The National Research Council 2010 decadal survey of nuclear physics1 describes this very broad field as follows:
Nuclear physics today is a diverse field, encompassing research that spans dimensions from a tiny fraction of the nucleon volume to the enormous scales of astrophysical objects. Its research objectives include the desire not only to better understand the nature of the forces and masses that interact at the nuclear level, but also to describe the primordial matter that existed at the Big Bang, where those nuclear forces dominated interactions, as well as the nature of neutrinos and the liquid state of quarks and gluons that can now be produced in the most advanced colliding-beam accelerators.
The impact of nuclear physics extends well beyond furthering a body of scientific knowledge. Tools developed by nuclear physicists often have application to other sciences: medicine, computational science, and material research, among others. Its discoveries impact astrophysics, particle physics and cosmology. Finally, many of today’s major societal problems—energy, climate, national security, and nonproliferation—are addressed with tools, instruments, and techniques obtained from nuclear physics.
This chapter places an electron-ion collider (EIC) in the context of nuclear science, within the United States in particular, and describes the role of an EIC in maintaining U.S. leadership within the global nuclear science community.
1 National Research Council, 2012, Nuclear Physics: Exploring the Heart of Matter (NP2010), The National Academies Press, Washington, D.C.
A central goal of modern nuclear physics is to understand the structure of the proton and the neutron directly from the dynamics of quarks and gluons governed by quantum chromodynamics (QCD) and how nuclear interactions between protons and neutrons emerge from these dynamics. Remarkable advances have been made to date. For example, the interaction of protons and neutrons can be described with an effective field theory using the symmetries of QCD in conjunction with input from experimental measurements. Combined with modern many-body methods, the effective field theory treatment of nuclear forces is the basis of ab initio structure calculations of atomic nuclei. Low-energy properties of protons and neutrons, such as their masses and the strength of their weak interactions, can now be extracted directly from QCD using numerical simulations of lattice QCD (LQCD) theory, discussed in Chapter 6. Advances in accelerator science and technology have made it possible to illuminate the proton and neutron with beams of high-energy electrons. When probed at high energies, the proton and neutron reveal a substructure of quarks, antiquarks, and numerous gluons. High-energy collisions of heavy nuclei have made it possible experimentally to explore the transformation from hadronic matter to quark and gluon matter at densities several times the normal nuclear density or temperatures in excess of 2 trillion degrees Kelvin. Such a quark-gluon plasma is thought to have been the dominant form of matter in the universe shortly after the Big Bang. QCD studies of proton and neutron structure as well as quark-gluon plasma constitute essential pillars of fundamental nuclear physics in the United States, alongside studies of the extremes of nuclear structure, neutrino physics, and fundamental symmetries in nature.
As described in greater detail in Chapter 5, the quark-gluon structure of nucleons and nuclei is being studied using electron scattering at the Thomas Jefferson National Accelerator Facility (JLab), and using polarized proton collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory (BNL) in the United States. At JLab, the recent upgrade has increased the electron beam energy to 12 GeV. A major focus of the program is to image the valence quark distributions in protons and nuclei. This effort engages many of the questions described in Chapter 2, but with a focus on the valence quark sector of the target nucleon or nucleus. The energy upgrade of the Continuous Electron Beam Accelerator Facility (CEBAF) accelerator at JLab enables a program that includes measurements of real photon and meson production as well as semi-inclusive deep inelastic scattering (SIDIS). These experiments will lead to studies of generalized parton distributions, tomographic images of the quark distribution in the proton, and transverse momentum distributions. Spin-polarized electron scattering on polarized protons is an important element of the JLab experimental program,
complementing polarized proton-proton scattering experiments at RHIC; the latter have provided evidence for positive gluon polarization in the proton.
A different regime of nuclear physics is explored in heavy ion collisions at RHIC in the United States and the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN). These machines probe the properties of the hot quark-gluon plasma, similar to that which existed shortly after the Big Bang until the universe cooled to the point that neutrons and protons were formed. Experiments at RHIC and LHC have revealed that contrary to expectations, the quark-gluon plasma behaves as a nearly perfect fluid—that is, with extremely low viscosity. Understanding the properties and characteristics of the hot, dense QCD matter formed in high-energy heavy ion collisions is currently a major goal of nuclear physics.
An EIC would provide an important bridge between the existing JLab and RHIC programs, and it would connect with the LHC heavy ion program as well. It would deepen the understanding of the QCD structure of nucleons and nuclei by focusing on the crucial role of gluons in generating the mass and spin of the proton, it would determine the distribution of gluons in nuclei with unprecedented accuracy, and it would study the highly occupied state of gluons that is expected to be the initial state for the formation of a quark-gluon plasma. A dedicated theory program, involving both continuum and lattice QCD, would be required to predict and interpret the results. Combined, theory and experiment would lead to first-rate insights into how the observed world emerges from the basic laws of QCD.
In addition, the 12 GeV JLab program will investigate the spectroscopy of exotic hadrons—QCD bound states that cannot be interpreted as simple three-quark or quark-antiquark states. QCD allows for new kinds of exotic hadrons, the first of which has recently been discovered at the LHC. “Glueballs” are states that have thus far not been observed and are, to a good approximation, composed of only gluons. If they exist, these states could probe the unique nature of the gluon as a force carrier that can interact with itself. Another example, the so-called hybrid mesons are hypothesized quark-antiquark states that have nontrivial gluonic components—that is, gluon admixtures that modify the quantum numbers of the quark-antiquark pair. Study of such configurations of gluons could foreshadow interesting states of gluons that may be possible to study at an EIC.
Nuclear physics also includes high-priority programs in neutrino physics and fundamental symmetries. Neutrinos are messengers from hot and dense environments like the solar interior, type II supernova explosions, and cooling neutron stars. In a supernova explosion, most of the energy is carried in neutrinos, and neutrino scattering is an integral part of the dynamics of a supernova explosion. Neutrinos also provide an important window into fundamental symmetries and possible extensions of the Standard Model of particle physics. One central question is whether the neutrino is its own antiparticle, which would imply that neutrinos
would violate lepton number conservation. Evidence for lepton number violation is being sought in neutrinoless double beta decay experiments, and nuclear physicists are actively working toward a ton-scale detector of such processes. Electron accelerators have also made important contributions to the study of fundamental symmetries. JLab studies parity-violating electron scattering, and a series of past and planned experiments, Q-WEAK, Measurement of Lepton Lepton Elastic Reactions, and Solenoidal Large Intensity Device, study the evolution of the fundamental electroweak coupling, and search for physics beyond the Standard Model. An EIC would naturally extend this program, studying fundamental symmetries at higher energies.
Complementary to research efforts in QCD, neutrinos, and fundamental symmetries, understanding the extremes of nuclear structure is currently a major focus of study worldwide, and world-leading new capabilities will be made available at the Facility for Rare Isotope Beams (FRIB) at Michigan State University. This facility will explore nuclei at the limits of stability in terms of the number of protons and neutrons that can be added to the known isotopes. Very neutron rich nuclei are important for the formation of heavy elements in the universe and inform an understanding of the matter that is expected to exist in the outer layers of neutron stars. In the interior of neutron stars, gravity compresses nuclear matter to densities beyond those that occur in nuclei. In this regime, one expects a change in degrees of freedom with increasing density from nucleonic to deconfined quark matter in the interior, with possible Bose condensates of mesons playing a role.
Nuclear physics in the United States is a substantial field of physical science. The Division of Nuclear Physics of the American Physical Society (APS) has a membership in excess of 2,500, which accounts for over 5 percent of the total APS membership across all physical sciences. In the United States, about 90 universities produce about 115 Ph.D.s per year2 (see Chapter 6 for the societal impact of U.S. nuclear physics Ph.D. production) in the four experimental research focus areas of hadronic physics, heavy ion physics, nuclear structure and astrophysics, and fundamental symmetries and neutrinos. With CEBAF at JLab and RHIC at BNL offering unique, world-class facilities in hadronic physics and heavy ion physics, nuclear physics in the United States over the past several decades has provided strong scientific leadership internationally. FRIB, under construction at Michigan State University, will enhance U.S. leadership in nuclear structure physics for the
2Implementing the 2007 Long Range Plan, Report to NSAC by the Subcommittee, R. Tribble, chair, January 31, 2013.
next several decades. An EIC is vital as a next-generation facility beyond CEBAF and RHIC to maintain U.S. leadership in QCD.
U.S. strength in nuclear physics lies not only in its experimental programs enabled by world-class facilities but also in the caliber of its nuclear theory research. For example, the theoretical prediction of the necessary conditions to observe the quark-gluon plasma was essential to its discovery at RHIC. The gluon momentum and polarization distributions in the proton cannot be extracted from lepton scattering data without the QCD theoretical framework. Nuclear theory has played an essential role in developing and defining the EIC science program, described in Chapter 2. For example, the concepts of the color glass condensate and generalized parton distributions were respectively invented and co-invented by U.S. nuclear theorists. These now have become part of the universal language to describe high-energy lepton scattering from hadrons and are essential to defining the most important experiments at an EIC.
The U.S. QCD community, from both experimental and theoretical perspectives, has carefully considered the scientific opportunities that would be made possible with new facilities in a series of meetings and discussions that have extended over almost two decades.3,4,5,6,7,8,9 This culminated in the Nuclear Science Advisory Committee (NSAC) 2015 Long Range Plan for U.S. Nuclear Science,10 where a high-luminosity, polarized EIC was recommended as the top priority for new construction after the completion of FRIB. An EIC is universally accepted as the essential next-generation facility to explore the high-energy structure of nuclei, to image for the first time the gluons and sea quarks in hadronic matter and to complete the understanding of nuclear matter in terms of the fundamental quarks and gluons of QCD. The realization of an EIC would unify the U.S. QCD community, which at present is two distinct research communities studying hadronic physics and heavy ion physics. The U.S. QCD community amounts to about half of the field of nuclear physics in the United States. About 80 percent of U.S. uni-
3EPIC ‘99 Workshop, April 8-11, 1999, IUCF, Bloomington, Ind.
4Physics with an Electron Polarized Light-Ion Collider, MIT, September 14-15, 2000, AIP Conference Proceedings No. 588, Ed. R. Milner.
5Opportunities in Nuclear Science, 2002 DOE/NSF Long Range Plan for U.S. Nuclear Science.
6 A. Deshpande, R. Milner, R. Venugopalan, and W. Vogelsang, 2005, Ann. Rev. Nucl. and Part. Sc. 55:165.
7The Frontiers of Nuclear Science, 2007 DOE/NSF Long Range Plan for U.S. Nuclear Science.
8 D. Boer, M. Diehl, R. Milner, R. Venugopalan, and W. Vogelsang, 2011, Gluons and the Quark Sea at High Energies: Distributions, Polarizations, Tomography, Report on the Joint BNL/INT/JLab Program on the Science Case for an Electron-Ion Collider, September 13 to November 19, 2010, Institute of Nuclear Theory, University of Washington, Seattle.
9 A. Accardi et al., 2016, Electron-ion collider: The next QCD frontier, Eur. Phys. J. A 52:238.
10Reaching for the Horizon, 2015 DOE/NSF Long Range Plan for U.S. Nuclear Science.
versities in nuclear physics produce Ph.D.s in these areas.11 The realization of an EIC is absolutely crucial to maintaining the health of the field of nuclear physics in the United States.
A 2004 report12 on a study of education in nuclear science recommended that “the nuclear science community work to increase the number of new Ph.D.’s in nuclear science by approximately 20 percent over the next five to ten years.” In
11Assessment of Workforce Development Needs in the Office of Nuclear Physics Research Disciplines, Report to NSAC from the Subcommittee on Workforce Development, J. Cizewski (Chair), July 18, 2014.
12Education in Nuclear Science, Report to NSAC from the Subcommittee on Education, J. Cerny (Chair), November 2004.
part, this recommendation was motivated by the national security need for technical expertise in the areas of stockpile stewardship and nonproliferation. However, the annual number of new Ph.D.s in nuclear physics in the United States has been approximately constant in time since then. The most recent assessment,13 in 2014, reports that there is a substantial increase in the percentage of Nuclear Physics Early Career Awards to individuals who received their Ph.D.s outside the United States. In addition, an increasingly large fraction of nuclear science faculty members
13Assessment of Workforce Development Needs in the Office of Nuclear Physics Research Disciplines, Report to NSAC from the Subcommittee on Workforce Development, J. Cizewski (Chair), July 18, 2014.
has received Ph.D.s from non-U.S. institutions. Furthermore, the 2014 assessment specifically identifies workforce challenges in the areas of accelerator science and high-performance computing. See Chapter 6 for further discussion of the nuclear physics workforce.
U.S. nuclear physics user facilities are strong attractors to the global nuclear science community because of their unique and powerful capabilities. A significant fraction of the international QCD community is currently performing research in the United States at JLab and RHIC, with 36 percent of JLab users and 42 percent of RHIC users from institutions outside the United States. Chapter 5 describes international facilities where QCD research is performed, including the heavy ion program at the LHC at CERN, the Common Muon and Proton Apparatus for Structure and Spectroscopy (COMPASS) hadronic physics experiment also at CERN, as well as the future Facility for Antiproton and Ion Research (FAIR) in Europe and the Nuclotron-Based Ion Collider Facility (NICA) in Russia. FAIR and NICA will study hadronic collisions at lower energies than the range currently available at RHIC or planned for an EIC. Although EIC concepts at the High Intensity Heavy-Ion Accelerator Facility (HIAF) in China and at CERN in Europe have been discussed, there are currently no plans to build a machine outside the United States. The existing infrastructure at CERN, and elsewhere, could not be easily adapted for the construction of a polarized EIC, and resources are committed to other projects and facilities with different physics goals. These aspects will be discussed in detail in Chapter 5.
There is in fact already well-defined international interest in a U.S.-based EIC. Following the 2015 long-range planning exercise, the EIC Users Group was formed. Currently, 39 percent of the EIC User Group members are from institutions outside the United States, with total User Group membership presently consisting of more than 700 Ph.D. scientists (see Box 3.1). Multiple international groups are already participating in the EIC Generic Detector R&D program. Furthermore, in Europe, the Nuclear Physics European Collaboration Committee (NuPECC) of the European Science Foundation recently published its 2017 Long Range Plan,14 which expresses explicit interest in a U.S.-based EIC: “NuPECC highly recognizes the science of the EIC project, presently under study, representing an opportunity for a major step forward in the field of hadron physics.”
14 European Science Foundation, 2017 Perspectives in Nuclear Physics, NuPeCC 2017 Long Range Plan, http://www.nupecc.org/lrp2016/Documents/lrp2017.pdf.