Superallowed Fermi Beta Decay Research Articles

Exotic phenomena in medium mass N ≃ Z nuclei within the beyond-mean-field approach

Recent studies on properties of proton-rich nuclei in the A = 70 mass region brought insights into fundamental symmetries and interactions and produced characteristics required by the astrophysical scenarios on the rp-process path revealing coexistence phenomena which dominate the structure and dynamics of these exotic nuclei. Isospin mixing and related phenomena like Coulomb energy differences, mirror energy differences, triplet energy differences as well as superallowed Fermi beta decay in isovector triplets have been investigated. The interplay between isospin-symmetry-breaking and shape-coexistence effects on isospin-related phenomena in nuclei as well as weak interaction processes under terrestrial and stellar x-ray burst environment have been explored in the frame of the beyond-mean-field complex Excited Vampir variational model. We used an adequate model space and the effective interaction was derived from a G-matrix starting from the charge-dependent Bonn CD potential. Results concerning the influence of shape coexistence and mixing on the structure and β-decay properties of exotic nuclei will be reviewed.

Nuclear Structure Studied with Direct Reactions for Fundamental Symmetry Tests

Direct reaction studies provide crucial information on nuclear structure for nuclei that are used in weak interaction and fundamental symmetry tests. Motivated by the superallowed Fermi beta-decay studies, we have investigated the structure of Cr-50 and Zn-62, the daughters of the superallowed beta(+)/EC emitters Mn-50 and Ga-62, with (p,t) reactions. For both nuclei, we reassigned the location of the first excited 0(+) state. The data for these reassignments are reviewed and limits on the possible L = 0 transfer to the previously assigned 0(2)(+) states are determined. For Cr-50, we find no evidence for the presence of a 0(+) state at 3695 keV. For Zn-62, the data allow for a possible contribution of L = 0 and L = 2, indicating a possible 0(+)/2(+) doublet of states at 2342 keV. However, this relies on an enhanced cross section at only one data point, and thus the existence of a 0(+) state at this energy is not clear. A preliminary result from a gamma-gamma angular correlation measurement following the beta(+)/EC decay of Ga-62 weakly favours the existence of a 0(+) state, but requires further study with increased statistics.

No-core configuration-interaction model for the isospin- and angular-momentum-projected states

[Background] Single-reference density functional theory is very successful in reproducing bulk nuclear properties like binding energies, radii, or quadrupole moments throughout the entire periodic table. Its extension to the multi-reference level allows for restoring symmetries and, in turn, for calculating transition rates. [Purpose] We propose a new no-core-configuration-interaction (NCCI) model treating properly isospin and rotational symmetries. The model is applicable to any nucleus irrespective of its mass and neutron- and proton-number parity. It properly includes polarization effects caused by an interplay between the long- and short-range forces acting in the atomic nucleus. [Methods] The method is based on solving the Hill-Wheeler-Griffin equation within a model space built of linearly-dependent states having good angular momentum and properly treated isobaric spin. The states are generated by means of the isospin and angular-momentum projection applied to a set of low-lying (multi)particle-(multi)hole deformed Slater determinants calculated using the self-consistent Skyrme-Hartree-Fock approach. [Results] The theory is applied to calculate energy spectra in N~Z nuclei that are relevant from the point of view of a study of superallowed Fermi beta-decays. In particular, a new set of the isospin-symmetry-breaking corrections to these decays is given. [Conclusions] It is demonstrated that the NCCI model is capable to capture main features of low-lying energy spectra in light and medium-mass nuclei using relatively small model space and without any local readjustment of its low-energy coupling constants. Its flexibility and a range of applicability makes it an interesting alternative to the conventional nuclear shell model.

The influence of pairing correlations on the isospin symmetry breaking corrections of superallowed Fermi beta decays

Within the framework of quasi-particle random phase approximation, the isospin breaking correction of superallowed 0+ → 0+ beta decay and unitarity of Cabibbo-Kobayashi-Maskawa mixing matrix have been investigated. The broken isotopic symmetry of nuclear part of Hamiltonian has been restored by Pyatov’s method. The isospin symmetry breaking correction with pairing correlations has been compared with the previous results without pairing. The effect of pairing interactions has been examined for nine superallowed Fermi beta decays; their parent nuclei are 26Al, 34Cl, 38K, 42Sc, 46V, 50Mn, 54Co, 62Ga, 74Rb.

The ft values of the superallowed Fermi beta decays

In this work, the ft values of the 0+→0+ superallowed Fermi beta decays have been investigated. In the calculations, the shell model has been modified by Pyatov’s restoration method because of the isospin breaking and the transition matrix elements have been found by means of the quasi-particle random phase approximation (QRPA).

The Influence of Pairing Correlations on the Isospin Symmetry Breaking Corrections of Superallowed Fermi Beta Decays

The Influence of Pairing Correlations on the Isospin Symmetry Breaking Corrections of Superallowed Fermi Beta Decays

The isospin mixing and the superallowed Fermi beta decay

In the present work, the isospin admixtures in the nuclear ground states of the parent nuclei and isospin structure of the isobar analog resonance (IAR) states have been investigated by studying the 0 + →0 + superallowed Fermi β decays using Pyatov’s restoration method. Within the random phase approximation (RPA), in this method, the effect of isospin breaking due to the Coulomb forces has been evaluated, taking into account the effect of pairing correlations between nucleons.

Super-allowed Fermi beta-decay: CKM unitarity

Analysis of the super-allowed Fermi beta-decay of nine bodies from 10C through 54Co yields |Vud|2 = 0.9514 ± 0.0014 and a unitarity test of the first row of the Cabibbo–Kobayashi–Maskawa matrix of |Vud|2 + |Vus|2 + |Vub|2 = 0.9996 ± 0.0017 in agreement with the standard model but in disagreement with alternative analyses of such data and with a study of the beta-decay of the free neutron.

Precise half-life measurement for the superallowed 0(+)-->0(+) beta emitter (74)Rb: first results from the new radioactive beam facility (ISAC) at TRIUMF.

Presently, the world data for superallowed beta decay leads to a result in disagreement (at the 98% confidence level) with the predictions of the minimal standard model for the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. Precise data for the superallowed 0(+)-->0(+) beta decay of (74)Rb would provide a critical test of the nucleus-dependent isospin symmetry-breaking corrections that must be calculated for these superallowed Fermi beta decays. The present work reports the first precise measurement of the half-life for (74)Rb ( t(1/2) = 64.761+/-0.031 ms). The data were obtained at the radioactive beam facility (ISAC) at TRIUMF using a beam of approximately 4000 (74)Rb ions s(-1).

Effect of nucleon structure variation in super-allowed Fermi beta-decay

There is a well known anomaly between the value of the Fermi decay constant extracted from super-allowed Fermi beta-decay of nuclear isotriplets and that required by unitarity of the Cabibbo-Kobayashi-Maskawa matrix. This discrepancy remains at the level of a few tenths of a percent after the most rigorous investigation of conventional nuclear and radiative corrections. Within the framework of the quark-meson coupling model of nuclear matter, which has been previously applied successfully to phenomena such as nuclear saturation and nuclear charge symmetry violation, we show that it is possible to understand a significant fraction of the observed anomaly.

Nuclear mismatch in super-allowed Fermi beta-decay

Abstract If super-allowed Fermi beta-decay is to be consistent with the conserved vector current and with unitarity of the Cabibbo-Kobayashi-Maskawa matrix it is necessary to understand a mismatch, as between the parent and daughter members of the isotriplets involved, that is worth several tenths of a percent in the transition rates. Present theoretical estimates of this mismatch fall short of what is needed by a factor of order 2 or more. It has been suggested that this shortfall is due to inadequate allowance for the effects of the spectator nucleons, but the present study shows that such effects are worth only a few hundredths of one percent.

G v , CKM unitarity, neutron decay; WR?

Superallowed Fermi beta-decay is analyzed in the light of new experimental data, new methodology and new treatment of the radiative and charge-dependent corrections. The following values are recommended:G v /(ħc) 3=(1.13769±0.00091)×10−5 GeV−2;G * /(ħc) 3 =(1.15128±0.00084) × 10−5GeV−2 whereG v is purged of radiative corrections suitably for combining withG μ to gain: ¦V ud ¦=0.97539±0.00080 which leads to:¦V ud ¦ 2 +¦V us ¦ 2 +¦V ub ¦2=1.0000+0.0017 and whereG * is the operational constant, incorporating the inner radiative correction, suitable for use in ordinary nuclear physics. It is noted that present results on the beta-decay of the neutron, when confronted by the above values, are inconclusive as to possible conflict with the minimal standard model and that judgement as to, for example, the intervention of a right-handed sector, should be suspended. The valueG * =1.2657±0.0030 is also recommended.

Nuclear deformation and super-allowed Fermi beta-decay

The Q EC values for the eight bodies from whose super-allowed Fermi beta-decay we derive our most accurate knowledge of the vector coupling constant are now determined with such precision that the associated uncertainties in the phase-space factor amount, on that account, to an average of only 0.030%; we must therefore consider all possible influences at the 0.01% level. Although the bodies in question are of J = 0 they possess, or may possess, large intrinsic deformations that: (i) affect the radial distribution of the charge whose field operates upon the emitted positron; (ii) affect the leptonic-nucleonic convolution of the shape factor. These effects are evaluated in an approximation that exaggerates their influence on the phase-space factor; using for the nuclear deformation values deriving from Q 2+-values measured by the re-orientation effect in Coulomb excitation the resultant magnitude of the change in the phase-space factor due to the deformation is found to be everywhere less than 0.01% although approaching that figure in some cases.

Methodology for superallowed Fermi beta-decay Part. I. Preliminaries and data

The group of papers, of which this is the first, discusses superallowed Fermi beta-decay in the context of the eight accurately-measured examples: 14O, 26Al m, 34Cl, 38K m, 42Sc, 46V, 50Mn and 54Co. The present paper, following a general introduction, considers the best current figures for the Q EC-values and t-values of the eight bodies of concern. It is pointed out that we are here interested not so much in the individual Q EC-values and t-values of the individual members of the set of eight as with the way in which all those values jointly determine a single final number, namely the vector coupling constant G V. In this case a single statistical criterion, based on the total number of input data involved for the entire set (at present 69), should be adopted when considering the admissibility, on statistical grounds, of any single measurement of any single quantity rather than a criterion relating just to the measurements of that single quantity. It is shown that this overall group criterion results in a very satisfactory statistical pattern for all the data relating to the eight bodies taken as a whole. A critical discussion is presented of the role of atomic excitations in the determination of Q EC-values and beta-decay rates.

The nuclear-structure dependence of radiative corrections in superallowed Fermi beta-decay

We give a shell-model calculation of the nuclear-structure-dependent axial-vector contribution of the Born graphs to the radiative correction in superallowed Fermi β-decay. The impact of the correction is to give a modest improvement in the test of the conserved-vector-current (CVC) hypothesis, and a slightly smaller value of the quark-mixing matrix element, V ud. The test of the unitary of the first row of the Kobayashi-Maskawa matrix gives: V ud 2 + ν us 2 + V ub 2 = 0.9962 ± 0.0016, violating the three-generation standard model. Hints from neutron-only data suggest that nuclear-structure corrections in superallowed Fermi β-decay are still not fully under control.

Radiative corrections to 10C superallowed Fermi beta decay.

In view of new data on the {sup 10}C superallowed Fermi {beta} decay, the radiative corrections have been reevaluated. In particular we calculate and include the nuclear-structure-dependent part of the axial-vector-induced contribution to the {ital O}({alpha}) radiative correction. The resulting {ital V}{sub {ital u}{ital d}} is appreciably larger than a value recently published, which was based on the same data.

Branching ratio for 10C superallowed Fermi beta decay.

The branching ratio for $^{10}\mathrm{C}$ superallowed Fermi \ensuremath{\beta} decay has been measured accurately by a newly developed method. The result is 1.473\ifmmode\pm\else\textpm\fi{}0.007 %. The Ft value is derived as 3065.4\ifmmode\pm\else\textpm\fi{}14.7 sec, which is consistent with the Ft values determined accurately for heavier nuclei and with predictions of conserved vector current hypothesis. The method developed here can be applied to the high precision \ensuremath{\beta}-\ensuremath{\gamma} spectroscopy.

The vector coupling constant and the Kobayashi-Maskawa matrix

Abstract Data on superallowed Fermi beta decay are analyzed using recent developments in our understanding of radiative corrections and using a novel largely-empirical approach to the nuclear mismatch correction. The resultant value for the operational vector coupling constant G1v is: G′ v /( h 3 = (1.1510±0.0005) × 10 −5 GeV −2 . The test for the unitarity of the first row of the Kobayashi-Maskawa matrix gives: |Vud|2 + |Vus|2 + |Vud|2 = 0.9989±0.0012.

Gv, KM and mt

Analysis of superallowed Fermi beta decay using a new approach to the nuclear mismatch factor and evaluation of the radiative corrections using renormalization group methods and including nuclear-structure-dependent terms in the “inner” radiative correction leads to | V ud| 2+| V us| 2+| V ub| 2 = 0.9999 ± 0.0012 for the first row of the Kobayashi-Maskawa matrix using a mass of 130 GeV for the top quark. We may, alternatively, assuming unitarity of the KM matrix, infer a top quark mass of (1.40±80) GeV from beta-decay data alone.