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 through experiments at the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, Virginia, and the Paul Scherrer Institute (PSI) in Switzerland. The research helps to address overarching questions about describing neutrons and protons in terms of their quark and gluon constituents, interactions that arise when two or three bodies are present in other media, the properties of dense nuclear matter, and the structure of nucleons. Answering these and related questions is a complex task that requires the dedicated experimental observations and careful testing of theoretical predictions against measured observations that are the goal of this project. 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 (UofSC) is based at the Thomas Jefferson National Accelerator Facility (JLab) in Newport News, Virginia, and the Paul Scherrer Institute (PSI) in Switzerland. At PSI, the group is performing the MUSE experiment, which will address the proton-radius puzzle in simultaneous measurements of the electron-proton and muon-proton elastic cross sections with positively and negatively charged leptons. Each of the four sets of data will allow the extraction of the proton charge radius. In combination, the data test possible differences between the electron and muon interactions and additionally two-photon exchange effects. The group’s program at JLab will continue to provide crucial high precision, polarized and unpolarized observables that will address present problems in strong QCD. The elastic and transition form-factor will trace the evolution from meson baryon to dressed-quark degrees of freedom, establish the early onset of precocious scaling that further manifests the three-quark structure of baryons, and address the dynamical mass generation and confinement problem. The cross-sections for lambda deuteron elastic scattering will provide new and independent constraints on the two- and three-body hyperon-nucleon force, which is essential to understand the properties of neutron stars. The cross-sections for J/ψ photoproduction off the deuteron will give a direct estimate of the J/ψ-N cross-section and a very first glimpse into the gluon structure of deuterons. Completing meson-photoproduction measurements will settle in an almost model-independent way lingering problems in N* physics and verify the SU(6)×O(3)-predicted three-quark baryon structure. The properties of hadrons are modified in the medium and bound proton electromagnetic form factors may differ from those in the vacuum, as revealed for the first time by the group's earlier measurements. The experiments on nuclear targets at MAMI and JLab, respectively, will confirm or refute these results. The observations of such phenomena are potentially transformative in how we view hadrons and their interactions in nuclear matter.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.