Nucleon Structure and Nuclear Force Studies

Kent State University, Kent OH

Investigators

Abstract

The general objective of this project is to study the structure of nucleons, i.e., neutrons and protons, and to investigate certain interesting aspects of the nuclear force. It is recognized now that nucleons are not fundamental particles. They are comprised of smaller particles, called quarks. The quarks are held together inside the nucleon by the exchange of particles called gluons. It is generally believed that the quarks and gluons are fundamental particles and are not comprised of even smaller particles. The quarks move inside the nucleon and, by the nature of the uncertainty principle of quantum mechanics, other quark-antiquark pairs are continually being created and destroyed inside the nucleon. The net result is that the nucleon is a fairly complicated collection of various kinds of quarks and the gluons. In order to understand exactly which quarks are there and how they are moving, it is necessary to measure detailed characteristics of the nucleons. One of the goals of this project is to extend earlier measurements of the charge distribution inside the neutron. The neutron is overall electrically neutral, but has a core of positive charge surrounded by an outer region of negative charge. Precise measurement of this charge distribution provides one of the most sensitive tests of what quarks are present in the neutron and how they are moving. This experiment will be performed at the Jefferson National Accelerator Facility in Virginia. Another goal of this project is to try to further our understanding of the source of the intrinsic angular momentum, or spin, of the proton. In a very simple model of the proton, it can be considered to be comprised of just three, so-called, valence quarks, and it is possible to see how these three quarks, with their own intrinsic spins, can combine to give the known spin of the proton; however, a series of experiments performed at electron accelerators in the U.S.A. and Europe have shown that the quark spins actually provide only about 1/3 of the total spin of the proton. The remaining 2/3 must come from the motions of the quarks and from the gluons that are being exchanged between the quarks. This group is involved in a series of experiments to be performed with colliding proton beams at the relativistic heavy-ion accelerator (RHIC) at Brookhaven National Laboratory on Long Island, New York to try to measure the contribution of the gluons to the spin of the proton. Even though the overall spin of the proton is well known, these measurements will, similar to the measurements of the charge distribution of the neutron, enable us to better understand what is happening inside the nucleon. Finally, it is also a goal of this project to perform an experiment to study the short-range nature of the nuclear force inside nuclei. This experiment will be performed at the Jefferson Laboratory and is a follow-up to a similar experiment performed at Brookhaven. That earlier experiment pioneered a technique for taking "snapshots" of two nucleons interacting at very small separation distances, and reconstructing their relative motions. The Jefferson Laboratory experiment will allow us to do this type of reconstruction with much greater precision.