Progress towards a high precision measurement of the neutron spin--electron angular correlation in polarized neutron beta decay with ultra-cold neutrons

摘要

A measurement of the angular correlation between the neutron spin and the direction of emission of the electron in polarized neutron decay (the "A" asymmetry), when combined with results from the neutron lifetime, provides a determination of the Cabibbo-Kobayashi-Maskawi (CKM) matrix element Vud. The value of Vud determined from neutron beta decay can also be compared with the value determined from measurements of the superallowed 0+ to 0+ nuclear beta decay, and the value determined by requiring that the CKM matrix be unitary. This provides a sensitive way to search for new physics beyond the standard model. Four measurements of the "A" asymmetry using cold neutron beams at reactor were carried out since 1985, all of which quote a combined statistical and systematic uncertainty of about 1% in the determination of "A." Unfortunately, the agreement between these four measurements is poor and the results also disagree with both the 0+ to 0+ nuclear beta decay and the CKM unitarity results. In order to understand the origin of these discrepancies, a high precision (0.2%) measurement of "A" using ultra-cold neutrons (UCN) is in progress at the Los Alamos Neutron Science Center (the UCNA experiment), with very different systematic effects than those in the reactor experiments.The essential elements of the UCNA experiments, including a dedicated spallation-driven solid deuterium UCN source developed by the collaboration, the UCN guides, and the superconducting spectrometer (SCS) including the beta detector system, are described, focusing on the UCN source and the spectrometer. The developed UCN source produced the highest stored density of UCN ever. The UCN were successfully transported out of the source along several meters of diamond-coated quartz guide tubes through the field of a 7-Telsla pre-polarizing magnet. The SCS magnet was successfully commissioned, demonstrating 10^-4 uniformity over 3 meters. The beta detector package including a Multi-Wire Proportional Chamber (MWPC) backed with a plastic scintillator detector was constructed. Studies of the energy response, linearity, timing response, and position resolution of the detector package were carried out using a home-built 135keV electron gun and a Helmholtz spectrometer, and the results are presented. Studies of systematics for the UCNA experiment based on a full GEANT4 Monte Carlo simulation are presented. A large and uncharacterized part of the systematics, the electron backscattering of low energy electrons, was measured in detail using the 135keV electron gun. The results from the electron backscattering studies are presented.