New result for the neutron ${\hat{I}}^2$-asymmetry parameter $A_0$ from UCNA

Abstract

The neutron ${\hat{I}}^2$-decay asymmetry parameter $A_0$ defines the correlation between the spin of the neutron and the momentum of the emitted electron, which determines $\hat{I}»=\frac{g_A}{g_V}$, 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 $\hat{I}»\equiv \frac{g_A}{g_V}=-1.2783(22)$. Combination with the previous UCNA result and accounting for correlated systematic uncertainties produces $A_0=-0.12015(34{)}_{\mathrm{stat}}(63{)}_{\mathrm{syst}}$ and $\hat{I}»\equiv \frac{g_A}{g_V}=-1.2772(20)$.

9 pages, 7 figures, updated to as-published version