Almost the entire mass of an atom is concentrated in its tiny nucleus. The atomic nucleus is made of nucleons, which are either positively charged (protons) or electrically neutral (neutrons). These subatomic particles, however, are not elementary but themselves composite objects made of quarks held together by the strong force via the exchange of glue particles (gluons). The properties of protons and neutrons are a manifestation of that strongest force on Earth. The present theory of that force is Quantum Chromodynamics.
Our research program at the University of South Carolina is based the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. There, we use high-energy electron and photon beams along with sophisticated particle detectors as powerful microscopes of the subatomic world. We try to answer questions like: What is the structure of the nucleon? What are and how can we excite its internal degrees of freedom? Once excited, in which way does the nucleon decay back to its ground state? Does the structure of the nucleon change if it is embedded in dense nuclear matter? Do other subatomic particles change their properties if they are produced in dense nuclear matter? The experimental study of these properties gives insight into the nature of the strong force and allows for rigorous tests of the underlying theory.