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Register a new userBeta spectrum of unique first-forbidden decays as a novel test for fundamental symmetries
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Within the Standard Model, the weak interaction of quarks and leptons is characterized by certain symmetry properties, such as maximal breaking of parity and favored helicity. These are related to the $V-A$ structure of the weak interaction. These characteristics were discovered by studying correlations in the directions of the outgoing leptons in nuclear beta decays. These days, correlation measurements in nuclear beta decays are intensively studied to probe for signatures for deviations from these symmetries, which are an indication of Beyond Standard Model physics. We show that the structure of the energy spectrum of emitted electrons in unique first-forbidden $beta$-decays is sensitive to the symmetries of the weak interaction, and thus can be used as a novel probe of physics beyond the standard model. Furthermore, the energy spectrum gives constraints both in the case of right and left coupling of the new symmetry currents. We show that a measurement with modest energy resolution of about 20 keV is expected to lead to new constraints on beyond the standard model interactions with tensor symmetry.
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Nuclear beta decays between (J^pi,T) = (0^+,1) analog states yield the best value for the Vud element of the Cabibbo-Kobayashi-Maskawa matrix. Current world data establish the corrected Ft values of 14 separate superallowed transitions to a precision of order 0.1% or better. The validity of the small theoretical correction terms is confirmed by excellent consistency among the 14 Ft values and by recent measurements that compare pairs of mirror superallowed transitions. With consistency established, the results now yield |Vud| = 0.97420(21). This value is consistent with the considerably less precise results obtained from beta decays of the neutron, the pion and T=1/2 mirror nuclei, which are hampered by experimental challenges.
The photon spectrum accompanying the orbital K-electron capture in the first forbidden unique decay of 81Kr was measured. The total radiation intensity for the photon energies larger than 50 keV was found to be 1.47(6) x 10^{-4} per K-capture. Both the shape of the spectrum and its intensity relative to the ordinary, non-radiative capture rate, are compared to theoretical predictions. The best agreement is found for the recently developed model which employs the length gauge for the electromagnetic field.
A new test of Lorentz invariance in the weak interactions has been made by searching for variations in the decay rate of spin-polarized 20Na nuclei. This test is unique to Gamow-Teller transitions, as was shown in the framework of a recently developed theory that assumes a Lorentz symmetry breaking background field of tensor nature. The nuclear spins were polarized in the up and down direction, putting a limit on the amplitude of sidereal variations of the form |(Gamma_{up} - Gamma_{down})| / (Gamma_{up} + Gamma_{down}) < 3 * 10^{-3}. This measurement shows a possible route toward a more detailed testing of Lorentz symmetry in weak interactions.
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.
Four 1cm^3 CdZnTe semiconductor detectors were operated in the Gran Sasso National Laboratory to explore the feasibility of such devices for double beta decay searches as proposed for the COBRA experiment. The research involved background studies accompanied by measurements of energy resolution performed at the surface. Energy resolutions sufficient to reduce the contribution of two-neutrino double beta decay events to a negligible level for a large scale experiment have already been achieved and further improvements are expected. Using activity measurements of contaminants in all construction materials a background model was developed with the help of Monte Carlo simulations and major background sources were identified. A total exposure of 4.34 kg.days of underground data has been accumulated allowing a search for neutrinoless double beta decay modes of seven isotopes found in CdZnTe. Half-life limits (90% C.L.) are presented for decays to ground and excited states. Four improved lower limits have been obtained, including zero neutrino double electron capture transitions of Zn64 and Te120 to the ground state, which are 1.19*10^{17} years and 2.68*10^{15} years respectively.
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