Nucleon Structure and Nuclear Force Studies

Kent State University, Kent OH

Investigators

Abstract

There are two main research areas that will be explored with this research grant. Both of these areas will provide training and dissertation research data for Ph.D. students from Kent State. First, the group will be investigating the structure of the nucleon, i.e., of the neutron and proton. The primary project here is to study the distribution of electrical charge inside the neutron. Although the neutron has no net electrical charge, the three quarks that make up the neutron have electrical charge: plus charge for the "up" quark, and minus charge for the two "down" quarks, with the charges of all three summing up to zero. Since the up quark tends to be closer to the center of the neutron, and the down quarks more on the surface, there is a distribution of charge within the neutron, with more plus charge near the center, and more minus charge near the surface. Precision measurements of this charge distribution will be made at Thomas Jefferson Laboratory, using equipment developed at Kent State. These measurements are of considerable scientific importance, because they provide constraints on theoretical models attempting to describe the quark structure of the neutron. The second project is to study the origin of the intrinsic angular momentum, known as the spin, of the proton. This project has been underway for several years with the STAR detector system at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory; although the total spin of the proton is known, its origin in terms of the interactions and motions of the quarks that makeup the proton is not well understood. Measurements performed with the RHIC accelerator are now providing a clearer picture of the structure of the proton. The second main area of research is the study of the "correlated pair" structure of nuclei. In the dominant model of nuclear structure, the neutrons and protons move independently of each other in stable quantum orbits inside the nucleus, much like the orbits of the electrons surrounding the nucleus. The developers of this model, called the nuclear shell model, shared the Nobel Prize in 1963. Recent research at Brookhaven National Laboratory and Thomas Jefferson Laboratory has shown that about 20% of the time, a given neutron or proton, instead of moving in an independent orbit, will be part of a pair of particles moving back-to-back at high velocity. These are called "correlated pairs" because their velocities are similar in size, but back-to-back. A fascinating result from that earlier research is that neutron-proton pairs occur 20 times more often than neutron-neutron pairs or proton-proton pairs. This finding has important implications for understanding neutron stars. These results came from the study of the carbon nucleus. The group will be extending that work to the helium nucleus, and observing correlated pairs for a larger range of velocities than were explored in previous work. The broader impacts of this research include an increased understanding of the physical universe distributed through refereed journals of nuclear physics and the training of doctoral-level physicists needed in industry, medical physics, and academia.