Superallowed 0+ --> 0+ nuclear beta decays: 2014 critical survey, with precise results for Vud and CKM unitarity

Abstract

A new critical survey is presented of all half-life, decay-energy and branching-ratio measurements related to 20 superallowed 0 + → 0 + β 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in 2008, we have added results from 24 new publications and eliminated 9 references, the results from which having been superceded by much more precise modern data. We obtain world-average f t-values for each of the eighteen transitions that have a complete set of data, then apply radiative and isospinsymmetry-breaking corrections to extract "corrected" Ft values. Fourteen of these Ft 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 Ft values consistent with conservation of the vector current (CVC). Only one set of calculations satisfactorily meets this condition. The resultant average Ft value, when combined with the muon liftime, yields the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, V ud = 0.97417 ± 0.00021. The unitarity test on the top row of the matrix becomes0.99978 ± 0.00055 if the Particle Data Group recommended value for Vus 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 Vus/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 Ft-value data we also set limits on the possible existence of scalar interactions.

Experimental determination of $V_{us}$ from kaon decays

“…The test of CKM unitarity requires a value for V ud . A preliminary update of the survey of experimental data on 0 + → 0 + β decays from Hardy and Towner [19] was presented at this conference, giving V ud = 0.97420( 21) [20]. The update includes a few new measurements, including BR and Q EC measurements for 14 O.…”

Leptonic decay-constant ratio $f_K/f_\pi$ from clover-improved $N_f=2+1$ QCD

“…If one assumes first-row unitarity V 2 ud +V 2 us + |V ub | 2 = 1 of the CKM-matrix together with the result from [17] that |V ub | = 4.12(37)(06) • 10 −3 is very small compared to the other two matrix elements, we obtain from our result, Eq. (3.4), V ud = 0.9736(04) stat (11) syst (01) exp , V us = 0.2281 (18) stat (48) syst (03) exp .…”

High precision mass measurements of the isomeric and ground states of $^{44}$V: improving constraints on the IMME parameters of the A=44, $\text{0}^{\text{+}}$ quintet

“…This would have consequences for nuclear astrophysics (e.g., for the rp-process). Another important application of such interactions is the calculation of nuclear structure corrections to superallowed Fermi β-decay between isobaric analogue states, used for the tests of the CVC hypothesis and the CKM matrix unitarity tests [21,23,24].…”