Session D8: Mini-Symposium on Physics with an Electron-Nucleus Collider

Nuclear Physics with an Electron-Ion Collider

An overview is given of the science program with an Electron--Ion Collider (EIC) with CM energies in the range $10^3$--$10^4 \, {\rm GeV}^2$, as envisaged by the U.S.\ and international nuclear physics community and endorsed in the 2007 NSAC Long--Range Plan. It includes precision studies of nucleon structure in QCD (gluon spin, quark flavor decomposition, gluon and sea quark spatial distributions or GPDs, parton orbital motion or TMDs), the fundamental quark/gluon structure of nuclei (nuclear gluons and EMC effect, nuclear quark/gluon radii from coherent scattering, hadronization in the nuclear medium), and the physics of high gluon densities at small x (saturation, Color Glass Condensate). Particular emphasis will be put on demonstrating the unique potential of an EIC for addressing these objectives, and discussing the proposed measurements in the context of the programs at other exisiting and planned facilities (JLab 12 GeV Upgrade, RHIC Spin, CERN COMPASS, LHC). A brief summary of the EIC machine concepts proposed by BNL and JLab and their basic parameters (energies, luminosity) will be presented, as well as the status and directions of the RD effort.   

The Electron-Ion Collider at BNL: Capabilities and Physics Highlights

Nuclei probed in DIS and dffractive processes in the high-energy (low-$x$) regime open a new precision window into fundamental questions in QCD. The proposed Electron-Ion Collider at BNL (eRHIC) is a new high-energy and high-luminosity electron-ion/proton machine. The proposed design provides unprecedented access to study deeply the nature of QCD matter and strong color fields. It will allow us to explore gluon saturation, one of the outstanding fundamental problems in QCD, and test the validity of the Color Glass Condensate approach. We will outline the compelling physics case for e+A collisions at eRHIC, and discuss briefly the status of machine and detector design concepts.   

Detector Studies for an Electron Ion Collider

A proposal has been made to build an Electron-Ion Collider (EIC) in the US in the next decade, capable of running at multiple electron and ion energies and capable of ion species up to Uranium. One of the important aspects of this programme is the ability to have a detector which is suitable for all energies and species. It must also be able to perform a number of difficult measurements, notably including very forward lepton and ion measurements. In this presentation I will outline the MC studies performed with the aim of designing a detector suitable for use at an EIC.   

The nucleon structure, what an Electron-Ion Collider will teach us

The question after the individual parton (quarks and gluons) contributions to the spin of the nucleon is even after 20 years of experimental efforts not yet solved. After several precise measurements in polarized DIS it is clear, that the spin of the nucleon cannot be explained by the contribution of the quarks alone. This is affirmed by the newest results from COMPASS, HERMES and JLAB on the inclusive spin structure function g1 and on the individual contributions from the different quark flavors from semi-inclusive DIS data. Measurements from the polarized pp-collider RHIC show that also the contribution from gluons is smaller than originally expected. Recent experimental evidence of exclusive reactions, especially DVCS, allows in the formalism of generalized parton distributions the study of an other nucleon spin component the orbital angular momentum. The most recent results on indications of the size of the orbital angular momentum of quarks from data and lattice measurements indicate a small contribution from quark orbital angular momenta to the spin of the proton. At the EIC it will not only be possible to measure all these contributions to the spin of the nucleon with unseen precision, but more importantly the range of all observables can be extended to smaller Bjoerken x, which allows to minimize the biggest uncertainty in these observables, the extrapolation to the unmeasured low-x region.   

Nucleon structure studies through exclusive reactions with an EIC at JLab

Hadrons in QCD are relativistic many-body systems, with a fluctuating number of elementary quark/gluon constituents and a very rich structure of the wave function, with distinct components in different kinematic regions. The 12 GeV energy upgrade at Jefferson Lab will allow a detailed study of the valence quark component. With an EIC at Jefferson Lab we enter the region where the many-body nature of hadrons, coupling to vacuum excitations, etc., become manifest. In this talk I will discuss the exciting prospects of studying the landscape of nucleon structure using exclusive reactions, and in particular the gluon and sea quark imaging of the nucleon.   

The Nucleon Form Factors measurement at EIC

The large luminosity of EIC will provide a unique possibility to study the proton and the neutron at very large momentum transfer where the QCD predictions could be tested. A nucleon Form Factor experiment up to 50 GeV$^{2}$ at EIC will be discussed.   

Possibility for a New Measurement of the Proton Elastic Form Factor Ratio at Very Low $Q^2$

The proton form factors at low $Q^2$ encode information about the peripheral structure of the proton as well as the interplay between the magnetic and electric charge distributions. Furthermore, low $Q^2$ form factor measurements impact high precision experiments, for example, the measurement of the hydrogen hyperfine splitting. Polarization transfer and beam target asymmetry measurements allow to determine the electric to magnetic form factor ratio with high precision down to $Q^2 \simeq 0.15$ GeV$^2$. A recently completed experiment at JLab has recently taken data with unprecedented precision down to $Q^2\simeq 0.3$ GeV$^2$, with a further experiment approved to extend the $Q^2$ range down to 0.015 GeV$^2$. At even lower $Q^2$ the beam-target asymmetry method is impeded by the need to detect either a very forward electron or to use a low energy beam which must traverse the high magnetic field of a polarized target. A recoil polarization measurement at lower $Q^2$ is impossible due to the very low energy of the recoil proton. We suggest an alternative measurement using colliding proton and electron beams which will allow a measurement of the form factor ratio to extremely low $Q^2$ ($\sim 10^{-4}$ GeV$^2$). The opportunity for this measurement will be discussed.   

Delta-Isobar Production in the Hard Photodisintegration of a Deuteron

Hard photodisintegration of the deuteron in delta-isobar production channels is proposed as a useful process in identifying the quark structure of hadrons and of hadronic interactions at large momentum and energy transfer. The reactions are modeled using the hard re scattering model, HRM, following previous works on hard breakup of a nucleon nucleon (NN) system in light nuclei. Here,quantitative predictions through the HRM require the numerical input of fits of experimental NN hard elastic scattering cross sections. Because of the lack of data in hard NN scattering into $\Delta$-isobar channels, the cross section of the corresponding photodisintegration processes cannot be predicted in the same way. Instead, the corresponding NN scattering process is modeled through the quark interchange mechanism, QIM, leaving an unknown normalization parameter. The observables of interest are ratios of differential cross sections of $\Delta$-isobar production channels to NN breakup in deuteron photodisintegration. Both entries in these ratios are derived through the HRM and QIM so that normalization parameters cancel out and numerical predictions can be obtained.   

Expected improvements in polarized parton distribution uncertainties from, proposed, Electron Ion Collider using a Global analysis approach.

Parton distribution functions (PDFs) are indispensable in any calculation of high energy processes involving hadrons. Global analysis of all the experimental data over a wide range of longitudinal momentum fraction of the partons, $x$, and a well resolved momentum scale, $Q^2$, is a way to extract the PDFs. A high luminosity ($>10^{33-34}$ $cm^{-2} s^{-1}$), high energy ($\sqrt(s)$ = 30 to 100 GeV) Electron-Ion-Collider (EIC) will allow to access the kinematic regime between that of HERA and of the fixed-target experiments with much higher statistics. Thus a global analysis including the EIC data will allow us to precisely determine the PDFs in a larger kinematic region. Since EIC will run with a polarized nucleon beam, an extraction of gluon polarization, $\Delta G$, using global analysis will be a major goal for the spin community at this facility. We will present results and improvements in uncertainties we can expect, coming from EIC, on polarized PDFs from global analysis.