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Register a new userPrecision measurement of the $beta$-asymmetry in spin-polarized $^{37}mathrm{K}$ decay
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Using TRIUMFs neutral atom trap, TRINAT, for nuclear $beta$ decay, we have measured the $beta$ asymmetry with respect to the initial nuclear spin in $^{37}mathrm{K}$ to be $A_beta=-0.5707(13)_mathrm{syst}(13)_mathrm{stat}(5)_mathrm{pol}$, a 0.3% measurement. This is the best relative accuracy of any $beta$-asymmetry measurement in a nucleus or the neutron, and is in agreement with the standard model prediction $-0.5706(7)$. We compare constraints on physics beyond the standard model with other $beta$-decay measurements, and improve the value of $V_mathrm{ud}$ measured in this mirror nucleus by a factor of 4.
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The half-life of tsups{37}K has been measured to be $1.23651(94)~mathrm{s}$, a value nearly an order of magnitude more precise than the best previously reported. The $beta^+$ decay of tsups{37}K occurs mainly via a superallowed branch to the ground-state of its $T=1/2$ mirror, tsups{37}Ar. This transition has been used recently, together with similar transitions from four other nuclei, as an alternative test of CVC and method for determining $V_{ud}$, but the precision of its $ft$ value was limited by the relatively large half-life uncertainty. Our result corrects that situation. Another motivation for improving the $ft$ value was to determine the standard-model prediction for the $beta$-decay correlation parameters, which will be compared to those currently being measured by the trinat{} collaboration at triumf. The new $ft$ value, $4605(8)~mathrm{s}$, is now limited in precision by the $97.99(14)%$ ground-state branching ratio.
We report a measurement of the nuclear polarization of laser-cooled, optically-pumped $^{37}mathrm{K}$ atoms which will allow us to precisely measure angular correlation parameters in the beta-decay of the same atoms. These results will be used to test the $V-A$ framework of the weak interaction at high precision. At the TRIUMF Neutral Atom Trap (TRINAT), a magneto-optical trap (MOT) confines and cools neutral $^{37}mathrm{K}$ atoms and optical pumping spin-polarizes them. We monitor the nuclear polarization of the same atoms that are decaying in situ by photoionizing a small fraction of the partially polarized atoms and then use the standard optical Bloch equations to model their population distribution. We obtain an average nuclear polarization of $P = 0.9913pm0.0008$, which is significantly more precise than previous measurements with this technique. Since our current measurement of the beta-asymmetry has $0.2%$ statistical uncertainty, the polarization measurement reported here will not limit its overall uncertainty. This result also demonstrates the capability to measure the polarization to $<0.1%$, allowing for a measurement of angular correlation parameters to this level of precision, which would be competitive in searches for new physics.
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.
Neutron beta decay is one of the most fundamental processes in nuclear physics and provides sensitive means to uncover the details of the weak interaction. Neutron beta decay can evaluate the ratio of axial-vector to vector coupling constants in the standard model, $lambda = g_A / g_V$, through multiple decay correlations. The Nab experiment will carry out measurements of the electron-neutrino correlation parameter $a$ with a precision of $delta a / a = 10^{-3}$ and the Fierz interference term $b$ to $delta b = 3times10^{-3}$ in unpolarized free neutron beta decay. These results, along with a more precise measurement of the neutron lifetime, aim to deliver an independent determination of the ratio $lambda$ with a precision of $delta lambda / lambda = 0.03%$ that will allow an evaluation of $V_{ud}$ and sensitively test CKM unitarity, independent of nuclear models. Nab utilizes a novel, long asymmetric spectrometer that guides the decay electron and proton to two large area silicon detectors in order to precisely determine the electron energy and an estimation of the proton momentum from the proton time of flight. The Nab spectrometer is being commissioned at the Fundamental Neutron Physics Beamline at the Spallation Neutron Source at Oak Ridge National Lab. We present an overview of the Nab experiment and recent updates on the spectrometer, analysis, and systematic effects.
The asymmetry parameter alpha_p^NM for a proton exclusively emitted in the Lambda p -> np process was, for the first time, measured in the non-mesonic weak decay of a polarized 5_La,bda_He hypernucleus by selecting the proton-neutron pairs emitted in the back-to-back kinematics. The highly polarized 5_Lambda_He was abundantly produced with the (pi+,K+) reaction at 1.05GeV/c in the scattering angular range of +-15$ degrees. The obtained value alpha_p^NM=0.31+-0.22, as well as that for inclusive protons, alpha_p^NM=0.11+-0.08+-0.04, largely contradicts recent theoretical values of around -0.6, although these calculations well reproduce the branching ratios of non-mesonic weak decay.
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