UCNtau: Progress Towards a 0.3 s Measurement of the Neutron Lifetime

High precision measurements of properties of the neutron provide an exciting window into the structure of the standard model of particle physics. A 0.3 s measurement of the neutron lifetime, along with a similarly precise angular correlation measurement, would allow for a determination the CKM matrix element Vud from neutron observables at a comparable precision as that derived from Superallowed Fermi decays. At the Los Alamos ultracold neutron source the UCNtau collaboration is using a magneto- gravitational trap to measure the neutron’s lifetime with a precision below 0.7 s. In situ counting and the asymmetric trap design make UCNtau systematically complementary to other physical bottle and cold-neutron beam experiments. To reach an ultimate precision of <= 0.3 s several key systematic effects such as phase space evolution, uncleaned quasi-bound neutrons, and pile-up must be controlled or characterized. The status of UCNtau’s data collection and effort to characterize leading systematic effects will be presented.