New results in Superallowed Nuclear Beta Decay

Abstract

The value of the Vud matrix element of the Cabibbo‐Kobayashi‐Maskawa matrix can be derived from nuclear superallowed beta decays, neutron decay and pion beta decay. Today, the most precise value of Vud (0.05% precision) comes from the nuclear decays; and its precision is limited not by experimental error but by the estimated uncertainty in theoretical corrections, which themselves are of order 1%. When combined with the accepted values for Vus and Vub, the result differs at the 98% confidence limit from the unitarity condition for the CKM matrix. The focus of recent work on superallowed beta decay has been to test and refine the small nuclear‐structure‐dependent corrections so that the precision on the value of Vud can be improved and the unitarity test sharpened. A consistent set of calculated corrections has been produced not only for the nine superallowed transitions currently measured to high precision, but for a number of additional superallowed transitions in nuclei not previously accessible to exac...