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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.

Unlike the proton, whose lifetime is longer than the age of the universe, a free neutron decays with a lifetime of about 15 minutes. Measuring the exact lifetime of neutrons is surprisingly tricky; putting them in a container and monitoring their decay can lead to errors because some neutrons will be lost owing to interactions with the container walls. To overcome this problem, Pattie et al. measured the lifetime in a trap where ultracold polarized neutrons were levitated by magnetic fields, precluding interactions with the trap walls (see the Perspective by Mumm). This more precise determination of the neutron lifetime will aid our understanding of how the first nuclei formed after the Big Bang. Science , this issue p. 627 ; see also p. 605 Ultracold polarized neutrons are levitated in a trap to measure their lifetime with reduced systematic uncertainty. The precise value of the mean neutron lifetime, τ n , 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.

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.

A new boron-coated CCD camera is described for direct detection of ultracold neutrons (UCN) through the capture reactions \\(^{10}\\)B (n,\\(\\alpha\\)0\\(\\gamma\\))\\(^7\\)Li (6%) and \\(^{10}\\)B(n,\\(\\alpha\\)1\\(\\gamma\\))\\(^7\\)Li (94%). The experiments, which extend earlier works using a boron-coated ZnS:Ag scintillator, are based on direct detections of the neutron-capture byproducts in silicon. The high position resolution, energy resolution and particle ID performance of a scientific CCD allows for observation and identification of all the byproducts \\(\\alpha\\), \\(^7\\)Li and \\(\\gamma\\) (electron recoils). A signal-to-noise improvement on the order of 10\\(^4\\) over the indirect method has been achieved. Sub-pixel position resolution of a few microns is demonstrated. The technology can also be used to build UCN detectors with an area on the order of 1 m\\(^2\\). The combination of micrometer scale spatial resolution, few electrons ionization thresholds and large area paves the way to new research avenues including quantum physics of UCN and high-resolution neutron imaging and spectroscopy.

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 is the simplest nuclear system that can be used to probe the structure of the weak interaction and search for physics beyond the standard model. Measurements of neutron lifetime and β-decay correlation coefficients with precisions of 0.02% and 0.1%, respectively, would allow for stringent constraints on new physics. The UCNτ experiment uses an asymmetric magneto-gravitational UCN trap with in situ counting of surviving neutrons to measure the neutron lifetime, τ n = 877.7s (0.7s) stat (+0.4/−0.2s) sys . We discuss the recent result from UCNτ, the status of ongoing data collection and analysis, and the path toward a 0.25 s measurement of the neutron lifetime with UCNτ.

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.

The UCNA experiment was designed to measure the neutron β-asymmetry parameter A0 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 A0 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 A0.