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.

The NPDGamma collaboration reports results from the first phase of a measurement of the parity violating up-down asymmetry Aγ with respect to the neutron spin direction of γ-rays emitted in the reaction n + p → d + γ using the capture of polarized cold neutrons on the protons in a liquid parahydrogen target. One expects parity-odd effects in the hadronic weak interaction (HWI) between nucleons to be induced by the weak interaction between quarks. Aγ in n + p → d + γ is dominated by a ∆I = 1, 3 S1 − 3 P1 parity-odd transition amplitude in the n-p system. The first phase of the measurement was completed at the Los Alamos Neutron Science Center spallation source (LANSCE) with the result Aγ = (−1.2 ± 2.1 stat. ± 0.2 sys.) × 10 −7 . We also report the first measurement of an upper limit for the parity allowed left right asymmetry in this reaction, with the result Aγ,LR = (−1.8 ± 1.9 stat. ± 0.2 sys.) × 10 −7 . In this paper we give a detailed report on the theoretical background, experimental setup, measurements, extraction of the parity-odd and parity-allowed asymmetries, analysis of potential systematic effects, and the LANSCE results. The asymmetry has an estimated size of 5 × 10 −8 and the aim of the NPDGamma collaboration is to measure it to 1 × 10 −8 . The second phase of the measurement will be performed at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.

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.

We report the first observation of the parity-violating gamma-ray asymmetry A np γ in neutronproton capture using polarized cold neutrons incident on a liquid parahydrogen target at the Spallation Neutron Source at Oak Ridge National Laboratory. A np γ isolates the ∆I = 1, 3 S1 → 3 P1 component of the weak nucleon-nucleon interaction, which is dominated by pion exchange and can be directly related to a single coupling constant in either the DDH meson exchange model or pionless effective field theory. We measured A np γ = (−3.0 ± 1.4(stat.) ± 0.2(sys.)) × 10 −8 , which implies a DDH weak πN N coupling of h 1 π = (2.6 ± 1.2(stat.) ± 0.2(sys.)) × 10 −7 and a pionless EFT constant of C 3 S 1 → 3 P 1 /C0 = (−7.4 ± 3.5(stat.) ± 0.5(sys.)) × 10 −11 MeV −1 . We describe the experiment, data analysis, systematic uncertainties, and implications of the result.

We report an upper bound on parity-violating neutron spin rotation in 4 He. This experiment is the most sensitive search for neutron-weak optical activity yet performed and represents a significant advance in precision in comparison to past measurements in heavy nuclei. The experiment was performed at the NG-6 slow-neutron beamline at the National Institute of Standards and Technology (NIST) Center for Neutron Research. Our result for the neutron spin rotation angle per unit length in 4 He is dφ/dz = [+1.7 ± 9.1(stat.) ± 1.4(sys.)] × 10 −7 rad/m. The statistical uncertainty is smaller than current estimates of the range of possible values of dφ/dz in n+ 4 He.