can map the probability for finding the various constituents as a function of the fraction they carry of the nucleon's overall momentum. Such detailed maps provide crucial tests for QCD calculations of the nucleon structure; a number of basic features have yet to be delineated or understood. At the same time, less-energetic projectiles must be used to obtain a lower-resolution, but more global. view of the nucleon's properties. These include its overall size and shape, distributions of charge and magnetism, and its deformability when subjected to external electric or magnetic fields. These lower-energy spatial maps serve as essential assembly drawings in understanding how the nucleon is actually put together.

The earliest spatial maps of nucleons revealed that both protons and neutrons have sizes on the order of one ten-trillionth of a centimeter (a distance named 1 fermi, in honor of Enrico Fermi. one of the greatest of nuclear physicists). These maps were made by scattering electrons elastically from hydrogen or deuterium targets, at sufficiently high energies that the electrons could resolve structures smaller than 1 fermi. Deuterium (whose nucleus comprises only one proton and one neutron) was needed to provide a neutron target because free neutrons are radioactively unstable, decaying with a lifetime of about 15 minutes. Electron scattering probes the distribution of electric charge and magnetism within the target; the early experiments had already provided a hint that smaller, electrically charged constituents resided inside the uncharged neutron.

It took a different kind of experiment, deep inelastic scattering, to provide the first definitive evidence for the quark substructure. In this process, as illustrated in Figure 2.2, an incident high-energy electron transfers not only momentum to the target nucleon (as in elastic scattering as well) but also a large quantity of energy. The energy lost by the electron is transformed into additional particles (mostly mesons) produced in the scattering. Even though these extra particles were not detected in the early experiments, new information was gained by noting that the dependences of the scattering probability on energy transfer and on momentum transfer were strongly correlated. The observed correlation suggested that the electron had actually scattered from a very small, charged object (quark) within the proton.

The techniques used in the early explorations of nucleon structure recently have been revitalized by technological advances in nuclear physics. These advances permit physicists to address more sophisticated scientific issues by allowing more detailed snapshots of the internal state of protons and neutrons. In addition, these methods have been supplemented by techniques with complementary sensitivities, using beams of high-energy hadrons.

Front Matter (R1-R16)

Summary and Recommendations (1-7)

1 Introduction (8-18)

2 The Structures of the Nuclear Building Blocks (19-46)

3 The Structure of Nuclei (47-79)

4 Matter at Extreme Densities (80-103)

5 The Nuclear Physics of the Universe (104-127)

6 Symmetry Tests in Nuclear Physics (128-149)

7 The Tools of Nuclear Physics (150-171)

8 Nuclear Physics and Society (172-196)

Appendix: Accelerator Facilities for Nuclear Physics in the United States (197-206)