Superallowed 0+ → 0+ nuclear β decays: 2014 critical survey, with precise results for Vud and CKM unitarity

A new critical survey is presented of all half-life, decay-energy, and branching-ratio measurements related to 20 superallowed 0(+)-> 0(+) beta decays. Included are 222 individual measurements of comparable precision obtained from 177 published references. Compared with our last review in 2008, we have added results from 24 new publications and eliminated 9 references, the results from which having been superseded by much more precise modern data. We obtain world-average f t values for each of the 18 transitions that have a complete set of data, then apply radiative and isospin-symmetry-breaking corrections to extract corrected F t values. Fourteen of these F t values now have a precision of order 0.1% or better. In the process of obtaining these results we carefully evaluate the available calculations of the isospin-symmetry-breaking corrections by testing the extent to which they lead to F t values consistent with conservation of the vector current. Only one set of calculations satisfactorily meets this condition. The resultant average F t value, when combined with the muon lifetime, yields the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa matrix, V-ud = 0.97417 +/- 0.00021. The unitarity test on the top row of the matrix becomes vertical bar V-ud vertical bar(2) + vertical bar V-us vertical bar(2) + vertical bar V-ub vertical bar(2) = 0.99978 +/- 0.00055 if the Particle Data Group (PDG) recommended value for V-us is used. However, recent lattice QCD calculations, not included yet in the PDG evaluation, have introduced some inconsistency into kaon-decay measurements of Vus and V-us/V-ud. We examine the impact of these new results on the unitarity test and conclude that there is no evidence of any statistically significant violation of unitarity. Finally, from the F t-value data we also set limits on the possible existence of scalar interactions.