Nuclear beta decay was important in motivating the Standard Model, and it continues to be of importance in searches for physics beyond the Standard Model. The simplest system for studying beta decay is the neutron, where there is a long history of increasingly accurate measurements of the parameters that characterize the decay. By measuring the neutron's lifetime (from the Ft value) and the neutron's beta asymmetry A, one has enough information to independently determine the weak vector and axial-vector couplings, GV and GA. Although recent accurate measurements of the neutron lifetime are in good agreement with each other, the same cannot be said for measurements of the neutron's beta asymmetry A. Since 1985, there have been four accurate (combined statistical and systematic uncertainty of about 1% each) measurements of A. Unfortunately, the agreement between these four A measurements is poor.
An approved Los Alamos experiment will again measure the A coefficient, this time utilizing Ultra-Cold Neutrons, dramatically reducing systematic errors otherwise present due to beam-induced backgrounds and incomplete polarization. The key features are 1) the ability to port the neutrons to a significant distance away from their production site (using total external reflection), 2) vetoing events when the beam pulse is on target (capitalizing on the pulsed production method), and 3) obtain 100% polarization using an axial magnetic field (where neutrons of the wrong spin do not have enough kinetic energy to overcome the mu.B potential).
Our group at Virginia Tech has responsibility for UCN transport for the Los Alamos experiment, the fast DAQ, and experimental coordination. The critical component of this system addressed by this proposal is the coating for transporting polarized neutrons. We have shown pulsed-laser-deposition of diamond films is suitable. At Virginia Tech, an existing excimer laser has been re-commissioned to produce suitable films. A system for coating both the guide tubes and central decay tube of the spectrometer has been developed. The successful campaign to make suitably coated guides for polarized UCNs has an impact, not just to this experiment, but to planned future experiments as well.
