New Results from the UCN$\tau$ Neutron Lifetime Experiment

A free neutron provides the simplest example of nuclear $\beta$-decay, leading to a unique suite of tests for fundamental parameters of electroweak theory. The neutron lifetime $\tau_n$, in conjunction with other neutron $\beta$-decay observables, can be used to extract $V_{ud}$ experimentally. The UCN$\tau$ experiment at Los Alamos National Laboratory (LANL) traps ultracold neutrons (UCN) in a magnetogravitational "bottle" for periods of time longer than the neutron lifetime. A permanent magnet Halbach array minimizes neutron losses, ensuring that the loss rates of UCN in UCN$\tau$ generate systematic corrections no greater than the statistical precision of the experiment. The addition of a buffer volume between the UCN source and the trap has improved the ability to reconstruct the number of initially stored neutrons. Potential sources of loss have been benchmarked by using the in-situ "dagger" and "active cleaner" detectors to probe UCN near the trapping threshold. We present the results of three independent, blinded, analyses utilizing $\sim 10^7$ neutrons collected over two calendar years of operation. UCN$\tau$ now reports a lifetime of $877.75 \pm 0.28_{\text{(stat.)}} + 0.22 / - 0.16_{\text{(sys.)}}$~s, directly probing recent developments in $\beta$-decay theory.