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Register a new userMass measurements and superallowed beta decay
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J.C. Hardy
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A recent Penning-trap measurement of the masses of 46V and 46Ti leads to a Qec value that disagrees significantly with the previously accepted value, and destroys overall consistency among the nine most precisely characterized T=1 superallowed beta emitters. This raises the possibility of a systematic discrepancy between Penning-trap measurements and the reaction-based measurements upon which the Qec values depended in the past. We carefully re-analyze (n,gamma) and (p,gamma) reaction measurements in the 24 leq A leq 28 mass region, and compare the results to very precise Penning-trap measurements of the stable nuclei 24Mg, 26Mg and 28Si. We thus determine upper limits to possible systematic effects in the reaction results, and go on to establish limits for the mass of radioactive 26Al, to which future on-line Penning-trap measurements can be compared. We stress the urgency of identifying or ruling-out possible systematic effects.
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Superallowed $0^+ to 0^+$ nuclear beta decay provides a direct measure of the weak vector coupling constant, $GV$. We survey current world data on the nine accurately determined transitions of this type, which range from the decay of $^{10}$C to that of $^{54}$Co, and demonstrate that the results confirm conservation of the weak vector current (CVC) but differ at the 98% confidence level from the unitarity condition for the Cabibbo-Kobayashi-Maskawa (CKM) matrix. We examine the reliability of the small calculated corrections that have been applied to the data, and conclude that there are no evident defects although the Coulomb correction, $delta_C$, depends sensitively on nuclear structure and thus needs to be constrained independently. The potential importance of a result in disagreement with unitarity, clearly indicates the need for further work to confirm or deny the discrepancy. We examine the options and recommend priorities for new experiments and improved calculations. Some of the required experiments depend upon the availability of intense radioactive beams. Others are possible with existing facilities.
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B. E. Glassman (Department of Physics
, Astronomy
, Nationaln Superconducting Cyclotron Laboratory
2019
textbf{Background}: Superallowed $0^+ rightarrow 0^+$ $beta$ decays of isospin $T=2$ nuclides can be used to test theoretical isospin symmetry breaking corrections applied to extract the CKM matrix element $V_{ud}$ from $T = 0,1$ decays by measuring precise $ft$ values and also to search for scalar currents using the $beta- u$ angular correlation. Key ingredients include the $Q_{textrm{EC}}$ value and branching of the superallowed transition and the half life of the parent. textbf{Purpose}: To determine a precise experimental $Q_{textrm{EC}}$ value for the superallowed $0^+ rightarrow 0^+$ $beta$ decay of $T=2$ $^{20}$Mg and the intensity of $^{20}$Mg $beta$-delayed $gamma$ rays through the isobaric analog state in $^{20}$Na. textbf{Method}: A beam of $^{20}$Mg was produced using the in-flight method and implanted into a plastic scintillator surrounded by an array of high-purity germanium detectors used to detect $beta$-delayed $gamma$ rays. The high-resolution $gamma$-ray spectrum was analyzed to measure the $gamma$-ray energies and intensities. textbf{Results}: The intensity of $^{20}$Mg $beta$-delayed $gamma$ rays through the isobaric analog state in $^{20}$Na was measured to be $(1.60 pm 0.04_{textrm{stat}} pm 0.15_{textrm{syst}} pm 0.15_{textrm{theo}}) times 10^{-4}$, where the uncertainties are statistical, systematic, and theoretical, respectively. The $Q_{textrm{EC}}$ value for the superallowed transition was determined to be $4128.7 pm 2.2$ keV based on the measured excitation energy of $6498.4 pm 0.2_{textrm{stat}} pm 0.4_{textrm{syst}}$ keV and literature values for the ground-state masses of $^{20}$Na and $^{20}$Mg. textbf{Conclusions}: The $beta$-delayed $gamma$-decay branch and $Q_{textrm{EC}}$ value are now sufficiently precise to match or exceed the sensitivity required for current low-energy tests of the standard model.
We report new shell-model calculations of the isospin-symmetry-breaking correction to superallowed nuclear beta decay. The most important improvement is the inclusion of core orbitals, which are demonstrated to have a significant impact on the mismatch in the radial wave functions of the parent and daughter states. We determine which core orbitals are important to include from an examination of measured spectroscopic factors in single-nucleon pick-up reactions. We also examine the new radiative-correction calculation by Marciano and Sirlin and, by a simple reorganization, show that it is possible to preserve the conventional separation into a nucleus-independent inner radiative term and a nucleus-dependent outer term. We tabulate new values for the three theoretical corrections for twenty superallowed transitions, including the thirteen well-studied cases. With these new correction terms the corrected Ft values for the thirteen cases are statistically consistent with one another and the anomalousness of the 46V result disappears. These new calculations lead to a lower average Ft value and a higher value of Vud. The sum of squares of the top-row elements of the CKM matrix now agrees exactly with unitarity.
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.
The measured $ft$-values for superallowed $0^{+} to 0^{+}$ nuclear $beta$-decay can be used to obtain the value of the vector coupling constant and thus to test the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. An essential requirement for this test is accurate calculations for the radiative and isospin symmetry-breaking corrections that must be applied to the experimental data. We present a new and consistent set of calculations for the nuclear-structure-dependent components of these corrections. These new results do not alter the current status of the unitarity test -- it still fails by more than two standard deviations -- but they provide calculated corrections for eleven new superallowed transitions that are likely to become accessible to precise measurements in the future. The reliability of all calculated corrections is explored and an experimental method indicated by which the structure-dependent corrections can be tested and, if necessary, improved.
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