Summary and Recommendations
Nuclear physics addresses the nature of matter making up 99.9 percent of the mass of our everyday world. It explores the nuclear reactions that fuel the stars, including our Sun, which provides the energy for all life on Earth. The field of nuclear physics encompasses some 3,000 experimental and theoretical researchers who work at universities and national laboratories across the United States, as well as the experimental facilities and infrastructure that allow these researchers to address the outstanding scientific questions facing us. This report provides an overview of the frontiers of nuclear physics as we enter the next millennium, with special attention to the state of the science in the United States.
The current frontiers of nuclear physics involve fundamental and rapidly evolving issues. One is understanding the structure and behavior of strongly interacting matter in terms of its basic constituents, quarks and gluons, over a wide range of conditions—from normal nuclear matter to the dense cores of neutron stars, and to the Big Bang that was the birth of the universe. Another is to describe quantitatively the properties of nuclei, which are at the centers of all atoms in our world, in terms of models derived from the properties of the strong interaction. These properties include the nuclear processes that fuel the stars and produce the chemical elements. A third active frontier addresses fundamental symmetries of nature that manifest themselves in the nuclear processes in the cosmos, such as the behavior of neutrinos from the Sun and cosmic rays, and in low-energy laboratory tests of these symmetries.
With recent developments on the rapidly changing frontiers of nuclear physics the Committee on Nuclear Physics is greatly optimistic about the next ten years. Important steps have been taken in a program to understand the structure
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
tools needed to produce such unusual nuclei in the laboratory, opening the door to new experiments on the properties of nuclear matter near the limits of binding.
The recommendations by this committee should be considered in the context of the careful planning in the nuclear physics community summarized by the Long Range Plans developed by the Nuclear Science Advisory Committee (NSAC). NSAC advises the two principal funding agencies for this field, the Department of Energy and the National Science Foundation. The Division of Nuclear Physics of the American Physical Society also played an important role, joining with NSAC to organize various town meetings for the purpose of gathering input from the community. The NSAC Long Range Plans have been prepared at about 6-year intervals (1979, 1983, 1989, and 1996). They have been influential in expressing new priorities of the field and in justifying new initiatives.1 The 1979 and 1983 Long Range Plans, for example, identified CEBAF and RHIC as the most promising new initiatives for decisively advancing the scientific frontiers of the field. The recent adoption of a similar planning process by the European nuclear physics community is an indication of the perceived effectiveness of the Long Range Plans.
In parallel with CEBAF and the construction of RHIC, the NSAC Long Range Plans have also identified and recommended several smaller targets of opportunity. Among those currently being implemented with agency funding are an upgrade to the capabilities for producing energetic beams of short-lived nuclei at Michigan State University, the construction of new detectors for studying solar neutrinos, and the adaptation of RHIC to the investigation of previously inaccessible aspects of the proton's structure.
Both the Department of Energy and the National Science Foundation support user facilities of world-class capability and both have strong university programs. DOE supports the largest user facilities and university groups, while NSF supports user facilities at universities and many university user groups. The committee believes that the continuing programs in the two agencies are essential to the field, with the DOE emphasis on national laboratory facilities and the NSF emphasis at the universities providing complementary strengths and opportunities.
Because there exists a tradition of successful deliberation and planning within the nuclear physics community, the Committee on Nuclear Physics chose to emphasize the science rather than the process in the recommendations presented
below. However, it would be remiss if it failed to bring into focus the funding stresses that now severely threaten the field.
At present it seems to be generally agreed by policymakers on all sides that the support of basic research is in the public interest, and there is considerable talk of increasing the corresponding budgets. However, the reality in nuclear physics, as in many other fields of research, is quite different. In 1996 the budget guidance provided by the DOE and NSF to help formulate the most recent Long Range Plan 2 for nuclear physics was for roughly constant manpower budgets. This goal has been undercut by the budgets of recent years. The cumulative result of a dollar-flat budget in the case of the DOE is that it now is 3 to 10 percent below the range of the guidance. In the case of NSF, there has been a larger decline, to about 15 percent below the 1996 guidance.
These decreases will curtail the utilization of new facilities and instrumentation and will jeopardize our nation's world-leading role in the field. This situation has arisen even as the efficient commissioning of CEBAF, the approaching completion of RHIC, new technical advances in the exploration of nuclei near the limits of binding, and discoveries in low-energy neutrino physics have made execution of the 1996 Long Range Plan all the more urgent, requiring the level of funding given in the guidance by the agencies.
Recommendation I: Discoveries in nuclear physics—new phenomena connected with the role of quarks and gluons in the nucleus, the structure and dynamics of nuclei, the nuclear physics of the cosmos, and the limits of the Standard Model—are within reach due to our recent investments in new facilities and instrumentation. With CEBAF having started on its research program of the quark-gluon structure of matter, RHIC about to embark on the study of matter at the limits of energy density, and with other recent advances in technical capabilities, a rich scientific harvest is limited by severely constrained budgets. The committee recommends the near-term allocation of resources needed to realize these unique experimental and theoretical opportunities.
Careful laboratory measurements of nuclear reactions that take place in stars have provided the foundation for some of the field's most important achievements in understanding the nuclear bases of the cosmos, including the solar neutrino problem and the origin of the light chemical elements in the Big Bang. Beams of exotic short-lived nuclei are opening up new opportunities for measuring nuclear properties and reactions in the poorly understood regions near the limits of stability. The properties of these barely stable nuclei have direct quantitative connections to the processes that fuel the stars and create the chemical
elements of our world. Beams of exotic nuclei hold great promise as tools for probing new nuclear properties and for testing fundamental symmetries at low energies. These considerations provide a compelling argument for constructing a next-generation facility that will use isotope separator online (ISOL) techniques to produce high-intensity, high-resolution beams of short-lived nuclei over a broad mass range.
Recommendation II: The committee recommends the construction of a dedicated, high-intensity accelerator facility to produce beams of short-lived nuclei. Such a facility will open up a new frontier in nuclear structure near the limits of nuclear binding and will strengthen our understanding of nuclear properties relevant to explosive nucleosynthesis and other aspects of the physics governing the cosmos.
Frontier research in nuclear physics relies on both large accelerators, such as CEBAF and RHIC, and smaller facilities, where specialized low-energy measurements can be made. These smaller facilities include several university and national laboratory accelerators where weak interaction, nuclear structure, and nuclear astrophysics studies are done. Both small and large accelerators rely critically on innovative instrumentation to make new discoveries. In the case of CEBAF and RHIC, the quality of the physics programs depends on specialized detectors. The development of much of this equipment is on a scale that is suitable for university laboratories, where graduate students can participate in the construction and gain experience with cutting-edge technology. Many of the equipment needs at the smaller facilities are equally specialized. Examples include atom and ion traps designed for precision studies of weak interactions and sensitive detector arrays for measuring nuclear reactions at the very low energies characteristic of stars like our Sun.
Recommendation III: The committee recommends continued investment in instrumentation for research. As new discoveries come to light and new ideas for experiments emerge, upgrades of detector systems at CEBAF and RHIC and instrumentation needs at smaller laboratories should be considered in accordance with their potential for new discoveries. NSAC is well positioned to provide DOE and NSF appropriate advice on relative priorities and specific major upgrades.
To foretell the course of a science beyond the near term is always difficult, as it depends both on the discoveries of the next few years and the doors that new advances in technology will open. The following represents some of the future options, among a number of attractive possibilities that can be perceived at the present time, for possible implementation in the early part of the next century.
CEBAF probes nuclei at length scales where the quark and gluon substructure of nuclei should first become apparent. It thus represents a first step in probing the relationship between standard nuclear physics based on protons,
neutrons, and mesons, and the underlying fundamental degrees of freedom—quarks and gluons. To understand the transition between these regimes, it may be necessary to extend the measurements to even finer resolution, such as that offered by a 15- to 30-GeV electron accelerator. The construction of a 25-GeV machine is now under discussion in Europe, and future upgrades of CEBAF are being considered in the United States.
RHIC is about to open a new door to ultrahigh energy densities in nuclear matter. The potential discovery there of a new phase of matter—a plasma of quarks and gluons—could point the way to issues requiring still higher beam intensities or energies. Construction of the Large Hadron Collider (LHC) at CERN in Europe has recently begun, with U.S. participation. Early in the next century, this facility will allow collisions of nuclei at 40 times the beam energy of RHIC. Future discoveries at RHIC will guide upgrades of RHIC and the participation of U.S. nuclear physicists in the LHC effort.
The impact of the discovery that neutrinos may have mass will be felt throughout physics. Thus, following the Sudbury Neutrino Observatory, there may be an urgent need to develop and deploy detectors capable of exploring the spectrum of lower-energy solar neutrinos, or of greatly improving the sensitivity to neutrinos from the next supernova neutrino burst. Terrestrial neutrino experiments have put important constraints on neutrino properties; a compelling case may arise for new terrestrial experiments.
Studies of fundamental symmetries in nuclei can isolate and enhance new phenomena beyond the Standard Model. In particular, new experimental searches for a neutron electric-dipole moment and precision measurements of beta-decay correlation coefficients can become the most stringent constraints on our understanding of fundamental symmetries. Promising possibilities exist for developing sources of cold and ultracold neutrons of unprecedented intensity.
Recommendation IV: Within the ten-year time frame envisioned for this report, new discoveries will provide strong arguments for one or more major new endeavors. Possible candidates include a higher-energy electron machine, capability for the study of heavy-ion collisions with increased energy densities, new detectors to explore mass effects on the solar and supernova neutrino fluxes, and an ultracold neutron facility providing an order-of-magnitude increase in the neutron densities for studies of fundamental symmetries. The committee recommends the continuation of frequent NSAC Long Range Plan efforts, to help retain the responsiveness of the field to the most promising new opportunities.
Nuclear physics not only advances the frontiers of knowledge but also makes remarkable contributions to the needs of society. The generation of nuclear energy, both for civilian power consumption and for nuclear weapons, has had a profound impact on our society in the last 50 years. Equally far-reaching has been the impact of nuclear physics in medicine; results of nuclear physics and
nuclear physics techniques, from magnetic resonance to detector technologies to the use of isotopes, have led to remarkable advances in diagnostic and therapeutic power. Nuclear diagnostic techniques have a growing and pervasive role in industry, national security, nonproliferation, geophysics, global climate research, and paleontology. Nuclear physics is the basis of important technologies in the design and preparation of materials. Through such applications, through the technical and intellectual intersections of nuclear physics with other fields of science, and through its intrinsic intellectual challenges, nuclear physics stands as one of the core sciences in the continuing advancement of knowledge.
Facilities and instrumentation are essential for progress, but science ultimately depends on the people who carry it out—on their individual creativity, drive, and enterprise. The scientists who conduct experiments and develop the theoretical framework for interpreting the results are the most essential components of the field. The continued intellectual vitality of nuclear physics as a science, and the continuation of the field's more direct contributions to societal needs, depend critically on the capacity to educate the next generation of physicists. Past performance has demonstrated that students trained in solving the enormously challenging problems of forefront physics research develop the array of skills needed to lead the nation in harnessing the rapidly advancing technology that often emerges from the research itself.
The remainder of this report summarizes the current status of the science of nuclear physics. Several items of more general interest are highlighted in boxes throughout the scientific chapters.