In January, 2018, Fornal and Grinstein proposed that a previously unobserved neutron decay branch to a dark matter particle (χ) could account for the discrepancy in the neutron lifetime observed in two different types of experiments. One of the possible final states discussed includes a single χ 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 ∼ 4π acceptance using a pair of detectors that observe a volume of stored Ultracold Neutrons (UCNs). We use the timing information of coincidence events to select candidate dark sector particle decays by applying a timing calibration and selecting events within a physically-forbidden timing region for conventional n → p + e - + ν̅ e decays. The summed kinetic energy ( E e + e − ) from such events is reconstructed and used to set limits, as a function of the χ mass, on the branching fraction for this decay channel.

The UCNA experiment was designed to measure the neutron β-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.

It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle (X) 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 X 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 similar to 4 pi acceptance using a pair of detectors that observe a volume of stored ultracold neutrons. The summed kinetic energy (E-e+(e)-) from such events is used to set limits, as a function of the X mass, on the branching fraction for this decay channel. For X masses consistent with resolving the neutron lifetime discrepancy, we exclude this as the dominant dark matter decay channel at >> 5 sigma level for 100 < E-e+(e)- < 644 keV. If the X + 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.

Background: The neutron beta-decay asymmetry parameter A(0) defines the angular correlation between the spin of the neutron and the momentum of the emitted electron. Values for A(0) permit an extraction of the ratio of the weak axial-vector to vector coupling constants, lambda g(A)/g(V), which under assumption of the conserved vector current hypothesis (g(V) = 1) determines g(A). Precise values for g(A) are important as a benchmark for lattice QCD calculations and as a test of the standard model. Purpose: The UCNA experiment, carried out at the Ultracold Neutron (UCN) source at the Los Alamos Neutron Science Center, was the first measurement of any neutron beta-decay angular correlation performed with UCN. This article reports the most precise result for A(0) obtained to date from the UCNA experiment, as a result of higher statistics and reduced key systematic uncertainties, including from the neutron polarization and the characterization of the electron detector response. Methods: UCN produced via the down scattering of moderated spallation neutrons in a solid deuterium crystal were polarized via transport through a 7 T polarizing magnet and a spin flipper, which permitted selection of either spin state. The UCN were then contained within a 3-m long cylindrical decay volume, situated along the central axis of a superconducting 1 T solenoidal spectrometer. With the neutron spins then oriented parallel or anti-parallel to the solenoidal field, an asymmetry in the numbers of emitted decay electrons detected in two electron detector packages located on both ends of the spectrometer permitted an extraction of A(0). Results: The UCNA experiment reports a new 0.67% precision result for A(0) of A(0) = -0.12054(44)(stat)(68)(syst), which yields lambda = 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)(stat)(63)(syst) and lambda = g(A)/g(V) = -1.2772(20). Conclusions: This new result for A(0) and g(A)/g(V) from the UCNA experiment has provided confirmation of the shift in values for g(A)/g(V) that has emerged in the published results from more recent experiments, which are in striking disagreement with the results from older experiments. Individual systematic corrections to the asymmetries in older experiments (published prior to 2002) were >10%, whereas those in the more recent ones (published after 2002) have been of the scale of <2%. The impact of these older results on the global average will be minimized should future measurements of A(0) reach the 0.1% level of precision with central values near the most recent results.

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 x 2 pi 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 +/- 0.00089(-0.00140)(+0.00123), from lambda = g(A)/g(V) = -1.27590 +/- 0.00239(-0.00377)(+0.00331). Complete details of the analysis are presented.

A detector efficiency model based on energy extraction from neutrons is described and used to compare 4He detectors with liquid scintillators (EJ301/NE-213). Detector efficiency can be divided into three regimes: single neutron scattering, multiple neutron scattering, and a transition regime in-between. For an average fission neutron of 2MeV, the amount of 4He needed would be about 1/4 of the amount of the mass of EJ301/NE-213 in the single-scattering regime. For about 50% neutron energy extraction (1MeV out of 2MeV), the two types of detectors (4He in the transition regime, EJ301 still in the single-scattering regime) have comparable mass, but 4He detectors can be much larger depending on the number density. A six-tube 11-bar-pressure 4He detector prototype is built and tested. Individual electrical pulses from the detector are recorded using a 12-bit digitizer. Differences in pulse rise time and amplitudes, due to different energy loss of neutrons and gamma rays, are used for neutron/gamma separation. Several energy spectra are also obtained and analyzed.

The accuracy of density measurements and position resolution in flash (40 ns) radiography of thick objects with 24 Gev/c protons is investigated. A global model fit to step wedge data is shown to give a good description spanning the periodic table. The parameters obtained from the step wedge data are used to predict transmission through the French Test Object (FTO), a test object of nested spheres, to a precision better than 1%. Multiple trials have been used to show that the systematic errors are less than 2%. Absolute agreement between the average radiographic measurements of the density and the known density is 1%. Spatial resolution has been measured to be 200 μm at the center of the FTO. These data verify expectations of the benefits provided by high energy hadron radiography for thick objects.

We describe a simple drift tube counter that has been used as a cosmic ray veto for the UCNA experiment, a first-ever measurement of the neutron beta-asymmetry using ultra-cold neutrons. These detectors provide an inexpensive alternative to more conventional scintillation detectors for large area cosmic ray anticoincidence detectors.

Received 10 November 2010DOI:https://doi.org/10.1103/PhysRevLett.105.219903© 2010 The American Physical Society

, plays animportant role in our understanding of the nucleon spinand flavor structure [1, 2]. It is a central target for highprecision lattice QCD calculations [3, 4] and an essentialparameter in effective field theories [5]. The axial-vectorweak coupling constant is also important in a variety ofastrophysical processes, including solar fusion reactionrates [6].The angular distribution of emitted electrons from po-larized neutron decay can be expressed as W(E) ∝ 1 +

A precise measurement of the neutron decay β asymmetry A₀ has been carried out using polarized ultracold neutrons from the pulsed spallation ultracold neutron source at the Los Alamos Neutron Science Center. Combining data obtained in 2008 and 2009, we report A₀ = -0.119 66±0.000 89{-0.001 40}{+0.001 23}, from which we determine the ratio of the axial-vector to vector weak coupling of the nucleon g{A}/g{V}=-1.275 90{-0.004 45}{+0.004 09}.

The design, construction, and performance of a type of sealed 3He drift tubes for neutron detection are presented. Because the 3He pressure is in the 25–300mbar range, the detector costs are not dominated by the 3He gas. Intrinsic neutron detection efficiencies up to 5% have been observed by using high-density polyethylene moderation. Sensitive measurements of the detector lifetime are achieved by monitoring the full-energy peak of the 3He(n, p)3H reaction as a function of time. The neutron peak position shows a 24-h cycle that may be explained by the physical adsorption of gases onto the wall. The estimated lifetimes of the detectors are sufficiently long and therefore, the design and the construction are robust and practical for applications such as fissile material detection.

We report the first measurement of an angular correlation parameter in neutron beta decay using polarized ultracold neutrons (UCN). We utilize UCN with energies below about 200 neV, which we guide and store for approximately 30 s in a Cu decay volume. The interaction of the neutron magnetic dipole moment with a static 7 T field external to the decay volume provides a 420 neV potential energy barrier to the spin state parallel to the field, polarizing the UCN before they pass through an adiabatic fast passage spin flipper and enter a decay volume, situated within a 1 T field in a 2x2pi solenoidal spectrometer. We determine a value for the beta-asymmetry parameter A_{0}=-0.1138+/-0.0046+/-0.0021.