This project focuses on a fundamental research challenge of modern nuclear physics, namely to understand the structure and interactions of the basic building blocks of the visible matter, protons and neutrons known collectively as nucleons, in terms of Quantum Chromodynamics (QCD), the theory of the strong forces in nature. Nucleons, are composed of a variety of quarks confined by the quanta of the strong nuclear fields known as gluons. In order to explore the nucleon structure we perform electron scattering experiments where sophisticated particle detectors are utilized as powerful microscopes in order to determine the detailed dynamics of quark constituents. This in turn provides an important input for testing and understanding QCD.
This project involves four experimental activities, three of which will take place at the Thomas Jefferson National Accelerator Facility (TJNAF) while the fourth one will occur at the MAMI Microtron in Mainz, Germany. At TJNAF, first, a high precision measurement of the nucleon excitation to the Delta resonance will shed light on the mechanisms responsible for the presence of non-spherical components in the nucleon wave-function, with an emphasis on the role of the pion cloud to the nucleon dynamics. Second, in a separate experiment we will explore deep inelastic electron scattering off triton (2 neutrons + 1 proton) and helium-3 (1 neutron + 2 protons) mirror nuclei to determine the ratio of the up over down valence quark momentum distributions in a free proton, and advance our understanding of the effect of the nuclear medium on these momentum distributions. Third, a measurement of the J/Psi meson electroproduction cross section near threshold off a proton will enable us to explore the non-perturbative gluonic interaction between the J/Psi and the proton. This measurement might reveal an enhancement of the cross section just above threshold which could be indicative of the important role conformal anomaly contribution plays in QCD.
At the MAMI Microtron, the first high precision measurement of the Virtual Compton Scattering reaction on the nucleon resonance will allow the investigation of the electric generalized polarizability of the proton, meaning how stiffly the nucleon responds to an external electric field. The experiment will also allow for the first exploration of the nucleon excitation through a purely electromagnetic photon channel.
The project will enhance the infrastructure for research by building instrumentation for nuclear physics experiments, primarily at TJNAF, and by supporting international collaborative research efforts. The research activities will also contribute to science education by providing topics for PhD theses in nuclear physics, thus training a new generation of nuclear scientists. Furthermore, the opportunities for training students at the undergraduate level with hands on hardware experience will advance nuclear science education, which is essential to ensure a broad, basic knowledge of nuclear science in society.
