Precise Measurement of the Ratio of (V-A) Coupling Constants in Free Neutron Beta Decay and Search for Non-Standard Model Couplings with the Nab and pNab Spectrometers

University Of Virginia Main Campus, Charlottesville VA

Investigators

Abstract

This grant supports the measurement of angular correlation coefficients describing neutron beta decay using the “Nab” spectrometer at Oak Ridge National Laboratory. In this experiment the "a" and "b" coefficients describe the proton and electron resulting from the decay of the neutron. The main purpose of the measurements is to test the Standard Model of Nuclear and Particle Physics, which has been extremely successful in describing most of the properties and behavior of the elementary particles to date but leaves open critical questions such as the reason for the dominance of matter over anti-matter in the universe. A discovery of Beyond the Standard Model physics would thus have important implications for Physics and Cosmology. Installation of the Nab spectrometer at the Spallation Neutron Source (SNS) at Oak Ridge will be finished by the end of summer 2021, and commissioning and data taking will commence and continue for the duration of the proposal period. The PI is the project manager for the Nab collaboration. This grant will fund the PI’s group, and they will participate in data taking, detector simulation, data analysis, and characterization of the proton detection efficiency. The PI is strongly committed to attracting members of underrepresented groups. The University of Virginia physics department recently became a new site for the APS bridge program, and the PI is serving as a mentor. Furthermore, the project has a valuable impact on higher education: most of the work is done by students at undergraduate and graduate level, and these students will earn valuable skills and experiences for their future careers. The redundancy inherent in the Standard Model description of the neutron beta decay process allows for sensitive checks of the model’s validity. One of the expected observables (the neutrino-electron-correlation coefficient “a”) will allow an extraction of , the upper left element of the Cabbibo-Kobayashi-Maskawa (CKM) matrix, in order to test the unitarity of the matrix. A recent re-evaluation of the world’s data suggests that CKM matrix unitarity is violated and so the results of the Nab experiment are eagerly anticipated. The other observable (The Fierz term “b”) allows us to test for scalar or tensor interactions at low energies, in a complementary way, and with sensitivity competitive to what is expected from the LHC. The current Long Range Plan for Nuclear Science devotes a paragraph to this effort (see LRP, page 75). 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.