ForewordThe study of the fundamental structure of nuclear matter is a central thrust of physics research in the United States. As indicated in Frontiers of Nuclear Science, the 2007 Nuclear Science Advisory Committee long range plan, consideration of a future Electron-Ion Collider (EIC) is a priority and will likely be a significant focus of discussion at the next long range plan. We are therefore pleased to have supported the ten week program in fall 2010 at the Institute of Nuclear Theory which examined at length the science case for the EIC. This program was a major effort; it attracted the maximum allowable attendance over ten weeks.This report summarizes the current understanding of the physics and articulates important open questions that can be addressed by an EIC. It converges towards a set of "golden" experiments that illustrate both the science reach and the technical demands on such a facility, and thereby establishes a firm ground from which to launch the next phase in preparation for the upcoming long range plan discussions. We thank all the participants in this productive program. In particular, we would like to acknowledge the leadership and dedication of the five co-organizers of the program who are also the co-editors of this report.David Kaplan, Director, National Institute for Nuclear Theory Hugh Montgomery, Director, Thomas Jefferson National Accelerator Facility Steven Vigdor, Associate Lab Director, Brookhaven National Laboratory iii Preface This volume is based on a ten-week program on "Gluons and the quark sea at high energies", which took place at the Institute for Nuclear Theory (INT) in Seattle from September 13 to November 19, 2010. The principal aim of the program was to develop and sharpen the science case for an Electron-Ion Collider (EIC), a facility that will be able to collide electrons and positrons with polarized protons and with light to heavy nuclei at high energies, offering unprecedented possibilities for in-depth studies of quantum chromodynamics. Guiding questions were• What are the crucial science issues?• How do they fit within the overall goals for nuclear physics?• Why can't they be addressed adequately at existing facilities?• Will they still be interesting in the 2020's, when a suitable facility might be realized?The program started with a five-day workshop on "Perturbative and Non-Perturbative Aspects of QCD at Collider Energies", which was followed by eight weeks of regular program and a concluding four-day workshop on "The Science Case for an EIC".More than 120 theorists and experimentalists took part in the program over ten weeks. It was only possible to smoothly accommodate such a large number of participants because of the extraordinary efforts of the INT staff, to whom we extend our warm thanks and appreciation. We thank the INT Director, David Kaplan, for his strong support of the program and for covering a significant portion of the costs for printing this volume. We gratefully acknowledge additional financial support provided by BNL and JLab.The program w...

The cross section for inclusive electron scattering by nuclear matter is calculated at high momentum transfers using a microscopic spectral function, and compared with that extrapolated from data on laboratory nuclei. It is found that the cross section obtained with the plane-wave impulse approximation is close to the observed data at large values of the energy loss, but too small at low values. In this regime final-state interactions are important; after including their effects theory and data are in fair agreement.It is necessary to treat nucleon-nucleon correlations consistently in estimating the final-state interactions. The effects of possible time dependence of the nucleon-nucleon cross section, giving rise to nuclear transparency, are also investigated. The y scaling of the response function is discussed to further elucidate the role of final-state interactions. the response, due to the momentum distribution in the initial state, is proportional to~q~, as it is in the case of strongly interacting quantum liquids. If the width of the folding function is finite, then it can be argued that, at large enough values of~q~, FSI can be neglected, and, as a consequence, the response will exhibit y scaling. In con-2328 1991 The American Physical Society SCATTERING OF GeV ELECTRONS BY NUCLEAR MATTER 2329 trast, in the case of the nuclear medium at high q, one has to use relativistic kinematics and therefore the width of the response due to the momentum distribution of particles in the initial state is roughly constant -k~. It then follows that FSI eA'ects can be neglected only if the folding width goes to zero at large q. The folding width is of the order of the imaginary part of the optical potential which is -60 MeV -kF/4 for several hundreds MeV nucleons. Therefore, FSI are not obviously negligible in scattering of multi-GeV electrons by nuclei.Ideally, one should start from a realistic relativistically covariant theory of nuclei; however, such a theory is not yet practicable due to difFiculties in treating pionexchange interactions.In the plane-wave impulse ap-

The null-plane pion wave function is used to compute the structure function for deep-inelastic unpolarized-lepton scattering. The old problems with such a phenomenology are that the computed structure fu~~ctions are almost independent of the Bjorken x variable, and that it is difficult to simultaneously reproduce the observed charge radius and pion decay constant. These are avoided by using constituent quarks with structure.

The fundamental theory of the strong interaction-quantum chromodynamics (QCD)—provides the foundational framework with which to describe and understand the key properties of atomic nuclei. A deep understanding of the explicit role of quarks and gluons in nuclei remains elusive however, as these effects have thus far been well-disguised by confinement effects in QCD which are encapsulated by a successful description in terms of effective hadronic degrees of freedom. The observation of the EMC effect has provided an enduring indication for explicit QCD effects in nuclei, and points to the medium modification of the bound protons and neutrons in the nuclear medium. Understanding the EMC effect is a major challenge for modern nuclear physics, and several key questions remain, such as understanding its flavor, spin, and momentum dependence. This manuscript provides a contemporary snapshot of our understanding of the role of QCD in nuclei and outlines possible pathways in experiment and theory that will help deepen our understanding of nuclei in the context of QCD.

We show that nuclear σ, ω, and π mesons can contribute coherently to enhance the electroproduction cross section on nuclei for longitudinal virtual photons at low Q 2 while depleting the cross section for transverse photons. We are able to describe recent HERMES inelastic lepton-nucleus scattering data at low Q 2 and small x using photon-meson and meson-nucleus couplings which are consistent with (but not determined by) existing constraints from meson decay widths, nuclear structure, deep inelastic scattering, and lepton pair production data. We find that while nuclear-coherent pion currents are not important for the present data, they could be observed at different kinematics. Our model for coherent meson electroproduction requires the assumption of mesonic currents and couplings which can be verified in separate experiments. The observation of nuclear-coherent mesons in the final state would verify our theory and allow the identification of a specific dynamical mechanism for higher-twist processes.