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.
We report here the outcomes of the research conducted by this group at Kent State University under the support of National Science Foundation award 0969129 for the period 7/01/2010 to 6/30/2014. This group, consisting of two faculty members and three graduate students at Kent State is in the general area of experimental nuclear physics. The research involved proposing, designing, executing, and analyzing the results from experiments performed at Jefferson National Accelerator Laboratory in Newport News, VA and at the RHIC accelerator at Brookhaven National Laboratory, NY. This group has a long experience with the development and use of large-volume scintillators for detecting neutrons emitted in medium-energy nuclear reactions, and much of this research involved using these detectors. The work performed at Jefferson Lab included an experiment designed to map out the charge distribution of the neutron. The neutron is overall neutral, but has both positive and negative charges within. In rough approximation, the neutron has a positive core surrounded by a negative outer layer. It is possible to measure the charge distribution by inelastic electron scattering from a light nucleus involving the knockout of a neutron. Such an experiment was performed at Jefferson Lab using the large volume neutron detectors developed by this group. Detailed mapping of this distribution of charge provides one of the most sensitive tests presently available of models of the internal structure of the neutron. These models generally involve the quark picture of the nucleon and take into account what quarks are present and their relative motions and interactions through the strong color force. Although there exist several such models, developed by different theoretical research groups, none of the present models are able to precisely describe the charge distributions of the neutron and the proton simultaneously. To this end, it is important to extend the measurements of the charge distributions of both the neutron and proton to higher momentum transfers, which via a Fourier transform, corresponds to smaller radii inside the nucleons. This group performed and reported results for the neutron to higher momentum transfers during this period and these results are shown in Fig. 1. This group is now involved in new experiments extending these important measurements to even higher momentum transfers. This group was involved also in experiments to study "short-range correlations" between nucleons inside nuclei. In the standard model of nuclear structure, the neutrons and protons move in quantum orbits in the nucleus independent of each other. The development of this picture is called the "nuclear shell model," and was awarded the Nobel Prize in 1963. But it has been known for some time, however, this can'tbe the whole picture. Nuclear structure theorists have suspected that some fraction of the time the nucleons form pairs interacting strongly with each other, in high-velocity back-to-back motion. By using electron scattering on nuclei involving knockout of nucleon pairs, one can study these interactions. Such experiments were conducted including the use of large volume neutron detectors developed by this group. A surprising result was obtained indicating the dominance of n-p (neutron-proton) interactions over n-n or p-p interactions at short ranges. This result is shown in Fig. 2. This work indicates the importance of the tensor part of the nuclear force inside nuclei (which depends on the relative orientations of the quantum "spins" of the neutron and proton) and has implications for many areas of nuclear physics, including nuclear astrophysics. Another activity of this group was the participation in an experiment to study inelastic electron scattering from the 3He nucleus in order to study the wavefunction of this 3-body nucleus and the reaction mechanisms of the scattering process. It is possible to perform exact 3-body calculations for such a process using the Faddeev calculations assuming certain models for the nuclear force. The results of the experiment and calculations are in good agreement indicating the general validity of this model. This work is important for testing the nuclear force models assumed and for the use of 3He as a target in other experiments. The results of this experiment are shown, compared to these calculations, in Fig. 3.
