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High Precision Measurement of the Superallowed 0^+ to 0^+ Beta Decay of ^{22}Mg  [PDF]
J. C. Hardy,V. E. Iacob,M. Sanchez-Vega,R. G. Neilson,A. Azhari,C. A. Gagliardi,V. E. Mayes,X. Tang,L. Trache,R. E. Tribble
Physics , 2003, DOI: 10.1103/PhysRevLett.91.082501
Abstract: The half-life, 3.8755(12) s, and superallowed branching ratio, 0.5315(12), for ^{22}Mg beta-decay have been measured with high precision. The latter depended on gamma-ray intensities being measured with an HPGe detector calibrated for relative efficiencies to an unprecedented 0.15%. Previous precise measurements of 0^+ to 0^+ transitions have been restricted to the nine that populate stable daughter nuclei. No more such cases exist, and any improvement in a critical CKM unitarity test must depend on precise measurements of more exotic nuclei. With this branching-ratio measurement, we show those to be possible for T_z = -1 parents. We obtain a corrected Ft-value of 3071(9) s, in good agreement with expectations.
First direct determination of the superallowed $β$-decay $Q_{EC}$-value for $^{14}$O  [PDF]
A. A. Valverde,G. Bollen,M. Brodeur,R. A. Bryce,K. Cooper,M. Eibach,K. Gulyuz,C. Izzo,D. J. Morrissey,M. Redshaw,R. Ringle,R. Sandler,S. Schwarz,C. S. Sumithrarachchi,A. C. C. Villari
Physics , 2015, DOI: 10.1103/PhysRevLett.114.232502
Abstract: We report the first direct measurement of the $^{14}\text{O}$ superallowed Fermi $\beta$-decay $Q_{EC}$-value, the last of the so-called "traditional nine" superallowed Fermi $\beta$-decays to be measured with Penning trap mass spectrometry. $^{14}$O, along with the other low-$Z$ superallowed $\beta$-emitter, $^{10}$C, is crucial for setting limits on the existence of possible scalar currents. The new ground state $Q_{EC}$ value, 5144.364(25) keV, when combined with the energy of the $0^+$ daughter state, $E_x(0^+)=2312.798(11)$~keV [Nucl. Phys. A {\bf{523}}, 1 (1991)], provides a new determination of the superallowed $\beta$-decay $Q_{EC}$ value, $Q_{EC}(\text{sa}) = 2831.566(28)$ keV, with an order of magnitude improvement in precision, and a similar improvement to the calculated statistical rate function $f$. This is used to calculate an improved $\mathcal{F}t$-value of 3073.8(2.8) s.
Superallowed 0+ --> 0+ nuclear beta decays: 2014 critical survey, with precise results for Vud and CKM unitarity  [PDF]
J. C. Hardy,I. S. Towner
Physics , 2014, DOI: 10.1103/PhysRevC.91.025501
Abstract: 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 ft-values for each of the eighteen transitions that have a complete set of data, then apply radiative and isospin-symmetry-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, Vud = 0.97417 +/- 0.00021. The unitarity test on the top row of the matrix becomes |Vud|^2 + |Vus|^2 + |Vub|^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/Vud. 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.
The measurement and interpretation of superallowed 0+ to 0+ nuclear beta decay  [PDF]
J. C. Hardy,I. S. Towner
Physics , 2013, DOI: 10.1088/0954-3899/41/11/114004
Abstract: Measurements of the decay strength of superallowed 0+ to 0+ nuclear beta transitions shed light on the fundamental properties of weak interactions. Because of their impact, such measurements were first reported 60 years ago in the early 1950s and have continued unabated ever since, always taking advantage of improvements in experimental techniques to achieve ever higher precision. The results helped first to shape the Electroweak Standard Model but more recently have evolved into sensitively testing that model's predictions. Today they provide the most demanding test of vector-current conservation and of the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. Here, we review the experimental and theoretical methods that have been, and are being, used to characterize superallowed 0+ to 0+ beta transitions and to extract fundamentally important parameters from their analysis.
New limit on fundamental weak-interaction parameters from superallowed beta decay  [PDF]
J. C. Hardy,I. S. Towner
Physics , 2004, DOI: 10.1103/PhysRevLett.94.092502
Abstract: A new critical survey of all world data on superallowed beta decays provides demanding tests of, and tight constraints on, the weak interaction. In confirmation of the conserved vector current (CVC) hypothesis, the vector coupling coupling constant is demonstrated to be constant to better than 3 parts in 10^4, and any induced scalar current is limited to fs \leq 0.0013 in electron rest-mass units. The possible existence of a fundamental scalar current is similarly limited to |Cs/Cv| \leq 0.0013. The superallowed data also determine the CKM matrix element to be Vud = 0.9738(4). With PDG values for Vus and Vub, the top-row test of CKM unitarity yields Vud^2 + Vus^2 + Vub^2 = 0.9966(14); although, if a recent result on Ke3 decay is used exclusively to obtain Vus, this sum becomes 0.9999(16). Limits on possible right-hand currents are given.
Isospin-breaking corrections to superallowed Fermi beta-decay in isospin- and angular-momentum-projected nuclear Density Functional Theory  [PDF]
W. Satula,J. Dobaczewski,W. Nazarewicz,T. R. Werner
Physics , 2012, DOI: 10.1103/PhysRevC.86.054316
Abstract: Background: The superallowed beta-decay rates provide stringent constraints on physics beyond the Standard Model of particle physics. To extract crucial information about the electroweak force, small isospin-breaking corrections to the Fermi matrix element of superallowed transitions must be applied. Purpose: We perform systematic calculations of isospin-breaking corrections to superallowed beta-decays and estimate theoretical uncertainties related to the basis truncation, time-odd polarization effects related to the intrinsic symmetry of the underlying Slater determinants, and to the functional parametrization. Methods: We use the self-consistent isospin- and angular-momentum-projected nuclear density functional theory employing two density functionals derived from the density independent Skyrme interaction. Pairing correlations are ignored. Our framework can simultaneously describe various effects that impact matrix elements of the Fermi decay: symmetry breaking, configuration mixing, and long-range Coulomb polarization. Results: The isospin-breaking corrections to the I=0+,T=1 --> I=0+,T=1 pure Fermi transitions are computed for nuclei from A=10 to A=98 and, for the first time, to the Fermi branch of the I,T=1/2 --> I,T=1/2 transitions in mirror nuclei from A=11 to A=49. We carefully analyze various model assumptions impacting theoretical uncertainties of our calculations and provide theoretical error bars on our predictions. Conclusions: The overall agreement with empirical isospin-breaking corrections is very satisfactory. Using computed isospin-breaking corrections we show that the unitarity of the CKM matrix is satisfied with a precision better than 0.1%.
The beta Decay of 32Cl: Precision gamma-Ray Spectroscopy and a Measurement of Isospin-Symmetry Breaking  [PDF]
D. Melconian,S. Triambak,C. Bordeanu,A. García,J. C. Hardy,V. E. Iacob,N. Nica,H. I. Park,G. Tabacaru,L. Trache,I. S. Towner,R. E. Tribble,Y. Zhai
Physics , 2011, DOI: 10.1103/PhysRevC.85.025501
Abstract: Background: Models to calculate small isospin-symmetry-breaking effects in superallowed Fermi decays have been placed under scrutiny in recent years. A stringent test of these models is to measure transitions for which the correction is predicted to be large. The decay of 32Cl decay provides such a test case. Purpose: To improve the gamma yields following the beta decay of 32Cl and to determine the ft values of the the beta branches, particularly the one to the isobaric-analogue state in 32S. Method: Reaction-produced and recoil-spectrometer-separated 32Cl is collected in tape and transported to a counting location where beta-gamma coincidences are measured with a precisely-calibrated HPGe detector. Results: The precision on the gamma yields for most of the known beta branches has been improved by about an order of magnitude, and many new transitions have been observed. We have determined 32Cl-decay transition strengths extending up to E_x~11 MeV. The ft value for the decay to the isobaric-analogue state in 32S has been measured. A comparison to a shell-model calculation shows good agreement. CONCLUSIONS: We have experimentally determined the isospin-symmetry-breaking correction to the superallowed transition of this decay to be (\delta_C-\delta_NS)_exp=5.4(9)%, significantly larger than for any other known superallowed Fermi transition. This correction agrees with a shell-model calculation, which yields \delta_C-\delta_NS=4.8(5)%. Our results also provide a way to improve the measured ft values for the beta decay of 32Ar.
The beta decay of 38Ca: Sensitive test of isospin symmetry-breaking corrections from mirror superallowed 0+-to-0+ transitions  [PDF]
H. I. Park,J. C. Hardy,V. E. Iacob,M. Bencomo,L. Chen,V Horvat,N. Nica,B. T. Roeder,E. Simmons,R. E. Tribble,I. S. Towner
Physics , 2014, DOI: 10.1103/PhysRevLett.112.102502
Abstract: We report the first branching-ratio measurement of the superallowed 0+-to-0+ beta transition from 38Ca. The result, 0.7728(16), leads to an ft value of 3062.3(68)s with a relative precision of +/-0.2%. This makes possible a high-precision comparison of the ft values for the mirror superallowed transitions, 38Ca-to-38mK and 38mK-to-38Ar, which sensitively tests the isospin symmetry-breaking corrections required to extract Vud, the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, from superallowed beta decay. The result supports the corrections currently used, and points the way to even tighter constraints on CKM unitarity.
Separated Oscillatory Fields for High-Precision Penning Trap Mass Spectrometry  [PDF]
S. George,S. Baruah,B. Blank,K. Blaum,M. Breitenfeldt,U. Hager,F. Herfurth,A. Herlert,A. Kellerbauer,H. J. Kluge,M. Kretzschmar,D. Lunney,R. Savreux,S. Schwarz,L. Schweikhard,C. Yazidjian
Physics , 2007, DOI: 10.1103/PhysRevLett.98.162501
Abstract: Ramsey's method of separated oscillatory fields is applied to the excitation of the cyclotron motion of short-lived ions in a Penning trap to improve the precision of their measured mass. The theoretical description of the extracted ion-cyclotron-resonance line shape is derived out and its correctness demonstrated experimentally by measuring the mass of the short-lived $^{38}$Ca nuclide with an uncertainty of $1.6\cdot 10^{-8}$ using the ISOLTRAP Penning trap mass spectrometer at CERN. The mass value of the superallowed beta-emitter $^{38}$Ca is an important contribution for testing the conserved-vector-current hypothesis of the electroweak interaction. It is shown that the Ramsey method applied to mass measurements yields a statistical uncertainty similar to that obtained by the conventional technique ten times faster.
A new method of measuring the half-life of ^{10}C  [PDF]
K. K. H. Leung
Physics , 2009,
Abstract: A new measurement of the half-life of the positron emitter 10C has been made. The 10C decay belongs to the set of superallowed 0+ -> 0+ beta-decays whose Ft values offer stringent tests of the Conserved Vector Current (CVC) hypothesis and which can be used in calculating |Vud|, the largest element of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix of the Standard Model. The experimental method used four plastic scintillator detectors in a 3-fold coincidence mode in order to detect the unique 2x511keV+718keV gamma-ray decay signature of 10C. The half-life values obtained from the data were strongly dependent on the detector count rates, but a final value of 19.303 +/- 0.013 s was determined by extrapolating the dependence to zero count rate. This study highlights the significance of systematic errors due to high count rates in precision half-life experiments and raises doubts about both the two previously accepted values of the half-life of 10C.
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