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Summary This report provides a long-term assessment of and outlook for nuclear physics. The first phase of the report articulates the scientific rationale and objectives of the field, while the second phase provides a global context for the field and its long- term priorities and proposes a framework for progress through 2020 and beyond. The full statement of task for the committee is given in Appendix A. Nuclear physics today is a diverse field, encompassing research that spans dimensions from a tiny fraction of the volume of the individual particles (neutrons and protons) in the atomic nucleus to the enormous scales of astrophysical objects in the cosmos. Its research objectives include the desire not only to better under- stand the nature of matter interacting at the nuclear level, but also to describe the nature of neutrinos and the state of the universe that existed at the big bang and that can now be studied in the most advanced colliding-beam accelerators, where strong forces are the dominant interactions. The impact of nuclear physics extends well beyond furthering our scientific knowledge of the nucleus and nuclear properties. Nuclear science and its tech- niques, instruments, and tools are widely used to address major societal prob- lems in medicine, border protection, national security, nonproliferation, nuclear forensics, energy technology, and climate research. Further, the tools developed by nuclear physicists often have important applications to other basic sciences— medicine, computational science, and materials research, among others—while its discoveries impact astrophysics, particle physics, and cosmology, and help to describe the physics of complex systems that arise in many fields. In the second phase of the study, developing a framework for progress though 1
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2 Nuclear Physics 2020 and beyond, the committee carefully considered the balance between univer- sities and government facilities in terms of research and workforce development and the role of international collaboration in leveraging future investments. The committee sought to address the means by which the balance between the various objectives of nuclear physics could be sustainable in the long term. In summary, the committee finds that nuclear science in the United States is a vital enterprise that provides a steady stream of discoveries about the fundamental nature of subatomic matter that is enabling a new understanding of our world. The scientific results and technical developments of nuclear physics are also being used to enhance U.S. competition in innovation and economic growth and are having a tremendous interdisciplinary impact on other fields, such as astrophysics, biomedical physics, condensed matter physics, and fundamental particle physics. The application of this new knowledge is contributing in a fundamental way to the health and welfare of the nation. The committee’s findings and recommendations are summarized below. FOLLOWING THROUGH WITH THE LONG-RANGE PLAN The nuclear physics program in the United States has been especially well managed. Among the activities engaged in by the nuclear physics community is a recurring long-range planning process conducted under the auspices of the Nuclear Science Advisory Committee (NSAC) of the Department of Energy (DOE) and the National Science Foundation. This process includes a strong bottom-up empha- sis and produces reports every 5 to 7 years that provide guidance to the funding agencies supporting the field. The choices made in NSAC’s latest long-range plan, the Long Range Plan of 2007, have helped to move the field along and set it on its present course, and the scientific opportunities that process recognized as impor- tant will enable significant discoveries over the coming decade. Exploitation of Current Opportunities Carrying through with the investments recommended in the 2007 Long Range Plan is the consequence of careful planning and sometimes difficult choices. The tradition of community engagement in the planning process has served the U.S. nuclear physics community well. A number of small and a few sizable resources have been developed since 2007 that are providing new opportunities to develop nuclear physics. Finding: By capitalizing on strategic investments, including the ongoing upgrade of the continuous electron beam accelerator facility (CEBAF) at the Thomas Jefferson National Accelerator Facility and the recently com-
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Summary 3 pleted upgrade of the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory, as well as other upgrades to the research infrastructure, nuclear physicists will confront new opportunities to make fundamental discoveries and lay the groundwork for new applications. Conclusion: Exploiting strategic investments should be an essential compo- nent of the U.S. nuclear science program in the coming decade. The Facility for Rare Isotope Beams After years of development and hard work involving a large segment of the U.S. nuclear physics community and the DOE, a major, world-leading new accelerator is being constructed in the United States. Finding: The Facility for Rare Isotope Beams is a major new strategic invest- ment in nuclear science. It will have unique capabilities and will offer oppor- tunities to answer fundamental questions about the inner workings of the atomic nucleus, the formation of the elements in our universe, and the evolution of the cosmos. Recommendation: The Department of Energy’s Office of Science, in conjunc- tion with the state of Michigan and Michigan State University, should work toward the timely completion of the Facility for Rare Isotope Beams and the initiation of its physics program. Underground Science in the United States In recent decades the U.S. program in nuclear science has enabled important experimental discoveries such as the nature of neutrinos and the fundamental reac- tions fueling stars, often with the aid of carefully designed experiments conducted underground, where the backgrounds from cosmic radiation are especially low. The area of underground experimentation is a growing international enterprise in which U.S. nuclear scientists often play a key role. Recommendation: The Department of Energy, the National Science Foun- dation, and, where appropriate, other funding agencies should develop and implement a targeted program of underground science, including important experiments on whether neutrinos differ from antineutrinos, on the nature of dark matter, and on nuclear reactions of astrophysical importance. Such a
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4 Nuclear Physics program would be substantially enabled by the realization of a deep under- ground laboratory in the United States. BUILDING THE FOUNDATION FOR THE FUTURE Nuclear physics in the United States is a diverse enterprise requiring the coop- eration of many institutions. The subject of nuclear physics has evolved signifi- cantly since its beginnings in the early twentieth century. To continue to be healthy the enterprise will require that attention be paid to elements essential to the vitality of the field. Nuclear Physics at Universities America’s world-renowned universities are the discovery engines of the Ameri- can scientific enterprise and are where the bright young minds of the next gen- eration are recruited and trained. As with other sciences, it is imperative that the critical value-added role of universities and university research facilities in nuclear physics be sustained. Unfortunately, there has been a dramatic decrease in the number of university facilities dedicated to nuclear science research in the past decade, including fewer small accelerator facilities at universities as well as a reduction in technical infrastructure support for university-based research more generally. These developments could endanger U.S. nuclear science leadership in the medium and long term. Finding: The dual role of universities—education and research—is impor- tant in all aspects of nuclear physics, including the operation of small, medium, and large facilities, as well as in the design and execution of large experiments at the national research laboratories. The vitality and sustain- ability of the U.S. nuclear physics program depend in an essential way on the intellectual environment and the workforce provided symbiotically by universities and the national laboratories. The fraction of the nuclear sci- ence budget reserved for facilities operations cannot continue to grow at the expense of the resources available to support research without serious damage to the overall nuclear science program. Conclusion: In order to ensure the long-term health of the field, it is critical to establish and maintain a balance between funding of operations at major facilities and the needs of university-based programs. A number of specific recommendations for programs to enhance the universi- ties are discussed in the report. Many of these suggestions are not costly but could
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Summary 5 have significant impact. An example of a modest program that would enhance the recruitment of early career nuclear scientists and could be provided at relatively low cost is articulated in the following recommendation: Recommendation: The Department of Energy and the National Science Foundation should create and fund two national competitions: one a fellow- ship program for graduate students that would help recruit the best among the next generation into nuclear science and the other a fellowship program for postdoctoral researchers to provide the best young nuclear scientists with support, independence, and visibility. Nuclear Physics and Exascale Computing Enormous advances in computing power are taking place, and computers at the exascale are expected in the near future. This new capability is a game-changing event that will clearly impact many areas of science and engineering and will enable breakthroughs in key areas of nuclear physics. These include providing new under- standings of, and predictive capabilities for, nuclear forces, nuclear structure and reaction dynamics, hadronic structure, phase transitions, matter under extreme conditions, stellar evolution and explosions, and accelerator science. It is essential for the future health of nuclear physics that there be a clear strategy for advancing computing capabilities in nuclear physics. Recommendation: A plan should be developed within the theoretical com- munity and enabled by the appropriate sponsors that permits forefront computing resources to be exploited by nuclear science researchers and establishes the infrastructure and collaborations needed to take advantage of exascale capabilities as they become available. Striving to Be Competitive and Innovative Progress in science has always benefited from cooperation and from competi- tion. For U.S. nuclear physics to flourish it must be competitive on the international scene, winning its share of the races to new discoveries and innovations. Providing a culture of innovation along with an understanding and acceptance of the appro- priate associated risk must be the goal of the scientific research enterprise. The committee sees one particular aspect of science management in the United States where increased flexibility would have large and immediate benefits. Finding: The range of projects in nuclear physics is broad, and sophisticated new tools and protocols have been developed for successful management of
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6 Nuclear Physics the largest of them. At the smaller end of the scale, nimbleness is essential if the United States is to remain competitive and innovative on the rapidly expanding international nuclear physics scene. Recommendation: The sponsoring agencies should develop streamlined and flexible procedures that are tailored for initiating and managing smaller- scale nuclear science projects. Prospects for an Electron-Ion Collider Accelerators remain one of the key tools of nuclear physics, other fields of basic and applied research, and societal applications such as medicine. Modifying exist- ing accelerators to incorporate new capabilities can be an effective way to advance the frontiers of the science. Of course it is the importance of the physics and of the potential discoveries enabled by the new capability that must justify the new investment. There is an initiative developing aimed at a new accelerator capabil- ity in the United States. Fortunately, the U.S. nuclear physics community has the mechanisms in place to properly evaluate this initiative. Currently there are sug- gestions that upgrades to either RHIC or CEBAF would enable the new capability. Finding: An upgrade to an existing accelerator facility that enables the col- liding of nuclei and electrons at forefront energies would be unique for studying new aspects of quantum chromodynamics. In particular, such an upgrade would yield new information on the role of gluons in protons and nuclei. An electron-ion collider is currently under scrutiny as a possible future facility. Recommendation: Investment in accelerator and detector research and development for an electron-ion collider should continue. The science opportunities and the requirements for such a facility should be carefully evaluated in the next Nuclear Science Long-Range Plan. Nuclear physics is a discovery-driven enterprise motivated by the desire to understand the fundamental mechanisms that account for the behavior of matter. Nevertheless, for its first hundred years, the new knowledge of the nuclear world has also directly benefited society through many innovative applications. As we move into the second century of nuclear physics the recommendations above will ensure a thriving and healthy field that continues to benefit society from new applications. Recently the stewardship of the nation’s isotope program has been placed in the DOE Office of Nuclear Physics. This reorganization is appropriate and provides a fresh opportunity for the nuclear physics community to serve society by
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Summary 7 applying its sciences to the most important of today’s problems in energy, health, and the environment. The isotopes program under the auspices of that office is expected to benefit rapidly from new innovations and developments. NSAC and its subcommittees have provided insightful reports that constitute a roadmap for the revitalized isotopes program. This advice is timely, coming when important decisions must be made. The committee sees these developments as an excellent example of how society’s investments in nuclear physics can help resolve difficult challenges that face the nation.
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