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

Although ninety-eight percent of the mass of ordinary matter is due to the strong subatomic force, many features of this force remain a puzzle. This award supports the study of the strong force by probing the substructure of matter. The PIs and their students will investigate questions about the emergence of bound three-quark systems and hence most of the visible mass in the universe, the nature of neutron stars, and the properties of dense nuclear matter. 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 foster discoveries in and advance the 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 PSI, the group participates in 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 data sets will allow the extraction of the proton charge radius. In combination, the data test possible differences between the electron and muon interactions and allow for the study of radiative corrections, particularly two-photon exchange effects. The PIs’ research program at JLab will continue to provide crucial high-precision, polarized, and unpolarized observables that will pin down present problems in strong QCD. The elastic and transition form-factor measurements will trace the evolution from meson-baryon to dressed-quark degrees of freedom, probe the early onset of precocious scaling that reveals the three-quark structure of baryons, and hence address the emergence of bound three-quark states, the dynamical generation of mass, and the confinement problem. The PIs and their students will extract from their experimental data the cross section for lambda-deuteron elastic scattering. This observable will provide new and independent constraints on the two- and three-body hyperon-nucleon force, which is essential to understanding the composition of the core of a neutron star. The cross-sections for J/Ψ photoproduction off deuteron will give a direct estimate of the J/Ψ-N cross-section and a very first glimpse into the gluon structure of deuterons. 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 group’s experiments on nuclear targets at MAMI and JLab 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. The award will support the group’s efforts to establish the physics case in support of the 22-GeV upgrade of JLab. This work would lead to the only foreseeable observables that could continuously connect the strong force into the regime where the Standard Model is firmly verified. 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.