for the Vud element of the Cabibbo-Kobayashi-Maskawa matrix. Current world data establish the corrected Ft values of 14 separate superallowed transitions to a precision of order 0.1% or better. The validity of the small theoretical correction terms is confirmed by excellent consistency among the 14 Ft values and by recent measurements that compare pairs of mirror superallowed transitions. With consistency established, the results now yield |Vud| = 0.97420(21). This value is consistent with the considerably less precise results obtained from beta decays of the neutron, the pion and T=1/2 mirror nuclei, which are hampered by experimental challenges.