Collaborative Research: Beta-decay Angular Correlation Measurements with Cold and Ultracold Neutrons

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This award funds a collaborative research program involving two separate angular correlation measurements with neutrons. These measurements will enable us to extract fundamental information concerning the weak interaction which causes neutron decay, including the axial coupling constant of the nucleon. This coupling constant appears in a very wide variety of nuclear and particle physics contexts, from being a critical input to high precision calculations of the solar fusion rates, to being a target for higher precison lattice QCD calculations, to placing, together with the neutron lifetime, limits on the existence of new particle physics interactions. The first experiment, UCNA, measures the beta-asymmetry, or the correlation between the neutron polarization and the decay beta-particle's momentum. UCNA is the first experiment to use ultracold neutrons, or neutrons with energies below about 300 neV that can be stored in material bottles, for this kind of measurement. Ultracold neutrons have advantages both for neutron polarimetry and neutron-generated backgrounds, key sources of systematic error in existing neutron angular correlation experiments. The UCNA experiment is currently operating, with one more run period planned at the neutron densities currently available at the Los Alamos ultracold neutron source. The targeted precision is below 0.4%, roughly a factor of 2 more precise than the current world average for the beta-asymmetry. The second experiment, Nab, uses an unpolarized beam of cold neutrons (with temperatures near 30K) to measure the electron-neutrino angular correlation, with a planned sensitivity at about the 0.1% level for the electron-neutrino correlation. This experiment should also be able to place important limits on extensions to the standard model of particle physics for scalar and tensor couplings. Key to the success of these experiments is the involvement of undergraduate and graduate students, as well as postdoctoral scholars. These young scientists will receive excellent training in experimental techniques, particle detectors, and data analysis.