Measurement of the Neutron Beta Decay Asymmetry by the UCNA Experiment

In free neutron beta decay, the direction of the emitted electron is correlated with the polarization of the decaying neutron. The asymmetry between spin-aligned and spin-antialigned electron emissions is a function of $\lambda$, the ratio of axial-vector and vector coupling constants. Together with the neutron lifetime, $\lambda$ can be used to determine $V_{ud}$ in the CKM matrix. Ultracold neutrons (UCN) are advantageous for measuring the decay asymmetry as they can be nearly 100\% polarized and minimize production of neutron-induced backgrounds. Presented here are results from the first measurement of the asymmetry using UCN, performed by the UCNA collaboration in 2007, and improvements for the higher-statistics measurements of this year.