Nuclear Physics with MEIC
Understanding the internal structure of hadrons and nuclei on the basis of the fundamental theory of strong interactions, Quantum Chromodynamics (QCD), is one of the central problems of modern nuclear physics, as explained e.g. in the U.S. Department of Energy Office of Science Nuclear Science Advisory Committee’s 2007 Long Range Plan . It is the key to understanding the dynamical origin of mass in the visible universe and the behavior of matter at astrophysical temperatures and densities. It is an essential element in describing nuclear structure and reactions from first principles, a project with numerous potential applications to science and technology. Theoretical methods to apply QCD to hadronic and nuclear systems have made dramatic advances in the last two decades but rely crucially on new experimental information for further progress. Electron scattering has been established as a powerful tool for exploring the structure of matter at the sub-femtometer level (<1 fm=10-15 m). Historically, such experiments provided the first proof of the extended nature of the proton and revealed the presence of pointlike constituents, or quarks, at smaller scales, revolutionizing our understanding of strong interactions. Subsequent experiments established the validity of QCD and the presence of gluonic degrees of freedom at short distances and measured the basic number densities of quarks and gluons in the nucleon (proton, neutron). While much progress has been made, several key questions remain unanswered : I) What role do non-valence (“sea”) quarks and gluons play in nucleon structure? What are their spatial distributions? How do they respond to polarization? What is their orbital motion, and how does it contribute to the nucleon spin? The answers to these questions will provide essential information on the effective degrees of freedom emerging from QCD at distances of the order of the hadronic size (~1 fm). II) What are the properties of the fundamental QCD color fields in nuclei with nucleon number A > 1? What are the nuclear gluon and sea quark densities? To what extent are they modified by nuclear binding, quantum-mechanical interference, and other collective effects? These questions are the key to understanding the QCD origins of the nucleon-nucleon interaction at different energies, the role of non-nucleonic degrees of freedom, and the approach to a new regime of high gluon densities and saturation at high energies. III) How do colorless hadrons emerge from the colored quarks and gluons of QCD? What dynamics governs color neutralization and hadron formation? By what mechanisms does the color charge of QCD interact with nuclear matter? We are still far from understanding the fundamental processes by which high-energy radiation converts into hadronic matter. It is now widely accepted that a polarized ep/eA collider (Electron-Ion Collider, or EIC) with a variable ep center-of-mass (CM) energy in the range √s=20–70 GeV, and a luminosity of ~1034 cm-2s-1 over most of this range, would offer a unique opportunity to address these questions . Such a facility would provide the necessary combination of kinematic reach 11 (momentum transfer viz. spatial resolution, energy span), luminosity (precision, multi- dimensional binning, rare processes), and detection capabilities (resolution, particle identification) to study nucleon and nuclear structure through scattering experiments with a variety of final states. It would represent the natural next step after the high-luminosity fixed- target ep/eA experiments (SLAC , JLab 6 and 12 GeV [4,5]) and the high-energy HERA ep collider [6,7]. It would be the first ever high-energy electron-nucleus collider and open up qualitatively new possibilities to study QCD in the nuclear environment. Finally, polarized beams would allow one to investigate proton and neutron spin structure with unprecedented accuracy and kinematic reach; such measurements were so far possible only in fixed-target experiments (EMC, SMC, SLAC, HERMES, COMPASS, JLab; for a review see Refs. [8,9,10]) or polarized pp collisions at RHIC . In this chapter we briefly review what measurements with such a medium-energy EIC (or MEIC) could contribute to answering the above questions .
The present MEIC baseline is a traditional ring-ring collider [1,2,3] in which the colliding electron and ion beams are stored in two collider rings. This choice, which has evolved from an ERL-ring collider of an earlier design stage , was adopted in 2007  after it was realized that an electron-ERL/ion-ring design could not significantly improve luminosities beyond a ring-ring collider that utilized a high bunch repetition rate. Moreover, an ERL-ring collider scenario would add a substantial burden on the polarized electron source, and many-pass energy recovery is not well established. A brief comparison of two collider scenarios—ring-ring and ERL-ring—for the MEIC design is presented in section 3.7. The central part of the proposed facility is a set of figure-8 shape electron and ion storage rings as shown in Figure 3.1. The electron ring is made of normal conducting magnets and will store an electron beam of 3 to 11 GeV. The CEBAF SRF linac  serves as a full-energy injector into the electron collider ring, requiring no further upgrade for energy, beam current, or polarization beyond the 12 GeV upgrade. The ion collider ring is made of high-field superconducting magnets and will store a beam with energy of 20 to 100 GeV for protons or up to 40 GeV per nucleon for light to heavy ions. The ion beams are generated and accelerated in a new ion injector complex that will be described below. The two collider rings are stacked vertically and housed in the same underground tunnel as shown in Figure 3.2. They have nearly identical circumferences of approximately 1.4 km, occupying a compact footprint of 500 m by 170 m, which is actually smaller than that of CEBAF, as shown in a Jefferson Lab site map in Figure 3.3. In addition, there is depicted in the figures a large figure-8 ring (in light grey color in Figure 3.1 and dashed red line in Figure 3.3) which represents two high energy collider rings (2.5 km or larger) for a future energy upgrade for reaching up to 20 GeV electrons, and up to 250 GeV protons or 100 GeV/u ions. The upgraded high-energy collider can use the same experimental halls and, possibly, the detectors of MEIC, and the medium-energy ion collider ring would then serve as the final booster in staged acceleration of ion beams.