Background:The neutron β-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, λ ≡ 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 β-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 downscattering 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 .

Fornal and Grinstein recently proposed that the discrepancy between two different methods of neutron lifetime measurements, the beam and bottle methods, can be explained by a previously unobserved dark matter decay mode, n → X þ γ. We perform a search for this decay mode over the allowed range of energies of the monoenergetic γ ray for X to be dark matter. A Compton-suppressed high-purity germanium detector is used to identify γ rays from neutron decay in a nickel-phosphorous-coated stainless-steel bottle. A combination of Monte Carlo and radioactive source calibrations is used to determine the absolute efficiency for detecting γ rays arising from the dark matter decay mode. We exclude the possibility of a sufficiently strong branch to explain the lifetime discrepancy with 97% confidence. DOI: 10.1103/PhysRevLett.121.022505 There is nearly a five-standard-deviation disagreement [1,2] between measurements of the rate of neutron decay producing protons measured in cold neutron beam experiments [3-5] (888.0 AE 2.0 s) and free neutron lifetime in bottle experiments [6-8] (878.1 AE 0.5 s). The cold neutron beam method consists of counting the number of protons emitted from neutron β decay in a well-characterized neutron beam, and the bottle experiments measure the number of ultracold neutrons (UCNs) that remain inside a trap after a certain storage time. A longer lifetime from the beam measurements could point to the existence of possible other decay modes of the neutron where a proton is not produced. Serebrov has suggested that the discrepancy could be due to neutrons oscillating into mirror neutrons [9,10]. Recently, Fornal and Grinstein suggested in Ref.[11] that the neutron lifetime discrepancy can be explained if the neutron were to decay into a γ ray and a dark matter particle, X. The γ ray has an allowable energy range of 782 to 1664 keV, where it is bounded from above by the stability of 9 Be and bounded from below by requiring X to be stable.Here, we report the results of a search for γ rays arising from UCNs decaying inside a nickel-phosphorouscoated [12], 560 l stainless-steel bottle. The bottle is filled with UCNs from the Los Alamos UCN facility [13] parasitically during the running of the UCN τ experiment [7], with the source operated in production mode. The γ rays are detected in a lead shielded, Compton-scatteringsuppressed 140% high-purity germanium (HPGe) detector (Fig. 1). The Compton-scattering suppression is achieved by an anticoincidence with an annular bismuth germinate (BGO) detector surrounding the HPGe detector. The Compton suppression reduced the background in the low energy part of the spectrum by a factor of 1.7. A gate valve placed upstream controlled the loading of UCNs into the bottle. The background γ rates were measured with the UCNs in production mode and the gate valve closed. This resulted in a factor of 4 reduction in the continuum background in the region of interest (ROI).The energy calibration of the HPGe spectrum was obtained from a linear fit to 13γ-ray lines from source...

A cryogenic apparatus is described that enables a new experiment, nEDM@SNS, with a major improvement in sensitivity compared to the existing limit in the search for a neutron Electric Dipole Moment (EDM). This apparatus uses superfluid 4 He to produce a high density of Ultra-Cold Neutrons (UCN) which are contained in a suitably coated pair of measurement cells. The experiment, to be operated at the Spallation Neutron Source at Oak Ridge National Laboratory, uses polarized 3 He from an Atomic Beam Source injected into the superfluid 4 He and transported to the measurement cells where it serves as a co-magnetometer. The superfluid 4 He is also used as an insulating medium allowing significantly higher electric fields, compared to previous experiments, to be maintained across the measurement cells. These features provide an ultimate statistical uncertainty for the EDM of 2 − 3 × 10 −28 e-cm, with anticipated systematic uncertainties below this level.

We report an improved measurement of the free neutron lifetime τ n using the UCNτ apparatus at the Los Alamos Neutron Science Center. We count a total of approximately 38 × 10 6 surviving ultracold neutrons (UCNs) after storing in UCNτ's magnetogravitational trap over two data acquisition campaigns in 2017 and 2018. We extract τ n from three blinded, independent analyses by both pairing long and short storage time runs to find a set of replicate τ n measurements and by performing a global likelihood fit to all data while selfconsistently incorporating the β-decay lifetime. Both techniques achieve consistent results and find a value τ n ¼ 877.75 AE 0.28 stat þ 0.22= − 0.16 syst s. With this sensitivity, neutron lifetime experiments now directly address the impact of recent refinements in our understanding of the standard model for neutron decay.

It has been proposed recently that a previously unobserved neutron decay branch to a dark matter particle (χ ) 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 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. The summed kinetic energy (E e + e − ) from such events is used to set limits, as a function of the χ mass, on the branching fraction for this decay channel. For χ masses consistent with resolving the neutron lifetime discrepancy, we exclude this as the dominant dark matter decay channel at 5σ level for 100 < E e + e − < 644 keV. If the χ + 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.