The Committee on U.S. Based Electron Ion Collider Assessment finds that the science questions that an electron ion collider (EIC) would answer are central to completing our understanding of atomic nuclei as well as being integral to the agenda of nuclear physics today. These questions, about the fundamental building blocks of nuclei—neutrons and protons—and how they are held together in nuclei, are compelling. An EIC would build upon the heritage of more than a century of scattering experiments, discoveries, as well as on the insights and advances in accelerator science and technology. The increased understanding of nucleons, nuclei, and the underlying theory quantum chromodynamics (QCD) that an EIC would bring would have direct impact on particle physics, and improve our understanding of the most beautiful of all Yang Mills theories, QCD.1 Design and construction of an EIC would keep the United States at the forefront of new collider technologies. An EIC would contribute to basic energy sciences through the EIC goal of understanding the emergent behavior of dense gluonic systems, and to plasma physics and astrophysics, through the creation of a state with enormous but saturated gluon density, resembling but differing from the radiation dominated plasmas of explosive astrophysics.
The committee was tasked with evaluating the importance and urgency of the science that an EIC addresses to both nuclear science and the physical sciences more broadly. The committee’s task also included assessing the role of an EIC in
1 D. Gross, 2016, Quantum chromodynamics—The perfect Yang-Mills gauge theory, Int. J. Mod. Phys. A 31:1630008.
the global context, including its relationship to other facilities within the United States and around the world. Lastly, the committee was asked to assess the broader impacts of an EIC, including on U.S. science leadership. The full statement of task is included in Appendix A.
The committee had a wide range of scientific expertise, from nuclear physics, particle physics, astrophysics, accelerator science, and condensed matter physics. The committee also invited speakers from the nuclear physics, accelerator physics, and particle physics communities to provide additional expert input and insights. To better understand its task, the committee met with representatives of the Department of Energy (DOE) and the National Science Foundation and a congressional staffer.
During its deliberations, the committee studied long-range plans in nuclear and particle physics relevant to EIC science, not only in the United States but also in the Asian and European communities, and surveyed existing and planned facilities around the world that can address science similarly to an EIC. Accelerator and collider experts from the United States and the international community were consulted. Discussion of design specifications as they related to achieving the scientific goals was explored, but no detailed comparisons were made between the two existing designs.
The committee’s conclusions are organized into a set of nine findings, which it summarizes here.
Finding 1: An EIC can uniquely address three profound questions about nucleons—neutrons and protons—and how they are assembled to form the nuclei of atoms:
- How does the mass of the nucleon arise?
- How does the spin of the nucleon arise?
- What are the emergent properties of dense systems of gluons?
A better understanding of the weak and strong forces—two of four fundamental forces of nature—is central to nuclear physics. The strong force, so named because it holds together neutrons and protons tightly in the nuclei of atoms, is a subtle aspect of a more fundamental force, the color force, described by the well-established theory of QCD. These three questions are at heart of understanding how QCD shapes nuclei and their building blocks, nucleons; answering these questions is necessary to complete our understanding of the chemical elements, the elementary constituents of our physical world. The third question is perhaps the most exciting to nuclear scientists because it offers the opportunity for the most surprises, including new phases of matter and deep insights about quantum field theory.
Finding 2: These three high-priority science questions can be answered by an EIC with highly polarized beams of electrons and ions, with sufficiently high luminosity and sufficient, and variable, center-of-mass energy.
Based on documents the committee reviewed, input from speakers, and committee expertise, the committee concluded that, pending future machine and science studies, Figure 7.1 (cf. Figure 2.4) well summarizes the requirements on an EIC needed to answer the three compelling science questions discussed above. In addition to highly polarized beams, high luminosity—as shown in Figure 7.1—is needed to answer, by means of imaging, the question of how the spin and mass of the nucleon arise; and a high and variable center-of-mass energy, as shown in Figure 7.1 is essential to understanding the nature of gluons in nuclei. As the figure indicates, an EIC would also be useful in studying nuclear structure in terms of quarks and gluons—with the gluon saturation region explored at highest energies.
Finding 3: An EIC would be a unique facility in the world and would maintain U.S. leadership in nuclear physics.
The committee did a comprehensive survey of existing and planned accelerator facilities in both nuclear and particle physics around the world. An EIC, with its high energy and luminosity and highly polarized electron and ion beams, would be unique, from both the accelerator point of view and the science that it can address.
Finding 4: An EIC would maintain U.S. leadership in the accelerator science and technology of colliders and help to maintain scientific leadership more broadly.
The EIC is the only high-energy collider being planned for construction in the United States. Furthermore, its high design luminosity and highly polarized beams would push the frontiers of accelerator science and technology. For these reasons, building the EIC would also maintain U.S. leadership in accelerator collider science. Because of the importance of accelerators, this would broadly benefit the physical sciences.
Finding 5: Taking advantage of existing accelerator infrastructure and accelerator expertise would make development of an EIC cost effective and would potentially reduce risk.
Significant accelerator infrastructure and expertise exists at both the Brookhaven National Laboratory (BNL) and the Thomas Jefferson National Accelerator Facility (JLab). In particular, JLab has just completed the 12 GeV upgrade of its Continuous Electron Beam Accelerator Facility (CEBAF), which employs a polarized electron beam and uses superconducting accelerator technology. The Relativistic Heavy Ion Collider (RHIC) at BNL is able to collide a large variety of heavy ions over a wide range of energies and has pioneered collisions of high-energy polarized protons. Both BNL and JLab have proposed design concepts for an EIC that use existing infrastructure and both laboratories have significant accelerator expertise and experience.
Finding 6: The current accelerator R&D program supported by DOE is crucial to addressing outstanding design challenges.
While well-developed designs for an EIC exist at both BNL and JLab, design challenges remain for each. Neither of the existing designs can fully deliver on the three compelling science questions. The DOE research and development (R&D) investment has been and will continue to be crucial to retiring design risk in a timely fashion.
Finding 7: To realize fully the scientific opportunities an EIC would enable, a theory program will be required to predict and interpret the experimental
results within the context of QCD and, furthermore, to glean the fundamental insights into QCD that an EIC can reveal.
QCD provides the mathematical description of how quarks and gluons assemble nucleons and nuclei, as well as the other hadrons, and a full understanding of how it does so will complete our understanding of the building blocks of our physical world, atoms. In so doing, other insights and surprises about this rich theory are likely to be revealed, some with broad implications in our understanding of the quantum world. In order to take advantage of the full potential of the EIC, a theory program to match its scope is essential, comprising both continuum and lattice QCD.
Finding 8: The U.S. nuclear science community has been thorough and thoughtful in its planning for the future, taking into account both science priorities and budgetary realities. Its 2015 Long Range Plan identifies the construction of a high-luminosity polarized EIC as the highest priority for new facility construction following the completion of the Facility for Rare Isotope Beams (FRIB) at Michigan State University.
The 2015 Long Range Plan for Nuclear Science2 provided a clear and compelling discussion of the scientific scope of the field and a ranked list of priorities for the field. The frontiers of nuclear science encompass understanding the implications of QCD for nucleons and nuclei, nuclear structure and nuclear astrophysics, neutrinos, and insights from nuclear physics into the fundamental symmetries of nature. Because an EIC can answer fundamental questions in the first of these frontiers, the 2015 Long Range Plan made an EIC the highest priority for a new facility after the completion of FRIB. The importance of an EIC to the frontiers of nuclear science was also recognized in the 2007 Long Range Plan,3 where accelerator R&D for an EIC was recommended and has since been supported by DOE.
Finding 9: The broader impacts of building an EIC in the United States are significant in related fields of science, including in particular the accelerator science and technology of colliders and workforce development.
Beyond its impact on nuclear science, an EIC will help to maintain international leadership in accelerator science and technology of colliders. The accelerator-collider expertise in the United States now resides within the DOE Office of Nuclear
2 U.S. Department of Energy and National Science Foundation, Reaching for the Horizon: The 2015 Long Range Plan for Nuclear Science, October 2015, https://science.energy.gov/~/media/np/nsac/pdf/2015LRP/2015_LRPNS_091815.pdf.
3 U.S. Department of Energy and National Science Foundation, The Frontiers of Nuclear Science: A Long Range Plan, December 2007, https://science.energy.gov/~/media/np/nsac/pdf/docs/nuclear_science_low_res.pdf.
Physics, following the closing of the Fermilab Collider facilities and the absence of a plan by DOE/High Energy Physics to construct a new collider. Any future accelerator facilities with high energy or high luminosity will benefit significantly from the expertise developed for an EIC.
An EIC would have impact on other research areas including particle physics, astrophysics, theoretical and computational modeling as well as rich intellectual connections to atomic and condensed matter physics.
Lastly, with the exciting physics frontier program enabled by an EIC, nuclear science will continue to attract outstanding graduate students, more than half of whom will go on to science, technology, engineering, and mathematics jobs in industry and DOE National Nuclear Security Administration and Office of Science laboratories.
The committee concludes that the science questions regarding the building blocks of matter are compelling and that an EIC is essential to answering these questions. Furthermore, the answers to these fundamental questions about the nature of the atoms will also have implications for particle physics and astrophysics and possibly other fields. Because an EIC will require significant advances and innovations in accelerator technologies, the impact of constructing an EIC will affect all accelerator based sciences.
In summary, the committee concludes that an EIC is timely and has the support of the nuclear science community. The science that it will achieve is unique and world leading and will ensure global U.S. leadership in nuclear science as well as in the accelerator science and technology of colliders. The latter, the committee notes, would position the United States for future high-energy collider projects in other fields.