Systematic Error Studies for a Measurement of the Beta Asymmetry Parameter Using Ultracold Neutrons

The angular correlation between a neutron's spin and the initial direction of its emitted electron when undergoing beta decay is known as the beta asymmetry parameter. This parameter can be used to help search for physics beyond the Standard Model by determining the value of the up and down quark weak mixing angle and testing the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. The UCNA experiment at the Los Alamos Neutron Science Center seeks to obtain a precise measurement of the neutron beta asymmetry by studying the beta decay of a dense population of identically-polarized ultracold neutrons (UCN.) The UCN are produced in a solid deuterium source and are polarized by a static magnetic field. Electrons emitted from the beta decay of UCN travel outwards along solenoidal field lines towards one of two oppositely-placed units each made up of a multi-wire proportional chamber backed by a plastic scintillator detector. Accurate determination of the events in these detection units is essential to a high-quality measurement of the expected directional asymmetry. The results of systematic error studies of recent UCNA data are presented.