@article{gonzalez_fries_cude-woods_bailey_blatnik_broussard_callahan_choi_clayton_currie_et al._2021, title={Improved Neutron Lifetime Measurement with UCN τ}, volume={127}, ISSN={["1079-7114"]}, DOI={10.1103/PhysRevLett.127.162501}, abstractNote={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 self-consistently incorporating the β-decay lifetime. Both techniques achieve consistent results and find a value τ_{n}=877.75±0.28_{stat}+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.}, number={16}, journal={PHYSICAL REVIEW LETTERS}, author={Gonzalez, F. M. and Fries, E. M. and Cude-Woods, C. and Bailey, T. and Blatnik, M. and Broussard, L. J. and Callahan, N. B. and Choi, J. H. and Clayton, S. M. and Currie, S. A. and et al.}, year={2021}, month={Oct}, pages={162501} }
 @article{sun_adamek_allgeier_bagdasarova_berguno_blatnik_bowles_broussard_brown_carr_et al._2020, title={Improved limits on Fierz interference using asymmetry measurements from the Ultracold Neutron Asymmetry (UCNA) experiment}, volume={101}, ISSN={2469-9985 2469-9993}, url={http://dx.doi.org/10.1103/PhysRevC.101.035503}, DOI={10.1103/PhysRevC.101.035503}, abstractNote={The Ultracold Neutron Asymmetry (UCNA) experiment was designed to measure the β-decay asymmetry parameter, A₀, for free neutron decay. In the experiment, polarized ultracold neutrons are transported into a decay trap, and their β-decay electrons are detected with ≈4π acceptance into two detector packages which provide position and energy reconstruction. The experiment also has sensitivity to b_n, the Fierz interference term in the neutron β-decay rate. In this work, we determine b_n from the energy dependence of A₀ using the data taken during the UCNA 2011-2013 run. In addition, we present the same type of analysis using the earlier 2010 A dataset. Motivated by improved statistics and comparable systematic errors compared to the 2010 data-taking run, we present a new b_n measurement using the weighted average of our asymmetry dataset fits, to obtain b_n = 0.066±0.041_(stat)±0.024_(syst) which corresponds to a limit of −0.012 < b_n < 0.144 at the 90% confidence level.}, number={3}, journal={Physical Review C}, publisher={American Physical Society (APS)}, author={Sun, X. and Adamek, E. and Allgeier, B. and Bagdasarova, Y. and Berguno, D. B. and Blatnik, M. and Bowles, T. J. and Broussard, L. J. and Brown, M. A.-P. and Carr, R. and et al.}, year={2020}, month={Mar} }
 @article{callahan_liu_gonzalez_adamek_bowman_broussard_clayton_currie_cude-woods_dees_et al._2019, title={Monte Carlo simulations of trapped ultracold neutrons in the UCNτ experiment}, volume={100}, ISSN={2469-9985 2469-9993}, url={http://dx.doi.org/10.1103/PhysRevC.100.015501}, DOI={10.1103/PhysRevC.100.015501}, abstractNote={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.}, number={1}, journal={Physical Review C}, publisher={American Physical Society (APS)}, author={Callahan, Nathan and Liu, Chen-Yu and Gonzalez, Francisco and Adamek, E. and Bowman, J. D. and Broussard, L. and Clayton, S. M. and Currie, S. and Cude-Woods, C. and Dees, E. B. and et al.}, year={2019}, month={Jul} }
 @article{pattie_callahan_cude-woods_adamek_broussard_clayton_currie_dees_ding_engel_et al._2018, title={Measurement of the neutron lifetime using a magneto-gravitational trap and in situ detection}, volume={360}, ISSN={1095-9203}, url={https://doi.org/10.1126/science.aan8895}, DOI={10.1126/science.aan8895}, abstractNote={How long does a neutron live? 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, 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.}, number={6389}, journal={SCIENCE}, publisher={American Association for the Advancement of Science (AAAS)}, author={Pattie, R. W., Jr. and Callahan, N. B. and Cude-Woods, C. and Adamek, E. R. and Broussard, L. J. and Clayton, S. M. and Currie, S. A. and Dees, E. B. and Ding, X. and Engel, E. M. and et al.}, year={2018}, month={May}, pages={627–631} }
 @article{brown_dees_adamek_allgeier_blatnik_bowles_broussard_carr_clayton_cude-woods_et al._2018, title={New result for the neutron <mml:mi>β-asymmetry parameter A<mml:msub>0</mml:msub></mml:mi> from UCNA}, volume={97}, ISSN={2469-9985 2469-9993}, url={http://dx.doi.org/10.1103/physrevc.97.035505}, DOI={10.1103/physrevc.97.035505}, abstractNote={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≡gA/gV, which under assumption of the conserved vector current hypothesis (gV=1) determines gA. Precise values for gA 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. 
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 λ=gA/gV=−1.2783(22). Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces A0=−0.12015(34)stat(63)syst and λ=gA/gV=−1.2772(20). 
Conclusions: This new result for A0 and gA/gV from the UCNA experiment has provided confirmation of the shift in values for gA/gV 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 A0 reach the 0.1% level of precision with central values near the most recent results.}, number={3}, journal={Physical Review C}, publisher={American Physical Society (APS)}, author={Brown, M. A.-P. and Dees, E. B. and Adamek, E. and Allgeier, B. and Blatnik, M. and Bowles, T. J. and Broussard, L. J. and Carr, R. and Clayton, S. and Cude-Woods, C. and et al.}, year={2018}, month={Mar}, pages={035505} }
 @article{sun_adamek_allgeier_blatnik_bowles_broussard_brown_carr_clayton_cude-woods_et al._2018, title={Search for dark matter decay of the free neutron from the UCNA experiment: n -> X plus e(+)e(-)}, volume={97}, ISSN={["2469-9993"]}, DOI={10.1103/physrevc.97.052501}, abstractNote={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 (UCNs). 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 ≫ 5σlevel for 100 keV 90% confidence level.}, number={5}, journal={PHYSICAL REVIEW C}, author={Sun, X. and Adamek, E. and Allgeier, B. and Blatnik, M. and Bowles, T. J. and Broussard, L. J. and Brown, M. A-P and Carr, R. and Clayton, S. and Cude-Woods, C. and et al.}, year={2018}, month={May} }
 @article{seestrom_adamek_barlow_blatnik_broussard_callahan_clayton_cude-woods_currie_dees_et al._2017, title={Total cross sections for ultracold neutrons scattered from gases}, volume={95}, ISSN={2469-9993}, DOI={10.1103/physrevc.95.015501}, abstractNote={We have followed up on our previous measurements of upscattering of ultracold neutrons (UCNs) from a series of gases by making measurements of total cross sections on the following gases hydrogen, ethane, methane, isobutene, n-butane, ethylene, water vapor, propane, neopentane, isopropyl alcohol, and ^3He. The values of these cross sections are important for estimating the loss rate of trapped neutrons due to residual gas and are relevant to neutron lifetime measurements using UCNs. The effects of the UCN velocity and path-length distributions were accounted for in the analysis using a Monte Carlo transport code. Results are compared to our previous measurements and with the known absorption cross section for ^3He scaled to our UCN energy. We find that the total cross sections for the hydrocarbon gases are reasonably described by a function linear in the number of hydrogen atoms in the molecule.}, number={1}, journal={PHYSICAL REVIEW C}, author={Seestrom, S. J. and Adamek, E. R. and Barlow, D. and Blatnik, M. and Broussard, L. J. and Callahan, N. B. and Clayton, S. M. and Cude-Woods, C. and Currie, S. and Dees, E. B. and et al.}, year={2017}, month={Jan} }
 @misc{phillips_snow_babul_banerjee_baxter_berezhiani_bergevin_bhattacharya_brooijmans_castellanos_et al._2016, title={Neutron-antineutron oscillations: Theoretical status and experimental prospects}, volume={612}, ISSN={["1873-6270"]}, DOI={10.1016/j.physrep.2015.11.001}, abstractNote={The observation of neutrons turning into antineutrons would constitute a discovery of fundamental importance for particle physics and cosmology. Observing the n–n̄ transition would show that baryon number (B) is violated by two units and that matter containing neutrons is unstable. It would provide a clue to how the matter in our universe might have evolved from the B=0 early universe. If seen at rates observable in foreseeable next-generation experiments, it might well help us understand the observed baryon asymmetry of the universe. A demonstration of the violation of B–L by 2 units would have a profound impact on our understanding of phenomena beyond the Standard Model of particle physics. Slow neutrons have kinetic energies of a few meV. By exploiting new slow neutron sources and optics technology developed for materials research, an optimized search for oscillations using free neutrons from a slow neutron moderator could improve existing limits on the free oscillation probability by at least three orders of magnitude. Such an experiment would deliver a slow neutron beam through a magnetically-shielded vacuum chamber to a thin annihilation target surrounded by a low-background antineutron annihilation detector. Antineutron annihilation in a target downstream of a free neutron beam is such a spectacular experimental signature that an essentially background-free search is possible. An authentic positive signal can be extinguished by a very small change in the ambient magnetic field in such an experiment. It is also possible to improve the sensitivity of neutron oscillation searches in nuclei using large underground detectors built mainly to search for proton decay and detect neutrinos. This paper summarizes the relevant theoretical developments, outlines some ideas to improve experimental searches for free neutron oscillations, and suggests avenues both for theoretical investigation and for future improvement in the experimental sensitivity.}, journal={PHYSICS REPORTS-REVIEW SECTION OF PHYSICS LETTERS}, author={Phillips, D. G., II and Snow, W. M. and Babul, K. and Banerjee, S. and Baxter, D. V. and Berezhiani, Z. and Bergevin, M. and Bhattacharya, S. and Brooijmans, G. and Castellanos, L. and et al.}, year={2016}, month={Feb}, pages={1–45} }
 @article{wei_broussard_hoffbauer_makela_morris_tang_adamek_callahan_clayton_cude-woods_et al._2016, title={Position-sensitive detection of ultracold neutrons with an imaging camera and its implications to spectroscopy}, volume={830}, ISSN={0168-9002}, url={http://dx.doi.org/10.1016/j.nima.2016.05.058}, DOI={10.1016/j.nima.2016.05.058}, abstractNote={Position-sensitive detection of ultracold neutrons (UCNs) is demonstrated using an imaging charge-coupled device (CCD) camera. A spatial resolution less than 15μm has been achieved, which is equivalent to a UCN energy resolution below 2 pico-electron-volts through the relation δE=m0gδx. Here, the symbols δE, δx, m0 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.}, journal={Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment}, publisher={Elsevier BV}, author={Wei, Wanchun and Broussard, L.J. and Hoffbauer, M.A. and Makela, M. and Morris, C.L. and Tang, Z. and Adamek, E.R. and Callahan, N.B. and Clayton, S.M. and Cude-Woods, C. and et al.}, year={2016}, month={Sep}, pages={36–43} }
 @article{wang_hoffbauer_morris_callahan_adamek_bacon_blatnik_brandt_broussard_clayton_et al._2015, title={A multilayer surface detector for ultracold neutrons}, volume={798}, DOI={10.1016/j.nima.2015.07.010}, abstractNote={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.}, journal={Nuclear Instruments & Methods in Physics Research. Section A, Accelerators, Spectrometers, Detectors, and Associated Equipment}, author={Wang, Z. H. and Hoffbauer, M. A. and Morris, C. L. and Callahan, N. B. and Adamek, E. R. and Bacon, J. D. and Blatnik, M. and Brandt, A. E. and Broussard, L. J. and Clayton, S. M. and et al.}, year={2015}, pages={30–35} }
 @article{salvat_adamek_barlow_bowman_broussard_callahan_clayton_cude-woods_currie_dees_et al._2014, title={Storage of ultracold neutrons in the magneto-gravitational trap of the UCN tau experiment}, volume={89}, ISSN={1089-490X}, DOI={10.1103/physrevc.89.052501}, abstractNote={The UCN experiment is designed to measure the lifetime n of the free neutron by trapping ultracold neutrons (UCN) in a magneto-gravitational trap. An asymmetric bowl-shaped NdFeB magnet Halbach array confines low-field-seeking UCN within the apparatus, and a set of electromagnetic coils in a toroidal geometry provides a background holding field to eliminate depolarization-induced UCN loss caused by magnetic field nodes. We present a measurement of the storage time store of the trap by storing UCN for various times and counting the survivors. The data are consistent with a single exponential decay, and we find store = 860 19 s, within 1 of current global averages for n. The storage time with the holding field deactivated is found to be store = 470 160 s; this decreased storage time is due to the loss of UCN, which undergo Majorana spin flips while being stored. We discuss plans to increase the statistical sensitivity of the measurement and investigate potential systematic effects.}, number={5}, journal={PHYSICAL REVIEW C}, author={Salvat, D. J. and Adamek, E. R. and Barlow, D. and Bowman, J. D. and Broussard, L. J. and Callahan, N. B. and Clayton, S. M. and Cude-Woods, C. and Currie, S. and Dees, E. B. and et al.}, year={2014}, month={May} }