New result for the neutron $\beta$-asymmetry parameter $A_0$ from UCNA

TL;DR

This paper reports a new precise measurement of the neutron beta-decay asymmetry parameter $A_0$ using ultracold neutrons, refining the value of the ratio of axial-vector to vector weak coupling constants, $rac{g_A}{g_V}$.

Contribution

The study presents the first measurement of the neutron beta-decay asymmetry parameter $A_0$ with ultracold neutrons and improves systematic uncertainties and statistical precision.

Findings

Measured $A_0 = -0.12054(44)_{stat}(68)_{syst}$

Derived $rac{g_A}{g_V} = -1.2783(22)$

Combined result $A_0 = -0.12015(34)_{stat}(63)_{syst}$

Abstract

The neutron $\beta$-decay asymmetry parameter $A_0$ defines the correlation between the spin of the neutron and the momentum of the emitted electron, which determines $\lambda=\frac{g_{A}}{g_{V}}$, the ratio of the axial-vector to vector weak coupling constants. The UCNA Experiment, located at the Ultracold Neutron facility at the Los Alamos Neutron Science Center, is the first to measure such a correlation coefficient using ultracold neutrons (UCN). Following improvements to the systematic uncertainties and increased statistics, we report the new result $A_0 = -0.12054(44)_{\mathrm{stat}}(68)_{\mathrm{syst}}$ which yields $\lambda\equiv \frac{g_{A}}{g_{V}}=-1.2783(22)$. Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces $A_0=-0.12015(34)_{\mathrm{stat}}(63)_{\mathrm{syst}}$ and $\lambda\equiv \frac{g_{A}}{g_{V}}=-1.2772(20)$.