I. S. Towner

Comparative tests of isospin-symmetry-breaking corrections to superallowed 0(+) -> 0(+) nuclear beta decay

We present a test with which to evaluate the calculated isospin-symmetry-breaking corrections to superallowed 0{sup +}{yields}0{sup +} nuclear {beta} decay. The test is based on the corrected experimental Ft values being required to satisfy conservation of the vector current (CVC). When applied to six sets of published calculations, the test demonstrates quantitatively that only one set, the one based on the shell model with Saxon-Woods radial wave functions, provides satisfactory agreement with CVC. This test can easily be applied to any sets of calculated correction terms that are produced in future.

Precision measurements of the

The {beta}-asymmetry parameter A-tilde for the Gamow-Teller decay of {sup 60}Co was measured by polarizing the radioactive nuclei with the brute-force low-temperature nuclear-orientation method. The {sup 60}Co activity was cooled down to milliKelvin temperatures in a {sup 3}He-{sup 4}He dilution refrigerator in an external 13-T magnetic field. The {beta} particles were observed by a 500-{mu}m-thick Si PIN diode operating at a temperature of about 10 K in a magnetic field of 0.6 T. Extensive geant4 Monte Carlo simulations were performed to gain control over the systematic effects. Our result, A-tilde=-1.014(12){sub stat}(16){sub syst}, is in agreement with the standard-model value of -0.987(9), which includes recoil-order corrections that were addressed for the first time for this isotope. Further, it enables limits to be placed on possible tensor-type charged weak currents, as well as other physics beyond the standard model.

^{60}

A precision measurement of the γ yields following the β decay of (32)Cl has determined its isobaric-analogue branch to be (22.47(-0.18)(+0.21))%. Since it is an almost pure-Fermi decay, we can also determine the amount of isospin-symmetry breaking in this superallowed transition. We find a very large value, δ(C) = 5.3(9)%, in agreement with a shell-model calculation. This result sets a benchmark for isospin-symmetry-breaking calculations and lends support for similarly calculated, yet smaller, corrections that are currently applied to 0+ → 0 + transitions for tests of the standard model.

Co

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.

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Central to the Electroweak Standard Model is the Cabibbo-Kobayashi-Maskawa (CKM) matrix, a unitary 3 × 3 matrix that relates the weak- to the mass-eigenstates of the three generations of quarks. The Standard Model itself does not prescribe the values of the individual matrix elements—they must each be determined experimentally—but it does insist that the full matrix must satisfy all conditions of unitarity. Thus, any conclusive experimental demonstration that the unitarity conditions are not met would prove the need for “new physics” extending beyond the current Standard Model. Conversely, if the experiment were instead to confirm unitarity, this result would circumscribe the scope of any possible new physics to an extent limited only by the quoted experimental uncertainties on the unitarity sum. Either way, there is compelling motivation here for precision measurements of the CKM matrix elements aimed at the most demanding tests possible of the matrixs unitarity.

-asymmetry parameter in search for tensor currents in weak interactions

A

Experimental validation of the largest calculated isospin-symmetry-breaking effect in a superallowed Fermi decay.

^{52}\mathrm{Cr}(p,t)^{50}\mathrm{Cr}

β decay of 32Cl: Precision γ-ray spectroscopy and a measurement of isospin-symmetry breaking

two-neutron pickup reaction was performed using the Q3D magnetic spectrograph at the Maier-Leibnitz-Laboratorium in Garching, Germany. Excited states in

Probing the Standard Model with Superallowed Nuclear Beta Decay

^{50}\mathrm{Cr}

A Search for

were observed up to an excitation energy of 5.3 MeV. Despite significantly increased sensitivity and resolution over previous work, no evidence of the previously assigned first excited