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

This award supports research on high precision measurements of neutron beta decay, a process caused by the weak nuclear force (or weak interaction). Although many nuclei undergo beta decay, neutron decay is important because it involves only a single nucleon, which simplifies the calculations and makes very precise predictions from the "standard model" of particle physics possible 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 that 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.

When neutron beta decay occurs, the decay process results in the emission of a proton, an electron and an antineutrino. This program involves a measurement of the distribution of directions for the emitted particles after neutron beta decay occurs (Nab) and a measurement of the average lifetime of the neutron (UCNτ). When taken together, these two measurements completely define the beta decay process.

During the award period, these two experiments are set to provide the most precise characterization of the weak interaction to date, potentially adding important confirmation that the standard model is not correct. An important aspect of the program is the training of students and post-docs, where the group contributes to a dissemination of the techniques and expertise of nuclear science in general, and neutron science in particular.

This training comes in part because of the specific nature of the research projects, which utilize nuclear physics analysis and technology. It also comes in part through the students’ involvement in research at national laboratories and nuclear facilities.

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 emitted anti-neutrino (the beta-neutrino correlation, a_βν), with the goal of almost an order of magnitude improvement in the precision of this parameter. The other, UCNτ, is situated at the ultracold neutron (UCN) source at Los Alamos National Science Center (LANSCE), and targets a factor of two improvement in the precision of the lifetime during the current funding period.

These precision levels enable a meaningful cross-check of the value of CKM matrix element V_ud determined from super-allowed decays, confirming discrepancies in the unitarity test of the standard model (referred to as the “Cabibbo Anomaly”). The significance of this anomaly has steadily increased over the past few years, driven by new particle physics data and refinement of theoretical analysis of kaon decays.

Analysis in 2022 suggests that new physics couplings to up quarks are required at greater than the 3σ level. The results of this award can confirm this result and finally eliminate nuclear structure as source of uncertainty. 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.

For Nab, the group is analyzing, for example, the effects of incomplete collection of the energy of decay electrons, a critical component of the systematic error budget for this experiment. This work has been on-going, with contributions from efforts providing the basis for the current energy calibration and interpretation of the signals from the Si detectors.

In the coming period the group will apply analysis tools and calibration infrastructure developed during the current period to during operation of the Nab experiment. UCNτ is designed to store ultracold neutrons, UCN, in a magneto-gravitational trap, where UCN do not make contact with material trap walls, dramatically reducing systematic uncertainties.

In 2021, UCNτ produced the most precise measurement of the neutron lifetime to date, with an uncertainty of 0.36 seconds. The focus now is an upgrade called UCNτ+, which will reduce uncertainties by another factor of two, a critical contribution to a high precision check of the Cabibbo Anomaly. The NCSU group is currently leading a simulation effort for the performance of an adiabatic loading procedure which is essential for the success of UCNτ+.

This work will be integrated with their ongoing effort to characterize sources of systematic uncertainty such as variations in the UCN spectrum and details of detector response.

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.