Summary

The Committee on U.S.-Based Electron-Ion Collider Science Assessment was asked by the Department of Energy (DOE), in its statement of task, to assess the scientific justification for a U.S. domestic electron-ion collider (EIC) facility and to evaluate the importance and urgency of the science that an EIC would address to both nuclear science and the physical sciences more broadly. The committee’s task also included assessing the role of an EIC in 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.

In summary, the committee finds a compelling scientific case for such a facility. The science questions that an EIC will answer are central to completing an understanding of atoms as well as being integral to the agenda of nuclear physics today. In addition, the development of an EIC would advance accelerator science and technology in nuclear science; it would also benefit other fields of accelerator-based science and society, from medicine through materials science to elementary particle physics.

Understanding of protons and neutrons, or “nucleons”—the building blocks of atomic nuclei—has advanced dramatically, both theoretically and experimentally, in the past half century. It is known that nucleons are made of fractionally charged “valence” quarks, as well as dynamically produced quark-antiquark pairs, all bound together by gluons, the carrier of the strong force. A central goal of modern nuclear physics is to understand the structure of the proton and neutron directly from the dynamics of their quarks and gluons governed by the theory of their interac-

tions, quantum chromodynamics (QCD), and how nuclear interactions between protons and neutrons emerge from these dynamics. With deeper understanding of the quark-gluon structure of matter, scientists are poised to reach a deeper picture of these building blocks, and atomic nuclei themselves, as collective many-body systems with new emergent behavior. Viewing nucleons and nuclei as complex interacting many-body systems gives rise to profound questions about the nature of ordinary matter.

Three central scientific issues that would be addressed by an EIC are as follows. The first is to understand in detail the mechanisms by which the mass of nucleons, and thus the mass of all the visible matter in the universe, is generated. The problem is that while gluons have no mass, and quarks are nearly massless, the nucleons that contain them are heavy; the total mass of a nucleon is some 100 times greater than the mass of the valence quarks it contains. The second is to understand the origin of the internal angular momentum or spin of nucleons, a fundamental property that underlies many practical applications, including magnetic resonance imaging (MRI). How the angular momentum, both intrinsic as well as orbital, of the internal quarks and gluons gives rise to the known nucleon spin is not understood. And third, the nature of gluons in matter—that is, their arrangements or states, and the details of how they hold matter together—is not well known. Gluons in matter are somewhat like dark matter in the universe—unseen but playing a crucial role. An EIC would potentially reveal new states resulting from the close packing of many gluons within nucleons and nuclei. These issues are fundamental to an understanding of the matter in the universe.

To pursue these questions requires peering into nucleons and nuclei with very-high-energy electrons, which would necessitate using the most powerful (in terms of its unique combination of resolving power and intensity) electron microscope ever to be built. The high energy is required to achieve the needed resolution, and the only practical way of reaching the needed energies is to collide counter-rotating beams of electrons with protons or atomic nuclei (ions). To carry out the scientific investigations, such a machine must be capable of colliding a beam of “polarized” electrons (all spinning in the same direction) of energies from 4 GeV up to possibly 20 GeV with a beam of polarized ions of energies from 30 GeV up to some 300 GeV at high “luminosity”—the measure of the rate at which collisions occur. In addition to achieving larger energy collisions of electrons and nucleons than would be attainable with a fixed target accelerator, a collider allows the center of mass of the target and projectile system to be tuned to be approximately at rest in the laboratory, allowing ready analysis of scattering events.

The immediate science that an EIC would enable is manyfold. It would permit “tomography” of nucleons and nuclei, in which one builds together many high-resolution, lower-dimensional slices, like in an MRI, to arrive at a composite multidimensional picture of their quark and gluon components. It would also be

a laboratory for studying QCD—the theory of quarks and gluons producing the strong forces holding matter together—with unprecedented depth, opening the study of the collective behavior of quarks and especially gluons. The situation is analogous to going from a knowledge of the Coulomb force between electric charges to seeing the complex phenomena that the force can produce, from superconductivity to weather. Understanding the collective physics of gluons offers the opportunity for the most surprises, including new phases of matter and deep insights about quantum field theory. Furthermore, the increased understanding of nucleons, nuclei, and QCD itself that an EIC would bring would have direct impact in particle physics, basic energy sciences, plasma physics, and astrophysics, as well as revealing connections to the study of materials and other fields of science.

The committee also finds that an EIC would be much more capable and much more challenging to build than earlier electron or polarized proton machines. The accelerator challenges are twofold: a high degree of polarization for both beams and high luminosity. It would be the most sophisticated and challenging accelerator currently proposed for construction in the United States and would significantly advance accelerator science and technology here and around the world. The committee’s study resulted in a set of nine findings, which are summarized here.

Hearing from experts on the science that an EIC would be able to carry out, the committee finds that

Consideration of the accelerator requirements to answer these questions leads to the second finding.

As a result of the comprehensive survey the committee made of existing and planned accelerator facilities in both nuclear and particle physics around the world, it finds that

An EIC would be the only high-energy collider planned for construction in the United States. Its high design luminosity and highly polarized beams would push the frontiers of accelerator science and technology. For these reasons, the committee finds that

The committee looked carefully at the requirements for building an EIC, and at the proposed design concepts for an EIC that uses existing infrastructure, accelerator expertise, and experience at both Brookhaven National Laboratory (BNL) and the Thomas Jefferson National Accelerator Facility (often referred to as the Jefferson Laboratory, or JLab), and finds that

Given the design challenges that remain, neither existing design can fully deliver on the three driving 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, and thus the committee finds that

The scientific challenges that would unfold with EIC require a robust theory program, not simply to design and interpret experiments, but also to develop the broad implications in an understanding of the quantum world, both through analytic theory as well as through lattice QCD simulations on large-scale computers. Thus, the committee finds that

The conclusion that the scientific advances made possible by an EIC would be profound culminates many years of study of the issues by the U.S. nuclear community. Accelerator R&D for an EIC was recommended in the Nuclear Science Advisory Committee’s 2007 Long Range Plan,¹ which continues to be supported by

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¹The Frontiers of Nuclear Science, 2007 DOE/NSF Long Range Plan for U.S. Nuclear Science.

the DOE. More recently, the 2015 Long Range Plan for Nuclear Science² provided a clear and authoritative discussion of the scientific scope of the field and a ranked list of priorities for the field. Thus, the committee finds that

Beyond its impact on nuclear science, an EIC will help to maintain international leadership in the accelerator science and technology of colliders. The accelerator-collider expertise in the United States now resides within the Office of Nuclear Physics at DOE. Future accelerator facilities with high energy or high luminosity will benefit significantly from the expertise developed for an EIC, and so the committee finds that

An EIC would have impact on other research areas, including particle physics, astrophysics, and theoretical and computational modeling, as well as rich intellectual connections to atomic and condensed matter physics. Enabled by an EIC, nuclear science would 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.

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 accelerator science and the technology of colliders.

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²Reaching for the Horizon, 2015 DOE/NSF Long Range Plan for U.S. Nuclear Science.