Beta Decay Measurements with Cold Neutron Beams and Ultracold Neutrons

North Carolina State University, Raleigh NC

Investigators

Abstract

This award will support three important experiments in neutron beta decay, a process in which a neutron decays because of the weak nuclear force (or weak interaction) into a proton, an electron and an anti-neutrino. The weak nuclear force is important because it powers our sun and helps determine the abundance of elements in the universe. Neutron decay in particular is important because it plays a key role in the formation of helium in the early universe. Additionally, neutron decay involves only a single nucleon and that makes it possible to calculate very precise predictions from the Standard Model of fundamental particles and interactions without complications from other nucleons. With very accurate measurements of neutron decay, we can perform a kind of particle physics forensics, looking for discrepancies in the Standard Model that can indicate the presence of new forces but without using high energy particle beams. This research is a part of the "precision frontier" for physics, and defines the cutting edge of our knowledge of the weak nuclear force and possible new interactions. An important aspect of this research is the training of students and post-docs. This contributes to the spreading of technical methods and expertise. This training comes from the specific nature of the projects, which use nuclear physics analysis and technology, and from the many opportunities students have to perform research at our national laboratories and nuclear facilities. Some of the techniques used have real-world applications in energy generation and medical imaging. One of the experiments supported by this award, Nab, will use an unpolarized beam of cold neutrons at the Spallation Neutron Source from the Oak Ridge National Laboratory to measure the angular correlation between the decay electron and the neutrino, with the goal of improving the precision of this correlation by over an order of magnitude improvement. The other two experiments, UCNA and UCNtau, are situated at the ultracold neutron (UCN) source at Los Alamos National Laboratory (LANL). The UCNA experiment measures the angular correlation between the neutron spin and the decay electron momentum (called the beta-asymmetry), and aims for a factor of 3 improvement over the precision of the global data set for the axial form factor. UCNtau is designed to measure the lifetime of the neutron, with the goal of a 1 second uncertainty during the proposal period. These programs will impact: (1) knowledge of the axial form factor at the level required for a the establishment of the CKM matrix element Vud from the neutron at the same level as the super-allowed decays, (2) knowledge of the neutron lifetime at a level such that it will not limit tests of cosmological 4He abundance models for some time, and (3) limits on tensor couplings at or above the level expected from CERN. For each of these experiments, the group brings its experience in beta decay measurements to bear on key sources of systematic error by developing instrumentation and analysis required by the experiments.