The Standard Model has already stood up against decades of intense experimental scrutiny. Why, then, is it not regarded as the best possible theory of nature, "the ultimate theory"? In spite of its successes, the Standard Model has about 19 parameters that are apparently arbitrary—numbers whose origins are not understood. Moreover, some of the parameters have values that are hard to understand. For example, in the present Standard Model, neutrino masses are all set to zero (in fact, the model would not require a big change to incorporate finite neutrino mass). The situation is much different from that of the massless photon, because there is no obvious symmetry in the Standard Model that accounts for massless neutrinos. In general, the particle masses are modified by interactions. Keeping physical neutrino masses zero seems to require a conspiracy: the magnitude of the bare neutrino masses and the strength of the interactions must be almost exactly tuned to give zero in the end. Physicists have come to regard theories that must be finely tuned as unnatural and unsatisfactory.

All the quarks are assigned a one-third unit of baryon number. So far, baryon number is absolutely conserved according to experiment. But, as with the zero neutrino masses, no underlying symmetry explains baryon-number conservation. If there were a symmetry, baryon number should be associated with a force, like electric charge, but experiments have found no force that couples to baryon numbers. A similar problem occurs for leptons: experiments seem to require conserved lepton numbers (lepton number discriminates between leptons from different generations) in the absence of an underlying symmetry. With no good reasons for these conservation laws, searches for violations of baryon and particularly lepton number continue to be actively pursued by nuclear physicists.

The unsatisfactory features of the Standard Model suggest specific areas for experiments to probe. For example, looking for conclusive evidence of nonzero neutrino mass is a major goal of nuclear physics.

Understanding the symmetries relevant to the behavior of the physical world, even approximate symmetries, organizes the underlying physics. The connection between symmetries and conservation laws is the key link in this understanding. Conservation laws for energy and momentum are understood as a consequence of underlying symmetries with respect to translations in time and space. If the laws of nature are the same everywhere in the universe and do not change in time, then energy and momentum must be conserved absolutely. The conservation laws for energy and momentum are so well tested and so well established that it is almost inconceivable that a violation will ever show up.

As experimental and theoretical tools grew more sophisticated, symmetries other than those of space-time were introduced in physics. These go under the name of internal symmetries. Electric-charge conservation is one example. Some related internal symmetries first studied in nuclear physics include charge independence

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)