Measurement of the neutron lifetime using a magneto-gravitational trap and in situ detection

The precise value of the mean neutron lifetime, τ n , plays an important role in nuclear and particle physicsand cosmology. It is used to predict the ratio of protons to helium atomsin the primordial universe and to search for physics beyondthe Standard Modelof particle physics. We eliminated loss mechanisms present in previous trap experiments by levitatingpolarized ultracold neutronsabove 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.