Superallowed0+→0+nuclearβdecays: 2014 critical survey, with precise results forVud

Abstract

Superallowed 0 + → 0 + nuclear β decays: 2014 critical survey, with precise results for V ud 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 + 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in 2008, we have added results from 24 new publications and eliminated 9 references, the results from which having been superceded by much more precise modern data.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 becomes |V ud | 2 + |Vus| 2 + |V ub | 2 = 0.99978 ± 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.

Global effective-field-theory analysis of new-physics effects in (semi)leptonic kaon decays

“…The most accurate value for |Ṽ e ud | is obtained from superallowed nuclear β transitions, in an analysis that also sets the most stringent limits on the non-standard scalar Wilson coefficient de S (via the Fierz interference term b F ) [47,74]. Combining this |Ṽ e ud | determination with the |Ṽ e us | value obtained from K e3(γ) allows one to test CKM unitarity, which…”

“…That allows one to combine K e3 and K µ3 SM extractions ofṼ us and to use the SM analysis of ref. [74] for V ud , cf. eq.…”

“…where we have simply used that the total phase space integral is given by • Nuclear β decays: current studies of superallowed nuclear transitions contain a SM analysis where V ud is extracted, finding |Ṽ e ud | = 0.97417(21), and a NP analysis where the Fierz term b F is bounded, b F = −0.0028(26) [74]. In our framework we need a combined extraction of both quantities.…”

Precision half-life measurement of Al25

“…where F t 0 + →0 + = 3072.27(72) s [4] is the average value of the 14 most precisely known pure Fermi 0 + → 0 + superallowed transitions and f A is the axial-vector part of the statistical rate function. f A was evaluated using the parametrization found in [32] and found to be 520.583 (32).…”

“…Currently, the most precise unitarity test is performed by taking the sum of the magnitudes of the squares of the three matrix elements in the top row (V ud , V us , V ub ). Among the three elements entering the sum, only V ud and V us play critical roles for the unitarity test [4]. Even though the current best values for these elements are consistent with CKM unitarity [4], there are a number of experimental [5,6] and theoretical efforts [7,8] centered on ensuring the reliability of every component entering into the unitarity test, including the determination of V ud since its extracted value has shifted in the past due to more precise measurements [9].…”