| In the standard model of particle physics the weak force has a purely vector -- axial-vector structure (V--A) and the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix is unitary. Measuring the decay parameters of the neutron, namely its lifetime taun and any of the angular correlation parameters, provides a way to test these assumptions. Precise measurements of taun and lambda ≡ GA/GV, the ratio of the axial-vector to vector coupling constants, could be combined to determine Vud, the largest element of the first row of the CKM matrix, which can be used to test the unitarity of the quark mixing matrix. The neutron-spin electron-momentum correlation parameter, A0, is to leading order determined by lambda and has the highest statistical sensitivity to lambda relative to the other correlation parameters. The previous and most precise measurement of A0 by Perkeo II is discrepant with the previous world average by ∼ 4sigma, where all previous measurements were performed with cold neutron beams. The UCNA collaboration designed and constructed an ultra-cold (UCN) neutron production facility at the Los Alamos Neutron Science Center (LANSCE) to exploit the different systematics associated with UCN to provide a complementary high precision measurement of Ao. In the following we present the experimental setup, analysis techniques, and results of the first UCN based measurement of the neutron beta-asymmetry parameter and the continuation of that experiment to higher precision. An initial proof-of-principle measurement, completed in 2007 and published in 2009, achieved a 4.4% precision on A o. In 2008/2009 a set of measurements were carried out to study the scattering systematics which were published in 2010 and resulted in a relative uncertainty of sigmaA = 1.3%. We also present limits on the tensor coupling coefficient CT /CA derived from beta-decay data from neutron and 0+ → 0 + systems. |
