of matter in terms of quarks and gluons. The United States has made two major and farsighted investments in this program. The Continuous Electron Beam Accelerator Facility (CEBAF) has recently come into operation and is now delivering beams of unprecedented quality. It will serve as the field's primary “microscope" for probing the building blocks of matter such as the nucleons (protons, neutrons) and the nuclei of atoms, at the small length scales where new physics phenomena involving quarks and gluons should first appear. It will provide new insights into the structure of both isolated nucleons and nucleons imbedded in the nuclear medium. The Relativistic Heavy Ion Collider (RHIC), whose construction is now nearing completion, will produce the world's most energetic collisions of heavy nuclei. This will allow nuclear physicists to probe the properties of matter at energies and densities similar to those characterizing the cores of neutron stars and the Big Bang. RHIC experiments should teach us about the expected transition to a new phase of nuclear matter in which the quarks and gluons are no longer confined within nucleons and mesons.
The theory supporting these new efforts has produced new bridges between quantum chromodynamics (QCD)—the theory of quarks and gluons—and the field's more traditional models of nuclear structure, which involve nucleons and mesons. Nuclear theorists have begun to construct "effective theories" that are equivalent to QCD at low energies, yet share many of the properties of traditional models that view nuclei as quantum fluids of protons and neutrons. This work is providing the field with new tools for more critically addressing the structure of nuclei and the properties of bulk nuclear matter.
An area that at present is generating intense interest is related to nuclear processes in the cosmos. Experiments measuring neutrinos from the Sun and from cosmic-ray interactions in Earth's atmosphere strongly suggest that neutrinos are massive, a result that would imply new physics beyond the current "Standard Model" of particle physics. U.S. nuclear physicists, who have worked in the field since initiating the first experiment more than 30 years ago, are currently partners in the Sudbury Neutrino Observatory, the first detector that will distinguish solar neutrinos of different types, or "flavors." Such experiments are part of a larger effort to carefully test the Standard Model at low energies. The nucleus is a powerful laboratory for probing many of the fundamental symmetries of nature, because it can magnify subtle effects that may hide beyond the direct reach of the world's most energetic accelerators.
Another frontier area is the study of how the nucleus changes when subjected to extreme conditions, such as very rapid rotation or severe imbalances between the numbers of neutrons versus protons. Exotic nuclei play essential roles in the evolution of our galaxy: the "parents" of about half of the heavy elements are very neutron-rich nuclei, believed to have been created within the spectacular stellar explosions known as supernovae, at temperatures in excess of a billion degrees. Remarkable advances in accelerator technology have now provided the