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 image states are used to determine image, the vector coupling constant; this, in turn, yields image, the up-down element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. When the value for image obtained from the latest survey of world data is combined with image and image, 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 image 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.