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Register a new userFinal results for the neutron $beta$-asymmetry parameter $A_0$ from the UCNA experiment
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The UCNA experiment was designed to measure the neutron $beta$-asymmetry parameter $A_0$ using polarized ultracold neutrons (UCN). UCN produced via downscattering in solid deuterium were polarized via transport through a 7 T magnetic field, and then directed to a 1 T solenoidal electron spectrometer, where the decay electrons were detected in electron detector packages located on the two ends of the spectrometer. A value for $A_0$ was then extracted from the asymmetry in the numbers of counts in the two detector packages. We summarize all of the results from the UCNA experiment, obtained during run periods in 2007, 2008--2009, 2010, and 2011--2013, which ultimately culminated in a 0.67% precision result for $A_0$.
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The neutron $beta$-decay asymmetry parameter $A_0$ defines the correlation between the spin of the neutron and the momentum of the emitted electron, which determines $lambda=frac{g_{A}}{g_{V}}$, the ratio of the axial-vector to vector weak coupling constants. The UCNA Experiment, located at the Ultracold Neutron facility at the Los Alamos Neutron Science Center, is the first to measure such a correlation coefficient using ultracold neutrons (UCN). Following improvements to the systematic uncertainties and increased statistics, we report the new result $A_0 = -0.12054(44)_{mathrm{stat}}(68)_{mathrm{syst}}$ which yields $lambdaequiv frac{g_{A}}{g_{V}}=-1.2783(22)$. Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces $A_0=-0.12015(34)_{mathrm{stat}}(63)_{mathrm{syst}}$ and $lambdaequiv frac{g_{A}}{g_{V}}=-1.2772(20)$.
We present a detailed report of a measurement of the neutron $beta$-asymmetry parameter $A_0$, the parity-violating angular correlation between the neutron spin and the decay electron momentum, performed with polarized ultracold neutrons (UCN). UCN were extracted from a pulsed spallation solid deuterium source and polarized via transport through a 7-T magnetic field. The polarized UCN were then transported through an adiabatic-fast-passage spin-flipper field region, prior to storage in a cylindrical decay volume situated within a 1-T $2 times 2pi$ solenoidal spectrometer. The asymmetry was extracted from measurements of the decay electrons in multiwire proportional chamber and plastic scintillator detector packages located on both ends of the spectrometer. From an analysis of data acquired during runs in 2008 and 2009, we report $A_0 = -0.11966 pm 0.00089_{-0.00140} ^{+0.00123}$, from which we extract a value for the ratio of the weak axial-vector and vector coupling constants of the nucleon, $lambda = g_A/g_V = -1.27590 pm 0.00239_{-0.00377}^{+0.00331}$. Complete details of the analysis are presented.
It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle ($chi$) could account for the discrepancy in the neutron lifetime observed in experiments that use two different measurement techniques. One of the possible final states discussed includes a single $chi$ along with an $e^{+}e^{-}$ pair. We use data from the UCNA (Ultracold Neutron Asymmetry) experiment to set limits on this decay channel. Coincident electron-like events are detected with $sim 4pi$ acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). The summed kinetic energy ($E_{e^{+}e^{-}}$) from such events is used to set limits, as a function of the $chi$ mass, on the branching fraction for this decay channel. For $chi$ masses consistent with resolving the neutron lifetime discrepancy, we exclude this as the dominant dark matter decay channel at $gg~5sigma$ level for $100~text{keV} < E_{e^{+}e^{-}} < 644~text{keV}$. If the $chi+e^{+}e^{-}$ final state is not the only one, we set limits on its branching fraction of $< 10^{-4}$ for the above $E_{e^{+}e^{-}}$ range at $> 90%$ confidence level.
The Ultracold Neutron Asymmetry (UCNA) experiment was designed to measure the $beta$-decay asymmetry parameter, $A_0$, for free neutron decay. In the experiment, polarized ultracold neutrons are transported into a decay trap, and their $beta$-decay electrons are detected with $approx 4pi$ acceptance into two detector packages which provide position and energy reconstruction. The experiment also has sensitivity to $b_{n}$, the Fierz interference term in the neutron $beta$-decay rate. In this work, we determine $b_{n}$ from the energy dependence of $A_0$ using the data taken during the UCNA 2011-2013 run. In addition, we present the same type of analysis using the earlier 2010 $A$ dataset. Motivated by improved statistics and comparable systematic errors compared to the 2010 data-taking run, we present a new $b_{n}$ measurement using the weighted average of our asymmetry dataset fits, to obtain $b_{n} = 0.066 pm 0.041_{text{stat}} pm 0.024_{text{syst}}$ which corresponds to a limit of $-0.012 < b_{n} < 0.144$ at the 90% confidence level.
The Advanced Molybdenum-based Rare process Experiment (AMoRE) aims to search for neutrinoless double beta decay (0$ ubetabeta$) of $^{100}$Mo with $sim$100 kg of $^{100}$Mo-enriched molybdenum embedded in cryogenic detectors with a dual heat and light readout. At the current, pilot stage of the AMoRE project we employ six calcium molybdate crystals with a total mass of 1.9 kg, produced from $^{48}$Ca-depleted calcium and $^{100}$Mo-enriched molybdenum ($^{48textrm{depl}}$Ca$^{100}$MoO$_4$). The simultaneous detection of heat(phonon) and scintillation (photon) signals is realized with high resolution metallic magnetic calorimeter sensors that operate at milli-Kelvin temperatures. This stage of the project is carried out in the Yangyang underground laboratory at a depth of 700 m. We report first results from the AMoRE-Pilot $0 ubetabeta$ search with a 111 kg$cdot$d live exposure of $^{48textrm{depl}}$Ca$^{100}$MoO$_4$ crystals. No evidence for $0 ubetabeta$ decay of $^{100}$Mo is found, and a upper limit is set for the half-life of 0$ ubetabeta$ of $^{100}$Mo of $T^{0 u}_{1/2} > 9.5times10^{22}$ y at 90% C.L.. This limit corresponds to an effective Majorana neutrino mass limit in the range $langle m_{betabeta}ranglele(1.2-2.1)$ eV.
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