High Precision Measurements of Beta Decay Using Stored Ultracold Neutrons and Cold Neutron Beams

North Carolina State University, Raleigh NC

Investigators

Abstract

This award supports high precision measurements of 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, in that it powers our sun and helps determine the abundance of elements in the universe. Neutron decay is also important because it involves only a single nucleon, making possible very precise predictions from the "standard model" of particle physics without the complications due to the presence of other nucleons. With very accurate measurements of neutron decay, we can perform a kind of particle physics "forensics", looking for discrepancies in our standard model which can indicate the presence of new forces, but without using high energy particle beams such as those at the Large Hadron Collider at CERN. 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. The PI and his group contribute to a new measurement of the neutron's lifetime (UCNtau) and an angular correlation measurement (Nab). The lifetime and angular correlations measurements probe different aspects of the weak interaction, and if successful, will sharpen our knowledge of critical components of the weak interaction and contribute to strong limits on extensions the standard model, at or above those which are being established at CERN. An important aspect of the project is the training of students and post-docs. This training comes in part because of the specific nature of the research projects, which utilize nuclear physics analysis and technology (which also has real-world applications in energy generation and medical imaging, for example) and in part from the many opportunities students working in the program have to perform research at national laboratories and nuclear facilities. One of the experiments supported by the 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 anti-neutrino (called the electron-anti-neutrino correlation), with the goal of over an order of magnitude improvement in the precision of this parameter. The other, UCNtau, is situated at the ultracold neutron (UCN) source at Los Alamos National Laboratory (LANL), and targets precision levels below 0.3 seconds during the current funding period. The impact of these programs being successful are: (1) knowledge of the axial form factor at the level required to determine the CKM matrix element Vud at the same level as the current theoretical uncertainty for the electro-weak radiative corrections, (2) knowledge of the lifetime at a level such that it will limit tests of our models for the cosmological He-4 abundance for some time, and (3) limits on tensor couplings at or above the level expected from CERN. Motivation for these measurements has been amplified over the last year by theoretical progress on the electroweak radiative corrections and lattice calculations of the axial coupling constant, with both opening new, higher precision probes for new physics. For each of these experiments, this 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. For Nab, for example, the group is analyzing the effects of incomplete collection of the energy of decay electrons and the calibration procedures, a critical component of the systematic error budget for this experiment. UCNtau is designed to store UCNs in a magneto-gravitational trap, where the UCNs do not contact with the trap walls. The top of the trap is open, permitting detectors to be lowered into the trap to measure UCN populations and determine the lifetime. The group will continue to develop UCN detectors for this purpose and focus additional effort on understanding systematic errors due to UCNs with high enough energies to slowly escape the trap and shift the measured value of the lifetime. These tasks are essential and ongoing parts of the planned experiments, with completed measurements and analysis incorporated into publications during the proposal period. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.