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Register a new userA new analysis of 14O beta decay: branching ratios and CVC consistency
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The ground-state Gamow-Teller transition in the decay of 14O is strongly hindered and the electron spectrum deviates markedly from the allowed shape. A reanalysis of the only available data on this spectrum changes the branching ratio assigned to this transition by seven standard deviations: our new result is (0.54 pm 0.02)%. The Kurie plot data from two earlier publications are also examined and a revision to their published branching ratios is recommended. The required nuclear matrix elements are calculated with the shell model and, for the first time, consistency is obtained between the M1 matrix element deduced from the analog gamma transition in 14N and that deduced from the slope of the shape-correction function in the beta transition, a requirement of the conserved vector current hypothesis. This consistency is only obtained, however, if renormalized rather than free-nucleon operators are used in the shell-model calculations. In the mirror decay of 14C a similar situation occurs. Consistency between the 14C lifetime, the slope of the shape-correction function and the M1 matrix element from gamma decay can only be achieved with renormalized operators in the shell-model calculation.
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Beta-decay branching ratios of 62Ga have been measured at the IGISOL facility of the Accelerator Laboratory of the University of Jyvaskyla. 62Ga is one of the heavier Tz = 0, 0+ -> 0+ beta-emitting nuclides used to determine the vector coupling constant of the weak interaction and the Vud quark-mixing matrix element. For part of the experimental studies presented here, the JYFLTRAP facility has been employed to prepare isotopically pure beams of 62Ga. The branching ratio obtained, BR= 99.893(24)%, for the super-allowed branch is in agreement with previous measurements and allows to determine the ft value and the universal Ft value for the super-allowed beta decay of 62Ga.
We have measured the beta-decay branching ratio for the transition from 21Na to the first excited state of 21Ne. A recently published test of the standard model, which was based on a measurement of the beta-nu correlation in the decay of 21Na, depended on this branching ratio. However, until now only relatively imprecise (and, in some cases, contradictory) values existed for it. Our new result, 4.74(4)%, reduces but does not remove the reported discrepancy with the standard model.
We present the full description of a measurement of the branching ratios for the beta-decay of 38Ca. This decay includes five allowed 0+ --> 1+ branches and a superallowed 0+ --> 0+ one. With our new result for the latter, we determine its ft value to be 3062.3(68) s, a result whose precision (0.2%) is comparable to the precision of the thirteen well known 0+ --> 0+ transitions used up till now for the determination of Vud, the up-down quark-mixing element of the CKM matrix. The 38Ca superallowed transition thus becomes the first addition to this set of transitions in nearly a decade and the first for which a precise mirror comparison is possible, thus enabling an improved test of the isospin-symmetry-breaking corrections required for the extraction of Vud.
Based on measurements the branching ratios for the decay of the recently discovered dibaryon resonance $d^*(2380)$ into two-pion production channels and into the $np$ channel are evaluated. Possibilities for a decay into the isoscalar single-pion channel are discussed. Finally also the electromagnetic decay of $d^*(2380)$ is considered.
A complete and critical survey is presented of all half-life, decay-energy and branching-ratio measurements related to 20 superallowed decays; no measurements are ignored, though some are rejected for cause and others updated. A new calculation of the statistical rate function is described and experimental ft values determined. The associated theoretical corrections needed to convert these results into Ft values are discussed, and careful attention is paid to the origin and magnitude of their uncertainties. As an exacting confirmation of the conserved vector current hypothesis, the Ft values are seen to be constant to 3 parts in 10^4. These data are also used to set new limits on any possible scalar interactions or right-hand currents. The average Ft value obtained from the survey, when combined with the muon lifetime, yields the CKM matrix element Vud = 0.9738(4); and the unitarity test on the top row of the matrix becomes |Vud|^2 + |Vus|^2 + |Vub|^2 = 0.9966(14) using the PDGs currently recommended values for Vus and Vub. We discuss the priorities for future theoretical and experimental work with the goal of making the CKM unitarity test more definitive.
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