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Register a new userFirst direct constraints on Fierz interference in free neutron $beta$ decay
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Precision measurements of free neutron $beta$-decay have been used to precisely constrain our understanding of the weak interaction. However the neutron Fierz interference term $b_n$, which is particularly sensitive to Beyond-Standard-Model tensor currents at the TeV scale, has thus far eluded measurement. Here we report the first direct constraints on this term, finding $b_n = 0.067 pm 0.005_{text{stat}} {}^{+0.090}_{- 0.061}{}_{text{sys}}$, consistent with the Standard Model. The uncertainty is dominated by absolute energy reconstruction and the linearity of the beta spectrometer energy response.
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In the standard model of particle physics, the weak interaction is described by vector and axial-vector couplings only. Non-zero scalar or tensor interactions would imply an additional contribution to the differential decay rate of the neutron, the Fierz interference term. We derive a limit on this hypothetical term from a measurement using spin polarized neutrons. This method is statistically less sensitive than the determination from the spectral shape but features much cleaner systematics. We obtain a limit of b = 0.017(21) at 68.27 C.L., improving the previous best limit from neutron decay by a factor of four.
We have observed beta-delayed proton emission from the neutron-rich nucleus 11Be by analysing a sample collected at the ISOLDE facility at CERN with accelerator mass spectrometry (AMS). With a branching ratio of (8.4 +- 0.6) 10^{-6} the strength of this decay mode, as measured by the B(GT)-value, is unexpectedly high. The result is discussed within a simple single-particle model and could be interpreted as a quasi-free decay of the 11Be halo neutron into a single-proton state.
The standard model predicts that, in addition to a proton, an electron, and an antineutrino, a continuous spectrum of photons is emitted in the $beta$ decay of the free neutron. We report on the RDK II experiment which measured the photon spectrum using two different detector arrays. An annular array of bismuth germanium oxide scintillators detected photons from 14 to 782~keV. The spectral shape was consistent with theory, and we determined a branching ratio of 0.00335 $pm$ 0.00005 [stat] $pm$ 0.00015 [syst]. A second detector array of large area avalanche photodiodes directly detected photons from 0.4 to 14~keV. For this array, the spectral shape was consistent with theory, and the branching ratio was determined to be 0.00582 $pm$ 0.00023 [stat] $pm$ 0.00062 [syst]. We report the first precision test of the shape of the photon energy spectrum from neutron radiative decay and a substantially improved determination of the branching ratio over a broad range of photon energies.
Precision measurements in neutron beta decay serve to determine the coupling constants of beta decay and allow for several stringent tests of the standard model. This paper discusses the design and the expected performance of the Nab spectrometer.
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.
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