UCNA: Measuring the β-asymmetry in neutron decay with ultracold neutrons*

The neutron provides a simple yet dynamic nuclear system to study the Standard Model of Particle Physics. The process of transforming a neutron into a proton, an electron, and an anti-neutrino contains a wealth of information in the decay rate and the kinematics of the emitted particles. Precision measurements of angular correlations in neutron β-decay can lend insight to the structure of the weak interaction and probe for physics beyond the Standard Model. The UCNA experiment at the Los Alamos Neutron Science Center is the only experiment to use ultracold neutrons to perform such a measurement, determining the β-asymmetry parameter A₀= -0.12054(44)_stat(68)_sys. The β-asymmetry describes the correlation between the neutron's spin and the emission direction of the out-going electron. To leading order A₀ is defined by the ratio of the axial-vector and vector weak couplings constants λ ≡ g_A / g_V. In this presentation I will discuss the final results from UCNA, their implications for searches of beyond the standard model physics, and prospects for a future UCNA+ experiment capable of a ≤0.2% precision.