Session JE: Advances in Fundamental Neutron Physics Experiments

A sub-second measurement of the neutron lifetime from the UCN$\tau$ experiment

The UCN$\tau$ experiment has completed the first sub-second measurement of the neutron lifetime where the systematic corrections are smaller than the uncertainties. Using a bowl made of permanent magnets to trap the neutrons from below and gravity to confined the neutrons from above UCN$\tau$ eliminates almost all systematic corrections arising from material interactions. A novel {\it in situ} counting scheme for the surviving neutrons eliminates any systematics associated with transport to an external detector. The successful upgrade of the ultracold neutron source at the Los Alamos Neutron Science Center (LANSCE) has allowed UCN$\tau$ to perform several 1~s measurements in configuration designed to explore possible systematic effects. We will report on the combined analysis of five 1-2 second blinded measurements of the neutron lifetime performed during the 2016-2017 LANSCE run cycle, which resulted in a value of $\tau_n =877.7 \pm (0.7)_{stat} (+0.3/-0.1)_{sys}$ in agreement with previous material bottle UCN storage experiments.   

Measurement of the beta asymmetry in neutron decay with UCNA: status in 2017 and new opportunities

UCNA is an experiment which measures the beta asymmetry in neutron decay, or the angular correlation between the spin of the neutron and the momentum of the emitted beta particle. When taken together with a measurement of the lifetime, the standard model provides rigorous predictions for the observables in neutron decay to about the $10^{-4}$ level. These high precision predictions set the stage for an effective probe for new physics above the energy scales reachable by the LHC. UCNA is the first experiment to measure angular correlations using ultracold neutrons (UCN). This approach is motivated by the opportunities UCN provide to control key sources of systematic error due to the neutron polarization and neutron-produced backgrounds. UCNA data was taken from 2007 to 2013, with the analysis of the 2011-2013 data completed this year. We present the status of the experiment, emphasizing the evolution of our understanding of the systematic errors. The recent upgrade of the LANSCE UCN source provides a significant opportunity to further improve the reach of UCNA in a next generation experiment, so we also outline strategies to push UCNA to precision levels complementary to the $0^+ \rightarrow 0^+$ decays, providing new constraints on BSM physics with post-LHC sensitivity.   

The Parity Violating Proton Asymmetry from Neutron Capture on Helium-3

While hadronic parity violation was observed almost 60 years ago in compound nuclei with large nuclear enhancements, a systematic characterization of weak interactions among strongly bound systems is still forthcoming. New theoretical frameworks, experimental facilities, and advanced technology have rejuvenated efforts to map out this "complexity frontier" of the Standard Model. A critical measurement in this campaign, the n-$^3$He experiment, was performed at Oak Ridge National Laboratory to obtain a specific combination of the five isospin-dependent couplings which characterize the hadronic weak interaction. We report the parity violating asymmetry from this experiment, the most precise hadronic asymmetry ever measured.