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.

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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.

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