Improved neutron lifetime experiment based on magnetic storage of ultracold neutrons

When freed from an atomic nucleus, the neutron decays via the weak interaction into a proton, electron, and antineutrino, with an average lifetime of about 15 minutes. The precise value of this lifetime depends on basic parameters of the standard model of particle physics, and detailed measurements of neutron decay determine these parameters. The most precise measurement of the neutron lifetime was recently reported by the UCNtau collaboration using an apparatus that confined ultracold neutrons through a combination of magnetic and gravitational fields. The final result, with a quarter-second uncertainty, appears to be statistics limited. This talk covers a major upgrade to the experiment to increase the number of trapped neutrons by an order or magnitude with a novel elevator loading mechanism. Here, ultracold neutrons are first loaded into a material volume "elevator," which is then swept through the magneto-gravitational trap leaving a population of trapped neutrons. The status of the upgraded experiment, called UCNtau+, will be described including results of commissioning at the Los Alamos ultracold neutron source.