Understanding of protons and neutrons, or "nucleons"â€"the building blocks of atomic nucleiâ€"has advanced dramatically, both theoretically and experimentally, in the past half century. 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 interactions, 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. The development of a U.S. domestic electron-ion collider (EIC) facility has the potential to answer questions that are central to completing an understanding of atoms and integral to the agenda of nuclear physics today. This study assesses the merits and significance of the science that could be addressed by an EIC, and its importance to nuclear physics in particular and to the physical sciences in general. It evaluates the significance of the science that would be enabled by the construction of an EIC, its benefits to U.S. leadership in nuclear physics, and the benefits to other fields of science of a U.S.-based EIC.

Understanding the behavior of large atomic nuclei (heavy ions) in high-energy collisions has been the focus of a concerted research effort over the past 10-15 years. Much of the latest progress in the field has centered around transverse momentum-dependent (or "unintegrated") parton distributions: in particular the prediction of the high-energy behavior of these distributions, in the form of the Balitsky-JIMWLK equations, and the development of the hybrid factorization framework, which connects the unintegrated parton distributions to predictions for experimentally measured cross sections. With the advent of high-energy proton-nucleus collisions at RHIC and the LHC, we are able to experimentally test these predictions for the first time. In this dissertation, I show two case studies of these predictions, to illustrate the use of the hybrid factorization at leading and next-to-leading order.First, as a simple example, I analyze the azimuthal angular correlation for a Drell-Yan process, the production of a lepton pair with an associated hadron. The correlation for back-to-back emission turns out to be determined by the low-momentum region of the unintegrated gluon distribution, and the correlation for parallel emission is determined by the high-momentum region. Accordingly, a proper prediction of the correlation at all angles requires a gluon distribution with physically realistic behavior at both high and low momenta. Furthermore, the properties of the central double peak that emerges in Drell-Yan production can provide some insight into the form of the gluon distribution.I'll then describe a numerical calculation of the cross section for inclusive hadron production, which incorporates all corrections up to next-to-leading order in the strong coupling. This calculation illustrates several obstacles presented by subleading terms, including the removal of divergences by renormalizing the integrated and unintegrated parton distributions. The results of the calculation are negative at high transverse momentum, which is surprising but may be mathematically reasonable, since the perturbative approximation to the cross section may break down under those kinematic conditions. However, it may be possible to make meaningful predictions for the nuclear modification ratio R_pA despite the negative cross section.Moving beyond next-to-leading order, it may be possible to cure the negativity of the inclusive hadron cross section by altering the formulas used. I'll show two possible methods of doing so: first, a straightforward resummation of selected higher-order terms corresponding to gluon loop diagrams is able to mitigate the negativity, though it requires some alterations of unclear theoretical origin. A more promising alternative seems to be use of exact kinematic definitions, incorporating terms which disappear in the infinite-energy limit; this constrains the kinematics to eliminate the region of phase space which most strongly contributes to the negativity. In this way, the calculation can be adapted to produce reasonable results at high transverse momentum.

A sample of proton-proton collisions at .sqrt. s = 63 GeV with a (trigger) charged particle of transverse momentum in the range 4 to 12 GeV/c and vertical bar rapidity vertical bar 1 is studied. The events were obtained with the Axial Field Spectrometer at the CERN ISR (experiment R807). Preliminary results are reported from an investigation of the distribution of the transverse momentum vectors of the observed charged particles in the central rapidity region associated with the trigger particle. It is found that the pattern of transverse momentum vectors of the observed charged particles in the central rapidity region associated with the trigger particle. It is found that the pattern of transverse momentum vectors of the charged particles observed in a given event is strongly dependent on the amount of transverse energy which is carried by the associated charged particles observed in the event. When the transverse energy is large ( 10 GeV/c within vertical bar rapidity vertical bar

We present results on the production of jets and "jetlike" clusters in 800-GeV/c proton-nucleus (pA) collisions. Events with high values of transverse energy in the central kinematic region were selected for nuclear targets of H, Be, C, Cu, and Pb. A jet-finding algorithm was used in analyzing the data. The A dependence of the jet and dijet cross sections was parametrized as A?. The values of? for events with "jetlike" cluster pairs found by the algorithm without any additional kinematic cuts reach a plateau of approximately 1.5 at dijet transverse energies> GeV. The collimation of observed "jetlike" clusters decreases with A, and the fragmentation is softer for heavier target nuclei. However, nuclear effects become less pronounced with the increasing cluster or cluster-pair transverse energy. We argue that the observed nuclear enhancement for the production of "jetlike" clusters is due to underlying event or/and soft-scattering contributions to the heavy-nuclei data. We show that the nuclear enhancement becomes consistent with a value of? within 0.10 from unity once the data are corrected for the underlying event or kinematic cuts enhancing clear jet structure are applied.