Superallowed nuclear β decay: symmetry breaking, CVC and CKM unitarity

Currently, the most restrictive test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix is anchored by nuclear beta decay. Precise measurements of the ft-values for superallowed beta transitions between analog 0+ states are used to determine GV, the vector coupling constant; this, in turn, yields Vud, the up-down element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. When the value for Vud obtained from the latest survey of world data is combined with Vus and Vub, the other top-row elements, it leads to the most demanding test available of the unitarity of that matrix. Important ingredients required in the extraction of Vud from experiment are calculated radiative corrections as well as corrections to account for the isospin symmetry breaking that occurs between the analog parent and daughter states of each superallowed transition. The latter are well supported by comparison with experiment. The current value for the unitarity sum is 0.99995(61), a result that is in remarkable agreement with Standard Model expectations.