Improved neutron beta decay asymmetry experiment at LANL*

Beta decay of a free neutron is the simplest form of "semi-leptonic" weak interaction and is free from nuclear structure effects. A critical measurement from the decay is the correlation (A_o) between the neutron's initial spin and emitted electron's momentum. The angular correlation coefficient, A₀, determines the axial neutron charge. Neutron lifetime and axial neutron charge determined are inputs to determine the magnitude of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element (V_ud) and provide a means to study physics beyond the Standard Model.

Los Alamos National Laboratory hosted the UCNA experiment to measure the beta-asymmetry parameter (A_o) by exploiting the ultra-cold neutron (UCN) source [1]. UCNs are produced at LANL via the down scattering of the moderated spallation neutrons in a solid deuterium crystal [2] and are polarized via transport through magnets. The UCNA+ experiment aims to take advantage of the improved source production, improved beta detector based on SiPMs, and improved calibration tools to achieve a precision of ~ 0.2%. In this contribution, I will discuss the experiment setup details and progress toward commissioning runs.