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Here, the precise value of the mean neutron lifetime, τn, 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.

In the UCN{\\tau} experiment, ultracold neutrons (UCN) are confined by magnetic fields and the Earth's gravitational field. Field-trapping mitigates the problem of UCN loss on material surfaces, which caused the largest correction in prior neutron experiments using material bottles. However, the neutron dynamics in field traps differ qualitatively from those in material bottles. In the latter case, neutrons bounce off material surfaces with significant diffusivity and the population quickly reaches a static spatial distribution with a density gradient induced by the gravitational potential. In contrast, the field-confined UCN -- whose dynamics can be described by Hamiltonian mechanics -- do not exhibit the stochastic behaviors typical of an ideal gas model as observed in material bottles. In this report, we will describe our efforts to simulate UCN trapping in the UCN{\\tau} magneto-gravitational trap. We compare the simulation output to the experimental results to determine the parameters of the neutron detector and the input neutron distribution. The tuned model is then used to understand the phase space evolution of neutrons observed in the UCN{\\tau} experiment. We will discuss the implications of chaotic dynamics on controlling the systematic effects, such as spectral cleaning and microphonic heating, for a successful UCN lifetime experiment to reach a 0.01% level of precision.

The precise value of the mean neutron lifetime, \\(\\tau_n\\), plays an important role in nuclear and particle physics and cosmology. It is a key input for predicting the ratio of protons to helium atoms in the primordial universe and is used to search for new physics beyond the Standard Model of particle physics. There is a 3.9 standard deviation discrepancy between \\(\\tau_n\\) measured by counting the decay rate of free neutrons in a beam (887.7 \\(\\pm\\) 2.2 s) and by counting surviving ultracold neutrons stored for different storage times in a material trap (878.5\\(\\pm\\)0.8 s). The experiment described here eliminates 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 and neutrons in quasi-stable orbits rapidly exit the trap. As a result of this approach and the use of a new in situ neutron detector, the lifetime reported here (877.7 \\(\\pm\\) 0.7 (stat) +0.4/-0.2 (sys) s) is the first modern measurement of \\(\\tau_n\\) that does not require corrections larger than the quoted uncertainties.

The neutron lifetime is important in understanding the production of light nuclei in the first minutes after the big bang and it provides basic information on the charged weak current of the standard model of particle physics. Two different methods have been used to measure the neutron lifetime: disappearance measurements using bottled ultracold neutrons and decay rate measurements using neutron beams. The best measurements using these two techniques give results that differ by nearly 4 standard deviations. In this paper we describe a new method for measuring surviving neutrons in neutron lifetime measurements using bottled ultracold neutrons that provides better characterization of systematic uncertainties and enables higher precision than previous measurement techniques. We present results obtained using our method.

Position-sensitive detection of ultracold neutrons (UCNs) is demonstrated using an imaging charge-coupled device (CCD) camera. A spatial resolution less than 15 \\(\\mu\\)m has been achieved, which is equivalent to an UCN energy resolution below 2 pico-electron-volts through the relation \\(\\delta E = m_0g \\delta x\\). Here, the symbols \\(\\delta E\\), \\(\\delta x\\), \\(m_0\\) and \\(g\\) are the energy resolution, the spatial resolution, the neutron rest mass and the gravitational acceleration, respectively. A multilayer surface convertor described previously is used to capture UCNs and then emits visible light for CCD imaging. Particle identification and noise rejection are discussed through the use of light intensity profile analysis. This method allows different types of UCN spectroscopy and other applications.

A multilayer surface detector for ultracold neutrons (UCNs) is described. The top \\(^{10}\\)B layer is exposed to the vacuum chamber and directly captures UCNs. The ZnS:Ag layer beneath the \\(^{10}\\)B layer is a few microns thick, which is sufficient to detect the charged particles from the \\(^{10}\\)B(n,\\(\\alpha\\))\\(^7\\)Li neutron-capture reaction, while thin enough so that ample light due to \\(\\alpha\\) and \\(^7\\)Li escapes for detection by photomultiplier tubes. One-hundred-nm thick \\(^{10}\\)B layer gives high UCN detection efficiency, as determined by the mean UCN kinetic energy, detector materials and others. Low background, including negligible sensitivity to ambient neutrons, has also been verified through pulse-shape analysis and comparisons with other existing \\(^3\\)He and \\(^{10}\\)B detectors. This type of detector has been configured in different ways for UCN flux monitoring, development of UCN guides and neutron lifetime research.

It is generally accepted that the main cause of ultracold neutron (UCN) losses in storage traps is the upscattering to the thermal energy range by hydrogen adsorbed on the surface of the trap walls. However, the data on which this conclusion is based are poor and contradictory. Here, we report a measurement, performed at the Los Alamos National Laboratory UCN source, of the average energy of the flux of upscattered neutrons after the interaction of UCN with hydrogen bound in semicrystalline polymer PMP (tradename TPX), [C\\(_{6}\\)H\\(_{12}\\)]\\(_n\\). Our analysis, performed with the MCNP code based on the application of the neutron scattering law to UCN upscattered by bound hydrogen in semicrystalline polyethylene, [C\\(_{2}\\)H\\(_{4}\\)]\\(_n\\), leads us to a flux average energy value of 26\\(\\pm3\\) meV in contradiction with previously reported experimental values of 10 to 13 meV and in agreement with the theoretical models of neutron heating implemented in the MCNP code.

The study of neutron cross sections for elements used as efficient ``absorbers'' of ultracold neutrons (UCN) is crucial for many precision experiments in nuclear and particle physics, cosmology and gravity. In this context, ``absorption'' includes both the capture and upscattering of neutrons to the energies above the UCN energy region. The available data, especially for hydrogen, do not agree between themselves or with the theory. In this report we describe measurements performed at the Los Alamos National Laboratory UCN facility of the UCN upscattering cross sections for vanadium and for hydrogen in CH\\(_2\\) using simultaneous measurements of the radiative capture cross sections for these elements. We measured \\(\\sigma_{up}=1972\\pm130\\) b for hydrogen in CH\\(_2\\), which is below theoretical expectations, and \\(\\sigma_{up} < 25\\pm9\\) b for vanadium, in agreement with the expectation for the neutron heating by thermal excitations in solids.

A new measurement of the neutron \\(\\beta\\)-decay asymmetry \\(A_0\\) has been carried out by the UCNA collaboration using polarized ultracold neutrons (UCN) from the solid deuterium UCN source at the Los Alamos Neutron Science Center (LANSCE). Improvements in the experiment have led to reductions in both statistical and systematic uncertainties leading to \\(A_0 = -0.11954(55)_{\\rm stat.}(98)_{\\rm syst.}\\), corresponding to the ratio of axial-vector to vector coupling \\(\\lambda \\equiv g_A/g_V = -1.2756(30)\\).

A precise measurement of the neutron decay \\(\\beta\\)-asymmetry \\(A_0\\) has been carried out using polarized ultracold neutrons (UCN) from the pulsed spallation UCN source at the Los Alamos Neutron Science Center (LANSCE). Combining data obtained in 2008 and 2009, we report \\(A_0 = -0.11966 \\pm 0.00089_{-0.00140}^{+0.00123}\\), from which we determine the ratio of the axial-vector to vector weak coupling of the nucleon \\(g_A/g_V = -1.27590_{-0.00445}^{+0.00409}\\).