3. Nucleons and Quarks

Nuclear Forces and Simple Nuclei

Measuring various properties of nuclear forces and tracing their origins to the fundamental interactions between quarts and gluons has been one of the major recent goals of nuclear physics. The long-range part of the nuclear force is known to be mediated by pions, the lightest of the mesons. However, our knowledge of the short-range parts is still incomplete. When two nucleons are separated by subfemtometer distances, their internal quark-gluon structures overlap. In such cases, description in terms of the quark-gluon exchange becomes necessary.

The force between two nucleons has been studied extensively over the years by scattering one nucleon from another, and the data have been used to constrain parameters in models of the force. In the past decade, a few successful parameterizations of the low-energy nucleon-nucleon force have emerged; they offer descriptions that differ in their assumptions about short-range behavior. It is an important challenge to experiment and theory to find ways to better understand this aspect of the nuclear force, where the interface with QCD is the most critical. Such information is provided, for instance, in experiments measuring meson production in nucleon-nucleon collisions.

In reactions at threshold energies, the two colliding nucleons must come essentially to rest, giving up all of their kinetic energy to produce the meson’s mass. The rate of such reactions is sensitive to the strong, short-range parts of nuclear forces. New experiments aim to obtain additional information on pion production by using spin-polarized beams, and to search for the threshold production of heavier mesons. These experiments also probe meson-nucleon interactions at very low energies and provide crucial tests of QCD-based techniques for deriving the effective nucleon-nucleon interaction.

The bound nuclear systems are shown as a function of the proton number Z (vertical axis) and the neutron number N (horizontal axis). The black squares represent the nuclei that are stable, in the sense that they have survived long enough since their formation in stars to appear on Earth; these form the “valley of stability.” The yellow color indicates man-made nuclei that have been produced in laboratories and live a shorter time. By adding either protons or neutrons, one moves away from the valley of stability, finally reaching the drip lines where nuclear binding ends because the forces between neutrons and protons are no longer strong enough to hold these particles together. Many exotic nuclei with very small or very large N/Z ratios are yet to be made and explored: they are indicated by the green color. The proton drip line is established by experiments up to Z = 83. In contrast, the neutron drip line is considerably further from the valley of stability and harder to approach. Except for the lightest nuclei where it has been reached, the position of the neutron drip line is estimated on the basis of nuclear models; it is uncertain due to the large extrapolations involved. Green and purple lines indicate the paths along which nuclei are believed to form in stars; only some of the dominant processes are shown. While these processes often pass near the drip lines, the nuclei decay rapidly within the star into more stable ones. One important exception to this stability plot occurs in extremely massive and compact aggregations of neutrons, neutron stars, under the combined influences of the nuclear forces and gravity.Page 51Suggested Citation:“3 The Structure of Nuclei.” National Research Council. 1999. Nuclear Physics: The Core of Matter, The Fuel of Stars. Washington, DC: The National Academies Press. doi: 10.17226/6288.

Unique information on the strong force between hadrons can be obtained by comparing the forces between two nucleons and between a nucleon and a lambda particle in which one of the quarks is a heavier strange quark. Any difference between these forces is entirely due to the change in a single quark. The force between the lambda particle and the nucleon is being mapped with the improved experimental capabilities at CEBAF, as well as through the investigation of bound nuclear systems called hypernuclei, in which a nucleon is replaced by a lambda particle.

Even the best available parameterization of the nucleon-nucleon force cannot accurately explain nuclear binding. In order to reproduce the binding energies of the simplest light nuclei, it is essential to add three-body forces to the pairwise interactions determined from nucleon-nucleon scattering. Such three-nucleon forces are expected because the nucleons are themselves composite objects whose constituents can be distorted by an external force. A more familiar example of such a three-body force is known from the analysis of orbits of artificial satellites. In the Earth-moon satellite system, the tides induced by the moon in oceans in turn alter Earth’s pull on the satellite. The nuclear three-body forces are believed to be rather weak, and it has not been possible yet to measure their small effects on the scattering of three nucleons. For now, the strengths of three-body forces have been adjusted to reproduce the binding energies of light nuclei. However, a satisfactory microscopic picture of the three-body force between nucleons is still lacking.

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