NSF-BSF: The Study of Nuclear Physics with Intermediate Energy Probes

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Although ninety-eight percent of the mass of ordinary matter is due to the strong subatomic force, the present theory of that force (Quantum Chromodynamics) is still not fully understood. This award supports the study of the strong force by probing the substructure of matter. Almost the entire mass of an atom is concentrated in its tiny nucleus, which is made of nucleons that are either positively charged (protons) or electrically neutral (neutrons). These subatomic particles, however, are not elementary but are themselves composite objects made of quarks held together by glue particles (gluons). The structure of nucleons is a manifestation of the strong force, which is indeed the strongest force known. This project helps to address overarching questions about the origin of most of the visible mass in the universe, the nature of neutron stars, the properties of dense nuclear matter, and the structure of nucleons. Answering these and related questions is a complex task requiring dedicated experimental observations and careful testing of theoretical predictions against measured observations. This research will help discovery in and advance understanding of nuclear physics, and will promote teaching, training, and learning. The preparation of junior scientists plays a central role in the supported activities.The nuclear physics research program at the University of South Carolina (USC) is based at the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, Virginia and the Paul Scherrer Institute (PSI) in Switzerland. At JLab, the group uses high-energy electron and photon beams along with sophisticated particle detectors, that were developed, in part, at USC, as powerful microscopes to study the structure and interaction of baryons. This research program will continue to provide crucial high-precision,polarized and unpolarized, electro- and photo-production observables that will pin down present problems in strong QCD. Complete and mostly complete meson-photoproduction measurements will settle in an almost model-independent way lingering problems in baryon spectroscopy and in particular verify the SU(6)xO(3) three quark baryon structure. Alternatively, the structure of baryons can be probed through their interactions in scattering processes. The study of strong interactions involving the strange quark is essential to understand the properties of neutron stars. This project carries a comprehensive study of hyperon photo-production off deuterons. The extensive set of single- and double-polarization observables will provide long-needed information from experiments on the hyperon-nucleon interaction. The properties of hadrons are modified in the medium as a consequence of the partial restoration of chiral symmetry and many-body effects. The bound proton electromagnetic form factors in He-4 may be modified compared to those in the vacuum as revealed for the first time by the group's earlier measurements. The upcoming USC experiment on He-4, H-2, and H-1 at JLab will confirm or refute these results. The observations of such phenomena are potentially transformative in the way we view hadrons and their interactions in nuclear matter. At PSI, the group is preparing the MUSE experiment. This precision experiment will address the proton-radius puzzle by scattering muons and electrons off the proton.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.