This award will support the North Carolina State University contribution to the Ultracold Neutron asymmetry A (UCNA) experiment. The goal of the UCNA experiment is to provide a high precision measurement of neutron beta-decay through a determination of the beta-asymmetry. The beta-asymmetry, known as "A", is the angular correlation between the neutron spin and the emission direction of the electron following beta-decay of the neutron, and is very sensitive to the value of the axial coupling constant in the charged weak interaction. This constant is a fundamental parameter governing the strength of the weak interaction, is an important target for lattice calculations, and is an input parameter in spin-structure studies of the nucleon. It is also of importance in astrophysics, being a critical input for energy generation in the sun and in fixing the magnitude of the neutron lifetime. The proposed measurements, if successful, will improve the precision of the beta-asymmetry over the most precise current experiments, and provide a value for the axial form factor competitive with the most precise current measurements of the neutron lifetime.
The UCNA experiment is unique, in that it is the first experiment to measure angular correlations in neutron decay using polarized ultracold neutrons (UCN). UCN have significant advantages for the control of systematic uncertainties intrinsic to angular correlations measurements, but experiments which use them have been limited by the available densities of UCN. The UCNA experiment, uses a dedicated solid deuterium source, also the first of it's kind, coupled to a spallation target for the 800 MeV proton beam in Area B of at Los Alamos. This source now produces UCN densities of about 1/cc, comparable to densities in experiments at the Institut Laue Langevin in Grenoble, France (the strongest UCN source in the world), with further upgrades on the way. The experimental facility developed for the UCNA experiment can potentially be extended to several other neutron beta-decay observables (such as the lifetime, and angular correlations sensitive to new tensor and scalar interactions) also under development in this project.
The broader impact of the proposed research stems in part from the fundamental impact of our planned measurements on the fields of nuclear and particle physics and astrophysics. The source development program also contributes to the broader impact of the project, potentially expanding the reach of our fundamental symmetries experiments, and by potentially shifting neutron and nuclear experiments back onto the university campus. Proof-of-principle tests of a UCN source constructed at the PULSTAR reactor at NCSU will be carried out in the first year of this project, permitting an assessment of the possible impact for these next-generation UCN sources in nuclear physics rearch.
