The evaluation of V-ud and its impact on the unitarity of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix

The determination of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element V-ud is reviewed. Data from 0(+) -> 0(+) superallowed beta decay in nuclei, neutron decay, beta decay of odd-mass mirror nuclei and pion beta decay are considered. Theoretical radiative and isospin-symmetry breaking corrections are applied. The most precise result comes from the nuclear 0(+) -> 0(+) decays, which yield a recommended value of vertical bar V-ud vertical bar = 0.974 25(22). We further summarize the data leading to the CKM matrix element V-us: K-l3 decays, K-l2 decays, hyperon decays and hadronic tau decay. Again SU(3)-symmetry breaking corrections (from lattice QCD) and radiative corrections are applied. We adopt values from K-l3 decay of vertical bar V-us vertical bar = 0.2246(12) and from K-l2 decay of vertical bar V-us/V-ud vertical bar = 0.2319(14). From the three data just cited, a least squares fit determines two CKM matrix elements: vertical bar V-ud vertical bar = 0.974 25(22) and vertical bar V-us vertical bar = 0.225 21(94). Data leading to the third member of the top row of the CKM matrix, Vub, are summarized as well but, being of order 10(-3), that matrix element contributes negligibly to the unitarity sum, vertical bar V-ud vertical bar(2) + vertical bar V-us vertical bar(2) + vertical bar V-ub vertical bar(2). We find this sum to be 0.999 90(60) showing unitarity to be satisfied to a precision of 0.06%. We discuss the constraints this result places on selected extensions to the standard model.