The precise value of the mean neutron lifetime, τ, plays an important role in nuclear and particle physics and cosmology. It is used to predict the ratio of protons to helium atoms in the primordial universe and to search for physics beyond the Standard Model of particle physics. We eliminated loss mechanisms present in previous trap experiments by levitating polarized ultracold neutrons above the surface of an asymmetric storage trap using a repulsive magnetic field gradient so that the stored neutrons do not interact with material trap walls. As a result of this approach and the use of an in situ neutron detector, the lifetime reported here [877.7 ± 0.7 (stat) +0.4/-0.2 (sys) seconds] does not require corrections larger than the quoted uncertainties.

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...

We present a detailed report of a measurement of the neutron β-asymmetry parameter A0, 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 × 2π 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.

The ultracold neutron (UCN) source at Los Alamos National Laboratory (LANL), which uses solid deuterium as the UCN converter and is driven by accelerator spallation neutrons, has been successfully operated for over 10 years, providing UCN to various experiments, as the first production UCN source based on the superthermal process. It has recently undergone a major upgrade. This paper describes the design and performance of the upgraded LANL UCN source. Measurements of the cold neutron spectrum and UCN density are presented and compared to Monte Carlo predictions. The source is shown to perform as modeled. The UCN density measured at the exit of the biological shield was 184(32) UCN/cm 3 , a four-fold increase from the highest previously reported. The polarized UCN density stored in an external chamber was measured to be 39(7) UCN/cm 3 , which is sufficient to perform an experiment to search for the nonzero neutron electric dipole moment with a one-standard-deviation sensitivity of σ(dn) = 3 × 10 −27 e·cm.

We present a review of the motivation and results of recent experiments which utilize ultracold neutrons for measurements of neutron beta decay. Because these experiments hinge critically on the available ultracold neutron source technology, we also review the status of ultracold neutron source development, emphasizing the Los Alamos ultracold neutron facility and the ongoing beta decay research program sited there.