Status of Analysis for the UCNA Experiment's 2011-2012 and 2012-2013 Data Sets

The UCNA Experiment at the Los Alamos Neutron Science Center (LANSCE) is the first measurement of the $\beta$-decay asymmetry parameter $A_0$ using polarized ultracold neutrons (UCN). $A_0$, which represents the parity-violating angular correlation between the direction of the initial neutron spin and the emitted decay electron's momentum, determines $\lambda=g_{A}/g_{V}$, the ratio of the weak axial-vector and vector coupling constants. A high-precision determination of $\lambda$ is important for weak interaction physics, and when combined with the neutron lifetime it permits an extraction of the CKM matrix element $V_{ud}$ solely from neutron decay. At LANSCE, UCN are produced in a pulsed, spallation driven solid deuterium source and then polarized via transport through a 7 T magnetic field. The UCN then travel to a decay storage volume situated within a 1 T solenoidal spectrometer with electron detectors at each end for measurement of decay electrons. Data collected during run periods in 2011-2012 and 2012-2013 are currently under analysis, for which the projected uncertainty on $A_0$ is $<0.6$\%. The status of this analysis will be presented in this talk.