The UCNA Experiment: Measuring Angular Correlations in Neutron Decay using Ultracold Neutrons

North Carolina State University, Raleigh NC

Investigators

Abstract

The UCNA experiment utilizes, for the first time, ultracold neutrons (UCN) to measure the angular correlation between the neutron spin and the electron momentum (the beta-asymmetry) in neutron beta-decay. A beta-asymmetry measurement, together with a measurement of the lifetime for neutron decay and the lifetime for muon decay, provides enough information for a model-independent extraction of the parameters characterizing semi-leptonic charged current weak interactions in the first generation of quarks. The proposed measurement can be used to determine Vud, the CKM matrix element important for nuclear beta-decay, and to provide various tests of the electroweak standard model. UCN are produced in a spallation-driven solid deuterium ultracold neutron source at Los Alamos, directed through a 7 Tesla polarizing magnet and spin-flipper, and then guided into a decay spectrometer. The spectrometer consists of a decay "cell" mounted within a uniform, 7 Tesla magnetic field. Charged particles emitted during the decay spiral around the field lines to detectors, positioned outside the decay volume to intercept all particles emitted during the decay. The utilization of ultracold neutrons for these measurements, instead of a more traditional cold neutron beam, has certain advantages: the neutrons can be polarized to essentially 100 percent, simply by passing them through a high field region, and backgrounds can be reduced to extremely low levels compared to those typically found at reactor-based experiments. In the proposal period we seek to establish a measurement of the beta-asymmetry at the one percent level, and then begin pushing for the ultimate precision achievable with this technique. The broader impact of our measurement lies in part because of its interest to the physics community at large: the UCNA experiment determines fundamental parameters in the electroweak standard model and provides a means to identify new physics below the weak interactions scale, a subject of general interest in physics. We also are in the process of developing UCN technology, which may have applications in diverse other fields, including as a probe for surface properties relevant to solid state physics, materials science and biophysics. Finally, our program will result in the training of undergraduate students, graduate students and post-doctoral associates in nuclear physics, neutronics and issues associated with neutron research in general.