Research in nuclear physics is an integral part of the search for knowledge and understanding of the world in which we live. All matter is composed of a hierarchy of building blocks. Living creatures, as well as our inanimate surroundings, are made of molecules, which are in turn made of atoms, whose mass resides almost entirely in the nuclei. The nuclei are composed of protons and neutrons, which ultimately consist of quarks and gluons. In the recent past, as our progress in understanding has reached down to ever smaller scales, each hierarchical level has developed its own subdiscipline, with its own distinct experimental and theoretical endeavors and new insights. Each subdiscipline has produced its own range of applications, benefiting the development of society and contributing to the scientific and technological base on which our industrial and economic strength rests.
The science of nuclear physics concerns itself with the properties of "nuclear" matter. Such matter constitutes the massive centers of the atoms that account for 99.9 percent of the world we see about us. Nuclear matter is within the proton and neutron building blocks of these nuclei, and appears in bulk form in neutron stars and in the matter that arose in the Big Bang. Nuclear physicists study the structure and properties of such matter in its various forms, from the soup of quarks and gluons present at the birth of our universe to the nuclear reactions in our Sun that make life possible on Earth.
Origins and Fundamentals
Our awareness of the very existence of a heavy nucleus at the center of the atom dates from the work of Rutherford in the first decades of this century. This
work was followed by basic, exciting developments: the discovery of neutrons, of nuclear reactions and the transmutations of elements, of isotopes, of the detailed nature of radioactivity. These discoveries followed in quick succession, in parallel with the developing insight that a revolutionary new framework—quantum mechanics—was needed to describe phenomena or the scales of the atom and the nucleus. This period also initiated our understanding of how nuclear processes fuel the Sun.
The early sequence of discoveries led, during World War II, to the Manhattan Project, which was based on the prior investigation of basic nuclear properties and played a key role in the history of our nation and the world. Applications of nuclear techniques to benefit human health started early, with major developments in this field continuing to the present.
The 1950s and 1960s saw the conceptual development of basic models of the atomic nucleus that provided a successful, if approximate, phenomenological framework for describing nuclear structure and reactions. The roots of many of today's urgent questions in nuclear physics can be traced to this period: Why were such model descriptions of the nucleus so successful? How do they arise as low-energy, long-distance representations of the more fundamental theory of quarks and gluons? How do the symmetries that govern the strong interactions constrain this framework and influence the different temperature regimes? What are the limitations of the models as smaller length scales and higher energies are probed? Studies of nuclear beta decay undertaken in this period helped establish the form of the weak interaction and guided the formulation of a Standard Model that has been astoundingly successful in uniting the weak and electromagnetic interactions. Yet this model remains incomplete. How can we find hints of the missing physics with low-energy precision experiments in nuclei testing the limitations of the Standard Model?
Scope of the Field
The fundamental questions that confront nuclear physics today have inevitably led the field to extend its horizons, both in the reach of its frontiers and in the scope of its research enterprise. The size and energy scales of present-day nuclear physics extend from the world of atomic and condensed matter physics to the more microscopic domain of high-energy physics, and at the large end of the scale, to the stars and the cosmos. It is this broad reach that makes nuclear physics so interesting to many scientists (more than 3,000 in the United States alone) and so integrally connected to other sciences. Nuclear physics both contributes to and benefits from other fields—for instance, from atomic physics for intricate table-top experiments, to high-energy physics for hall-size collider detectors. High-energy physics is concerned with the elementary particles and their interactions; it is the goal of nuclear physics to understand and explain why and
how these particles, through their interactions, group themselves together to form matter.
Nuclear phenomena are important in systems spanning the entire length scale indicated in Figure 1.1, because issues requiring the application of nuclear
physics methods recur at different scales with a correspondingly enormous range of energies needed for their study. All of the fundamental forces of nature come into play, each with its own special role in governing the behaviors observed over this huge span.
The central intellectual challenges around which contemporary nuclear physics research is organized drive the need for forefront accelerator facilities, sophisticated detector instrumentation, and high-powered computers, as well as the development of innovative experimental and theoretical methods. These new technologies and methods have, in turn, found rapid and widespread application, fulfilling society's needs in areas as diverse as health, security, energy production, and industrial efficiency. The application of new techniques from the physical sciences has enabled a major revolution in the life sciences, a process that will continue into the future.
Nuclear physics has accomplished a great deal in the recent past, as is discussed throughout this report. The committee lists here a few of the results of the past decade.
Significant advances have been made toward determining the internal structure of the building blocks of matter:
- Measurements of the distribution of quarks and antiquarks in nuclei show that these distributions are different from those in free protons or neutrons.
- Several experiments have demonstrated that neutrons and protons experience slightly, but distinctly, different nuclear forces; thus, these forces do not exactly obey the principle of charge symmetry.
- The theoretical analyses of measurements of the structure of the proton and neutron show that the spin of these particles does not have a simple origin in the quarks and that the polarization of the quark-gluon "sea" plays an important role.
New insights into the properties and structure of nuclei have been gained:
- Electron-scattering experiments have determined the structure of nuclei on a scale less than the size of individual nucleons; this advance is comparable with the first determinations of crystal structure through x-ray scattering.
- The limits of knowledge have been extended by creating in the laboratory a variety of new, short-lived elements and isotopes at the limits of nuclear stability: the first atoms of the heaviest elements through element number 112, new "magic" nuclei with unusual neutron-proton ratios, and exotic new structures such as nuclei with large, very diffuse, neutron halos.
- Superdeformed, highly elongated shapes have been discovered in nuclei
- undergoing rapid rotation; the states associated with these shapes are extremely stable.
- Advanced supercomputers and new mathematical techniques allow, for the first time, exact many-body calculations of the properties of light nuclei, starting with the basic interaction between nucleons, and establishing the importance of the nuclear three-body force.
New insights have been gained in studies of matter at very high energy densities:
- Experiments have shown that systems formed in energetic collisions of nuclei decay by copious emission of sizable fragments. These decays appear to have some of the features of a phase transition from liquid-like to gas-like behavior of nuclear matter.
- Theoretical work indicates that above a certain energy density quarks will be liberated from their confinement in protons and neutrons. Recent measurements at existing accelerators have demonstrated that the relevant densities will be reached in collisions at RHIC.
- Heavy-ion collisions at very high energies have been observed to be more than simple superpositions of independent nucleon-nucleon collisions. Notably, intriguing changes are observed in the production of heavy mesons, depending on the types of quarks they contain. These changes further signal the possibilities of new phenomena in dense nuclear matter.
New results regarding fundamental symmetries have been obtained through studies at low energies:
- Precision beta-decay experiments have limited the mass of the electron neutrino to about one hundred-thousandth of that of the electron.
- The exceedingly rare process of nuclear double beta decay, by the simultaneous emission of two electrons and two neutrinos, was directly measured for the first time. Limits on double beta decay with no neutrinos have been greatly improved, thus testing with unprecedented sensitivity whether the neutrino might be its own antiparticle.
The connection between nuclear properties and the nature of the universe has been further elucidated:
- Precision experiments have firmly established that fewer neutrinos reach the Earth from the Sun than would be expected on the basis of solar energy production, suggesting that neutrinos have mass and can change from one kind of neutrino to another.
- Major advances have been made in measuring the cross sections for processes in stars that are crucial to the formation of the elements, utilizing both intense beams of stable nuclei and new techniques based on beams of short-lived nuclei.
- Recent theoretical progress in understanding nuclear forces suggests that the mass at which neutron stars, the densest objects in our universe, collapse into black holes, is substantially lower than previously thought. This would imply that many compact objects detected by astronomers in binary systems, and previously thought to be neutron stars, are instead likely to be black holes.
- Recent advances in the nuclear physics of supernovae have led to a deeper understanding of their explosion mechanism and of the processes responsible for heavy element synthesis, and to the discovery of a new nucleosynthesis process driven by neutrinos.
Technical innovations at a number of laboratories have made possible further advances in experimental research expanding the horizons of the research effort. In many cases, the development of these technologies generates novel applications in areas outside of nuclear physics research:
- Superconducting technology has been applied to a number of new nuclear physics accelerators: new accelerating structures for both heavy ions and electrons and new superconducting magnets for cyclotrons and beam transport.
- The CEBAF accelerator, the first continuous-beam electron accelerator at multi-GeV energies, has been operating successfully as the world's finest electron microscope for studying the physics of the nucleus.
- New techniques have been developed to produce and separate copious beams of short-lived isotopes far off stability.
- “Cooled" beams in storage rings have been developed for high-resolution studies with internal targets and used successfully in important, high-precision experiments.
- Gaseous polarized targets, in which the nuclei of atoms have their spins aligned, have been developed and used in a number of fundamental experiments.
Finally, techniques and knowledge from nuclear physics have been applied to societal needs:
- Applications of new detection techniques for scanning radioisotopes in patients have expanded the horizons of diagnostic medicine.
- The use of polarized nuclear magnetic resonance of noble gases in tomography is providing enhanced images of the lung.
- Applications of accelerators to therapeutic medicine are making significant advances; treatments with beams of protons and neutrons are becoming routine.
- Accelerator mass spectroscopy is used increasingly in a number of fields beyond archaeology—for instance, in areas related to the environment, such as geochemistry, geophysics, and global climate history, and in studies of the origins of life, as well as in medical studies.
The focus of the field is on matter whose behavior is governed primarily by the strong interaction. This class of matter includes most of the known mass in the universe, ranging from the smallest stable particles with strong interactions—the proton and the neutron—through atomic nuclei, and up to neutron stars and supernovae. The goal is to understand how all of this strongly interacting matter is assembled, how the properties of and the limitations on the existing forms of matter are predetermined by the properties and interactions of its fundamental constituents. This is a formidable task, but a central one in seeking to understand our world and our universe. The major questions facing nuclear physics at the dawn of the new millennium are considered in the discussion that follows.
The strong interaction that binds nucleons together in nuclei is much more complex than the electromagnetic force that holds electrons in atoms, and atoms in molecules. Studies of scattering between two nucleons demonstrate that their low-energy interactions can be described in part in terms of the exchange of mesons, particles of medium mass. This insight is the basis for many successful models of nuclear structure. But our best present understanding is that the fundamental constituents of nuclei are quarks and gluons, whose interactions are described by quantum chromodynamics (QCD). Both nucleons and mesons are composites of quarks. In fact, the most remarkable property that follows from QCD is that individual quarks do not exist in isolation, but instead are always found bound with other quarks and antiquarks in such composite particles. This leads to some of the most fundamental questions in modern nuclear physics:
- How do the nucleon-based models of nuclear physics with interacting nucleons and mesons arise as an approximation to the quark-gluon picture of QCD?
- In probing ever-shorter distances within the nucleus, at what point must the description in terms of nucleons give way to a more fundamental one involving quarks and gluons?
- Does the nuclear environment modify the quark-gluon structure of nucleons and mesons?
A powerful new facility, CEBAF, was recently completed at the Thomas Jefferson National Accelerator Facility to allow examination of nuclei at length scales smaller than the size of the nucleon.
The classical nuclear physics models must also break down as more and more energy is crowded into the nuclear volume. At sufficiently high temperatures, the distinction between individual nucleons in a nucleus should disappear: the nucleons will melt, and their quark constituents will be free to roam over much of the nuclear volume. This is the state of nuclear matter we believe existed
in the early instants of the Big Bang: only as the universe expanded and cooled from its fiery start did nucleons coalesce from a sea of quarks and gluons. It is the goal of the Relativistic Heavy Ion Collider (RHIC), a major new facility in nuclear physics, to study matter at the highest energy density and in the process recreate and study this transition.
- What are the phases of matter formed when ordinary nuclei are heated to the very high temperatures at which quarks and gluons become deconfined from the nucleons and mesons?
- What are the experimental signatures for a transition to new phases in relativistic heavy-ion collisions?
- What are the implications for the analogous epoch in the Big Bang?
A few minutes after the Big Bang, long after nucleons had coalesced out of the early quark-gluon plasma, the universe cooled sufficiently to allow nucleons to condense into the first light nuclei. Since that time, nucleosynthesis has continued in the centers of stars such as our Sun—it is the fusion of light elements into heavier ones that is the source of solar and stellar energy. These new nuclei, expelled into space in violent stellar explosions or by stellar winds, form the raw material for the formation of new stars and new planets. Thus, nuclear physics processes are the source of the rich chemical abundance of the world that we see about us. Many of the heavy elements on Earth are the progeny of exotic, very short-lived, neutron-rich nuclei that previously could only exist in nature in violent astronomical environments, such as supernovae. Today's nuclear physics researchers have developed the technology to produce and study some of these exotic nuclei in the laboratory:
- What is the quantitative origin of the chemical elements in the Big Bang and continuing to the supernovae we observe in our galaxy and elsewhere?
- What is the influence on element production of the properties of exotic nuclei, especially those near the limits of nuclear stability that become accessible with the advent of intense radioactive beams?
- What qualitatively new features appear in this hitherto unexplored regime of nuclei, and how do they influence stellar properties?
While it is believed that nuclei can ultimately be described in terms of QCD, more empirical models of nuclear physics have provided a realistic framework for understanding a rich array of observed nuclear phenomena. These include shell structure, which makes some nuclei much more tightly bound than others; collective rotations and vibrations of many nucleons in the nucleus; transitions between regular and chaotic behavior in nuclear spectra; and weakly bound halo nuclei with an enormous increase in nuclear size. As in any physical system,
pushing nuclei to their limits reveals new features and leads to new insights and understanding.
- How do nuclei behave when pushed to the limits of their excitation energy, angular momentum, and nuclear binding?
- Can the apparently simple phenomenological models, which describe several nuclear properties so successfully, be related transparently to the basic interactions of the nuclear building blocks?
- Why do these simple models work so well?
In the framework provided by empirical models, nuclear physics has much in common with other subfields, such as condensed matter physics, where understanding the effective degrees of freedom provides the essential insight into the behavior of many-particle systems. The theoretical techniques used in nuclear physics—shell structure, collective coordinates, clustering and pairing, Monte Carlo and other large-scale numerical methods—are shared by many other subfields.
- How can the symbiosis of nuclear physics and other subfields be exploited to advance understanding of all many-body systems?
Early studies of beta decay in nuclear physics helped to provide the experimental foundation for the Standard Model of fundamental interactions. In particular, they pointed toward the existence of neutrinos, elusive particles with little, if any, mass, that interact only via the weak force. The feebleness of their interactions lets them escape unscathed from the interior of the Sun and makes neutrinos an excellent probe of the nuclear reactions that fuel the Sun deep in its interior. But several experiments have found far fewer solar neutrinos reaching the Earth than expected. Today, 30 years of effort in solar neutrino physics is culminating in the first detectors that can distinguish the different kinds of neutrino interactions, and thereby test the most far-ranging explanations suggested for the low flux. Experiments with these detectors over the next few years could well prove that neutrinos have mass.
- What is the reason for the low flux of solar neutrinos?
- Will the resolution of this problem demonstrate conclusively that neutrinos have new properties, such as a nonzero mass?
- What can studies of neutrinos from supernovae reveal about the properties of neutrino "families"?
- How will such studies help us understand stellar evolution, including the mechanism responsible for supernova explosions?
It is quite possible that the "new physics" lying beyond the Standard Model
may reside at energies well beyond the reach of direct accelerator experiments. Thus, our only opportunity to find this new physics may be through the "fingerprints" it has left on our low-energy world: subtle violations of the symmetries of the Standard Model. By exploiting the rich variety of nuclear species, nuclear physicists have developed an arsenal of precision techniques in searching for massive neutrinos, tiny changes in particle interactions when time's arrow is reversed, and other phenomena inconsistent with the Standard Model.
- What are the low-energy manifestations of physics beyond the Standard Model? How can precision experiments in nuclear physics reveal them?
Since the Second World War, the United States has played a world leadership role in nuclear physics. However, the contributions of other countries, particularly in Europe and Japan, have gradually increased and are now at least on an equal footing in most areas, and superior in some. This, of course, is as it should be. Science is an international undertaking, and societies with strong economies help lead the way in the pursuit of knowledge, both as an obligation and because this pursuit is in their long-term interest. Much scientific work is done in collaboration or in a spirit of friendly competition; the scientific process thrives in an atmosphere in which the implications of important new or surprising results can quickly be checked and extended by others. Unique facilities built by many nations are accessible to researchers from around the world. U.S. researchers can be found at work in laboratories of many countries—Germany, France, Finland, Japan, Canada, and Russia, to name a few. In turn, scientists from all parts of the world are carrying out their research at U.S. laboratories, both large and small. The fabric of science is intimately tied to worldwide cooperation, sometimes by formal agreements, but most often as a matter of course by informal arrangements.
A critical component of doing scientific research is the education of new scientists. A continuing flow of young scientists in training imparts a strong vitality to the process of questioning and searching for knowledge. The majority of scientists trained in nuclear physics go on to positions in industry, business, or government. Their knowledge, familiarity with modern technologies, and problem-solving skills provide excellent preparation for the requirements of our increasingly complex, technological society. The impact of the field on undergraduate and high school education, in providing students with an opportunity for hands-on contact with scientific research and with exposure to the horizons of high technology, is a continuing contribution.
Offshoots from basic research in nuclear physics have profoundly changed our daily lives. Nuclear energy and, by extension, nuclear weapons have had a deep impact on society. The uses of radioisotopes and magnetic resonance imaging in medicine are so widespread that rarely are they associated any longer with their origins in nuclear research. In these areas and in many others, inventions of new practical applications are ongoing—new radioisotopes and radiation detectors allow positron-emission tomography (PET) imaging of the human brain's functions, neutron beams serve as bomb detectors and scan for explosives, and accelerator-based mass spectrometry permits ultrasensitive detection of trace elements.
Proton and heavy-ion beams provide effective forms of cancer therapy. Accelerators developed by nuclear physicists for basic research are used extensively in materials studies. New concepts of fission reactors may create much safer and more efficient power reactors, and heavy-ion beams may move us a step closer to an inexhaustible supply of energy through inertial fusion reactors. Even in its seemingly esoteric mission for basic research, nuclear physics trains the technical manpower that invents, implements, and operates such applications in industry, medicine, and government, including national defense. Nuclear physics is one of the cornerstones of the nation's technological edifice.