executivesummaryofthewhitepaperontheelectronioncollider(eic)...

EIC White Paper Executive Summary 7/31/2012

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Executive Summary of the White Paper on the Electron Ion Collider (EIC) Understanding the fundamental structure of matter in the universe, and its evolution from its origin (the Big Bang) to its current state, are among the most central goals of human scientific endeavor. The evolution is intimately tied to the properties of fundamental particles that existed in each of its phases. It is the atom that is the building block of everything we see and sense around us, as well as ourselves — the world of the visible matter. Nuclear science is concerned with the properties of the atoms and the origin and structure of the visible matter, in particular, the atoms’ core, the nucleus. The nucleus was first discovered 1911 in the renowned Rutherford’s experiment, which revolutionized our view of atomic structure. In the last 100 years, nuclear science has played a critical role in our understanding of visible matter on all distance fronts: femtometer for hadron structure, nanometer for novel materials, to cosmological evolution at light years. Nuclear science has contributed to the development of almost every sector of human society from energy production and supply, medical diagnosis and treatment, to computer gadgets in our everyday life. At first glance, the structure of visible matter appears fairly simple: the mass of an atom is completely dominated by the mass of the nucleus, and it in turn by the mass of the nucleons (neutrons and protons) within it. Experimental investigations in the second half of the last century revealed that nucleons are themselves composed of more fundamental constituents called quarks, bound together by gluons. But the sum of these pieces does not add up to the whole: the mass of a nucleon receives negligible contributions from the masses of its constituents, nearly massless (up and down) quarks and the massless gluons. The same is true for most quark-gluon bound states, collectively called “hadrons.” Thus, the combined substance of everything we see in the visible world, including the most massive planets and stars, is a stunning manifestation of what John Wheeler called ‘mass without mass’ [1] — huge amounts of matter made of nearly massless building blocks. It turns out that the mass of a nucleon, and hence of the visible universe, is dominated by the energy associated with the motion and the interaction of quarks and gluons — in particular, the self-interactions among gluons that are at the heart of the fundamental theory known as Quantum Chromo-Dynamics (QCD). QCD attributes the forces among quarks and gluons to their ‘color’ charge. It is by the self-interaction of the gluons, the colored ‘force carrier,’ that QCD distinguishes itself from Quantum Electrodynamics (QED), where the force carriers (photons) carry no electric charge. These gluon properties give rise to Asymptotic Freedom – the idea that the closer these color charges are together, the weaker the strength of the color interaction responsible for binding quarks into ordinary visible matter. The discovery of this phenomenon in 1973 resulted in the 2004 Nobel Prize in Physics, and paved the way for using particle accelerators to study the strong color interactions at the sub-femtometer distance. QCD has been very successful in interpreting and predicting the phenomena of strong interactions taking place locally at sub-femtometer-to-attometer distances in the collisions of light-speed particles for over forty years. But, we have yet to explain why quarks and gluons are confined in hadrons – the confinement of color in QCD. While no one currently questions the validity of QCD as a Standard Model of the strong interactions, our knowledge and understanding of it is far from complete. There are still important overarching unanswered questions in QCD and hadron physics:

• How do gluons, and the sea quarks they spawn, contribute to hadronic properties? How does the proton spin originate from the confined constituents, quarks and gluons? What is the role of orbital motion of quarks and gluons in contributing to the proton spin?


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• How are quarks and gluons confined in hadrons? How do quarks and gluons distribute in space and move in a nucleon in the direction perpendicular to the nucleon’ light-speed motion? How does the motion of quarks and gluons correlate with the hadron properties?

• Is there a new regime of QCD matter at extremely high gluon density? Does the extreme high density of soft gluons in hadrons saturate? Does the saturation produce matter of universal properties in all hadrons and nuclei viewed at light speed?

• What is the landscape of quarks and gluons in a nucleus? How does the nuclear environment affect the distribution of quarks and gluons in nuclei? How does nuclear matter respond to a fast moving color charge passing through it?

The answers to these questions are crucial for understanding the nature of visible matter. The RHIC program at Brookhaven National Laboratory (BNL) is providing crucial information to help address the first question and hints to the third and fourth question, and the 12 GeV upgrade of CEBAF at Thomas Jefferson National Accelerator Facility (JLab) will address the first two questions in the kinematic region of the valence quarks. But an Electron-Ion Collider (EIC) will provide the future “Rutherford experiment” with a probe of high-precision, high-resolution and high intensity to address all of the above questions at one facility. The proposed EIC was designated in the 2007 Nuclear Physics Long Range Plan as “embodying the vision for reaching the next QCD frontier.” It would extend the QCD science programs established at both the CEBAF accelerator at JLab and RHIC at BNL in dramatic and fundamentally important ways. The EIC would be distinguished from all past, current, and contemplated facilities around the world by being at both the intensity and the versatility frontier, allowing the above questions to be tackled at one facility. In particular, the EIC design exceeds the capabilities of HERA, the only electron-proton collider to date, by adding a) polarized proton and light-ion beams; b) a wide variety of heavy ion beams; c) two to three orders of magnitude improvement in luminosity; d) wide energy variability. The scientific goals and the machine parameters for the EIC were proposed in deliberations at a community wide program held at the Institute of Nuclear Theory (INT) [2]. The physics goals were set by identifying critical questions in QCD that remain unanswered despite the significant experimental and theoretical progress made over the past decade. A White Paper being prepared for the 2013 Long Range Plan, [3] presents a summary of those scientific goals, and a brief description of the golden measurement programs and accelerator and detector technology advances required to achieve them. This is an executive summary of that White Paper. Realizing the EIC will also extend the US leadership in nuclear and accelerator science.

Science Highlights of the Electron Ion Collider

Nucleon Spin and its 3D Structure and Tomography Several decades of experiments on the deep inelastic scattering (DIS) of electron or muon beams have taught us a lot about how quarks and gluons (collectively called partons) share the momentum of a light-speed nucleon. They have not, however, resolved the question of how partons share the nucleon's spin. The earlier studies were also limited: they provided only a one-dimensional view of nucleon structure. The EIC is designed to yield much greater insight into nucleon structure (Figure 1, from left to right), by facilitating multi-dimensional maps of the distributions of partons in space, momentum (including momentum components transverse to the nucleon momentum), spin and flavor.


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Figure 1: Evolution of our understanding of the nucleon spin: in the 1980’s being only contributed to by constituent quarks, 1990s/2000 needing contributions from gluons, and the orbital motion of quarks, to the present day picture where all constituents and their motion contribute resulting in a rich internal dynamics and a 3D structure of the nucleon.

The 12 GeV upgrade of CEBAF at Jefferson Lab will start on such studies in the kinematic region of the valence quarks, but only experiments at the EIC would be able to explore the role of the gluons, dominant below x = 0.2 or so, and sea quarks in determining the hadron structure and its properties, and to resolve crucial questions, such as whether the substantial ``missing'' portion of nucleon spin resides in the gluons. And by probing transverse momentum, it should also illuminate the role of partons orbital motion within the nucleon.

The spin and flavor structure of the nucleon: After an intensive and worldwide experimental program over the past two decades we have learned that quarks carry ~30% of the proton spin while recent RHIC results indicate that the gluons spin contribution in the currently explored kinematic region is clearly non-zero but not yet sufficient to account for the missing 70%. With the unique capabilities to reach two orders of magnitude smaller in parton momentum fraction x and a wide range of momentum exchange Q2, the EIC would offer the most powerful tool to precisely quantify how the spin of gluons and that of quarks of various flavors contribute to the proton’s spin. The EIC could achieve this by colliding longitudinally polarized electrons and nucleons, with both inclusive DIS measurements, where only the scattered electron is detected, and semi-inclusive DIS (SIDIS) measurements, where, in addition, a hadron created in the collisions is to be detected and identified.

Figure 2: Left: The range in parton momentum fraction x vs. resolution Q2 (GeV2) accessible to the EIC for different center-‐ of-‐mass energy (√s) combinations (45 and 140 GeV) in comparison with the existing

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data. Right: The possible reduction in the measurement of the contribution from the gluon (ΔG) vs. that from quarks & antiquarks (ΔΣ) with the inclusive and semi-‐inclusive DIS experiment at the EIC is shown. The blue region indicates the uncertainty calculated in a next-‐to-‐leading perturbative QCD analysis of all available data [3] and the red region indicates what could be achieved with the EIC. Figure 2 (right) shows the possible reduction in uncertainties in the gluon’s and quark+antiquark’s contribution to the nucleon spin, evaluated between a parton momentum fraction x from 0.001 to 1.0, that could be achieved by the EIC as it turns on and its early phases of operation. The uncertainties calculated here are based on the state-of-the art theoretical treatment of all available data related to the nucleon spin puzzle in the world. Evidently the EIC will make a huge impact on our knowledge of these quantities. There is no other existing or anticipated facility comparable in its impact. Should the uncertainties be thus reduced then they will further emphasize the need to explore the contributions to the nucleon spin beyond those from quark and gluon’s intrinsic spins, i.e. from the motion of partons inside the nucleon leading to possible orbital angular momentum.

The confined parton motion inside the nucleon: The SIDIS measurements have two natural momentum scales: the large momentum transfer from the electron beam needed to achieve the desired spatial resolution, and the momentum of the produced hadrons transverse to the direction of the momentum transfer, which has a small value sensitive to the motion of confined partons. Remarkable theoretical advances over the past decade have led to a rigorous framework where information on the confined motion of the partons inside a fast moving nucleon is matched to transverse momentum dependent parton distributions (TMDs). In particular, TMDs are sensitive to correlations between the spin of the hadron and the motion of its partons, as well as between the spin of the parton and the motion of its fragmented hadrons. These correlations can arise from spin-orbit coupling among the partons, about which very little is known to date. With both electron and nucleon beams polarized at collider energies, the EIC will dramatically advance our knowledge of the motion of confined parton, gluon and sea quarks in ways not achievable at any existing or proposed facility. Figure 3 shows the transverse momentum distribution probabilities of the up and down quarks in x and y (spatial) direction, inside a transversely polarized proton (moving in the z direction while polarized in the x direction). While this is an intuitive model prediction for quarks, nothing is known about the spin and momentum correlations of the gluons and sea quarks. Measuring such distributions is a fundamentally different path to investigate the internal structure and dynamics of the proton which goes well beyond a one-dimensional snapshot of nucleon structure. Currently no data exists for extracting such a picture in the gluon-dominated region in the proton. The EIC would be crucial to initiate and realize such a program.

Figure 3: The transverse momentum distributions of up (left) and down (right) quarks in a transversely polarized proton moving in z direction while polarized in x direction. This is a model prediction calculation.

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The tomography of the nucleon -‐ spatial imaging of gluons and sea quarks: By choosing particular final states in electron-proton scattering, the EIC would be able to selectively probe the transverse spatial distribution of sea quarks and gluons in the fast moving proton as a function of the parton's longitudinal momentum fraction x. This spatial imaging is a powerful and complementary way to go beyond the one-dimensional snapshots of nucleons – the longitudinal momentum distributions of quarks and gluons. Such tomographic information promises insight into the dynamics of confinement of partons within nucleons. With its broad range of collision energies, its high luminosity and nearly hermetic detectors, the EIC could image the proton with unprecedented detail and precision from small to large transverse distances. The accessible parton momentum fractions x extend from a region dominated by sea quarks and gluons to one where valence quarks become important and where the connection to the precise images expected from the 12 GeV upgrade at Jefferson Lab is possible. This is exemplified in Fig. 4, which shows the expected accuracy for the spatial distribution of gluons as measured in the exclusive process: electron-proton è electron + J/ψ + proton. The tomographic images obtained from exclusive processes are encoded in generalized parton distributions (GPDs) that unify the concepts of parton densities and of elastic form factors and are extracted from cross sections. They contain detailed information about spin-orbit correlations and the angular momentum carried by partons, including their spin and their orbital motion. With the combined kinematic coverage of EIC and of the upgraded CEBAF, we shall obtain meaningful constraints on the quark orbital angular momentum contribution to the proton’s spin.

Figure 4: Projected accuracy of the transverse spatial distribution of gluons as extracted from exclusive J/Ψ production. bT is the distance of the gluon from the center of the proton, whereas the kinematic quantity xV=(MJ/ψ2+Q2)/(W2+Q2-‐MN2), where W2=(p+q)2 and MN2=p2, determines the gluon momentum fraction. The collision energies assumed for stage 1 and stage 2 are Ee = 5 GeV, Ep = 100 GeV and Ee = 20 GeV, Ep = 250 GeV, respectively. The intermediate xV bin is accessible with either energy setting and gives almost identical uncertainty bands in both cases.

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Nucleus, a laboratory for QCD

The atomic nucleus is a “molecule” in QCD, made of nucleons (protons and neutrons), which, in turn, are bound states of quarks and gluons. Understanding the emergence of nuclei from QCD is an ultimate long-term goal of nuclear physics. With its wide kinematic reach, as shown in Fig. 5 (Left), the capability to perform both inclusive and semi-inclusive DIS measurements, and the control over the nuclear species, the EIC would be the first experimental facility capable of exploring the internal 3-dimensional sea quark and gluon structure of a fast moving nucleus, and thus enable new understanding of nuclei at a sub-femtometer scale. More importantly, at the EIC, the nucleus itself would be an unprecedented QCD laboratory for discovering the properties of gluons at an extreme high gluon density, and for studying the propagation of a fast moving color charge in QCD matter.

Figure 5: Left: The resolution (Q2) vs. the parton momentum fraction (x) landscape of available data for lepton-‐nucleus (DIS) and Drell-‐Yan (DY) experiments at fixed target energies along with the kinematic reach of the EIC at two different center of mass energies. Right: Map of high energy QCD in resolution (Q2) vs. energy (Y=ln(1/x)) plane. The high gluon density saturation region is shown.

QCD at extreme parton density: The discovery of the sharp growth of gluon density in a nucleon when gluon momentum fraction x decreases, by HERA in Germany, triggered a world-wide effort to study QCD in regions of extreme gluon density. A continuing growth in the number of soft gluons would result in a violation of the unitarity bound of hadronic cross section. In QCD the large soft gluon density enables the non-linear process of gluon-gluon recombination to reduce its growth. Such a QCD self-regulation mechanism necessarily generates a dynamic scale from the interaction of high density massless gluons, known as the saturation scale, Qs, at which gluon radiation and recombination reach a balance where both the density of gluons and the effective QCD coupling strength α s(Q2) are expected to saturate, producing new and universal properties of hadronic matter. The saturation scale Qs separates the condensed and saturated soft gluon matter from the dilute but confined quarks and gluons in a hadron, as shown in Fig. 5 (Right). The existence of such a saturated soft gluon matter, often referred to as Color Glass Condensate, is a direct consequence of the QCD dynamics. The dynamic saturation scale Qs may be impossible to reach in electron-proton scattering without a multi-TeV proton beam. However,

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heavy ion beams provide precocious access to the saturation regime because the virtual photon in forward lepton scattering probes matter coherently over a characteristic length proportional to 1/xp, which can exceed the diameter of a Lorentz contracted nucleus. Then all the gluons at a given impact parameter of the nucleus, enhanced by the nuclear diameter proportional to A1/3 with the atomic weight A, contribute to the probed density, reaching saturation at far lower energies than would be needed in electron-proton collisions. While HERA, RHIC and LHC found evidence for saturated gluonic matter, the EIC would have the potential to seal the case, completing the process started at those facilities.

Figure 6: Left: The double ratio of the diffractive and total cross sections in electron-‐gold (eA) and electron-‐proton (ep) collisions defined in the text plotted as a function of the invariant mass MX2 of the produced particles. Right: Ratios of cross-‐section of eA vs. ep for J/Ψ (blue) and φ (red) production with (square) and without (circle) the saturation, and with the statistical uncertainties possible with measurements at the EIC.

Figure 6 (Left) shows a double ratio: the ratio of the coherent diffractive cross sections over the total DIS cross sections in electron-gold collisions over the same ratio in electron-proton collisions, as a function of invariant mass of produced particles, MX

2, at the EIC. Coherent diffractive events are events with a gap in rapidity in which no particles are produced and the beam nucleus remains intact. The gluon saturation greatly enhances the coherent diffractive cross sections and leads to a much larger double ratio, in comparison with calculations without the saturation physics, as shown in Fig. 6 (Left). Such measurement at the EIC would provide the first unambiguous evidence for the discovery of a novel QCD matter of saturated gluons. Figure 6 (Right) shows the nuclear over nucleon cross section ratio of vector meson production for the light ϕ (red) and heavy J/Ψ (blue) mesons as a function of initial size of the quark-antiquark pair that hadronizes into the observed vector meson at the EIC. QCD calculations of vector meson production with and without the reach of gluon saturation lead to two dramatically different manifestations, one of suppression and the other of enhancement, respectively. The effect is clearly amplified by the size of the produced vector meson, as shown in Fig. 6 (Right), and provides an excellent probe to diagnose the characteristics of the saturated gluons. The measurement in Fig. 6 (Right) would be another powerful probe for the existence of the new QCD matter of saturated gluons.

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Propagation of a color charge in QCD matter: One of the key evidences of the discovery of quark-gluon plasma (QGP) at BNL was the strong suppression of fast moving hadrons produced in the relativistic heavy ion collisions (from the so-called “jet quenching”), which is believed to be due to the energy loss of fast moving partons caused by their multiple interactions with the medium, the QGP. However, it has been puzzling that heavy meson production seems to be as much suppressed as that of the light meson, although a heavy quark is much less likely to loose its energy via medium induced radiation. By choosing a particular final state particle in electron-proton collisions, the EIC with its collider energies provides an excellent laboratory to study how a quark or a gluon hadronizes into an observed particle and to extract the universal fragmentation functions, which encode the key information on the hadronization. With its variety of nuclear species the nuclear matter at the EIC, in an electron-nucleus scattering, would provide a femtometer filter to test and to help select the right hadronization mechanism (see schematic in Fig. 7 (Left)). This would provide an independent and complimentary information essential for understanding the response of nuclear medium to a colored fast moving (heavy or light) quark at high energies. The EIC will thus offer the best testing ground for studying the multiple scattering and energy loss mechanism in QCD.

Figure 7: Left: (improvement of the figure) Interactions of the parton moving through cold nuclear matter. Right: Ratio of semi-‐inclusive cross section for producing a pion (red), composed of light quarks, and a D0 meson (blue) composed of heavy quarks in electron-‐Lead collisions to the same produced in electron-‐deuteron as function of z, the momentum fraction of the exchanging photon carried by the observed meson. Statistical uncertainties are shown in each case. Figure 7 (Right) shows the ratio of semi-inclusive meson production in electron-nucleus and electron-deuteron collisions for light meson-pion (red square symbols) and heavy meson-D0 (blue circle symbols) at the EIC with low (solid symbols) and high (open symbols) virtual photon energies, respectively, as a function of z - the momentum fraction of the virtual photon taken by the observed meson. The ratio is directly coupled to the amount of energy loss and the shape of fragmentation function. Unlike the suppression expected for pion production at all z, the ratio of heavy meson production could be larger than unity due to the distinct peak in heavy flavor fragmentation function. A study of such effects in light and heavy meson production over a broad z range would be possible with the EIC. Evidence of such a dramatic difference in multiplicity ratios (Figure 7, Right) at the EIC would shed light on the hadronization process and what governs the transition from quarks to hadrons.


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The distribution of quarks and gluons in the nucleus: The EMC experiment at CERN and experiments in the following two decades clearly revealed that the distribution of quarks in a fast moving nucleus is not a simple superposition of their distributions within nucleons. Instead, the ratio of nuclear over nucleon structure functions follows a non-trivial function of Bjorken xB, significantly away from unity, and shows a suppression (often referred to as the phenomenon of nuclear shadowing) as xB decreases. Amazingly, there is as of yet no knowledge whether the same holds true for gluons. With its much wider kinematic reach in both x and Q, the EIC could enable the measurement of the suppression (the shadowing) of the structure functions to a much lower value of x, approaching the novel region of gluon saturation. In addition, at the EIC one could, for the first time, reliably quantify the nuclear gluon distribution over a wide range of momentum fraction x. With its unprecedented luminosity, the EIC could then also provide the first imaging of 3-dimensional sea quark and gluon structure of a fast moving nucleus with sub-femtometer resolution.

Physics possibilities at the Intensity Frontier The subfield of Fundamental Symmetries (FS) in nuclear physics has an established history of key discoveries, enabled by either the introduction of new technologies or the increase in energy and luminosity of accelerator facilities. While the EIC is primarily being proposed for exploring new frontiers in QCD, it offers a unique new combination of experimental probes potentially interesting to the investigations in FS. For example, the availability of polarized beams at high energy and high luminosity, combined with a state-of-the-art hermetic detector, could extend Standard Model tests of the running of the weak-coupling constant far beyond the reach of the JLab12 parity violation program, namely toward the Z-pole scale previously probed at LEP and SLC.

The Electron Ion Collider and its Realization Two independent designs for a future EIC have evolved in the US. Both use the existing infrastructure and facilities available to the US nuclear science community. At Brookhaven National Laboratory (BNL) the eRHIC design (Figure 8, left) utilizes a new electron beam facility based on an Energy Recovery LINAC (ERL) to be built inside the RHIC tunnel to collide with RHIC’s high-energy polarized proton and nuclear beams. At Jefferson Laboratory (JLab) the ELectron Ion Collider (ELIC) design (Figure 8, right) employs a new electron and ion collider ring complex together with the 12 GeV upgraded CEBAF, now under construction, to achieve similar collision parameters. The EIC machine designs are aimed at achieving:

• Highly polarized (> 70%) electron and nucleon beams • Ion beams from deuteron to the heaviest nuclei (Uranium or Lead) • Variable Center of mass energies ranges from ~20-~70 GeV, upgradable to ~150 GeV • Collision luminosity ~1033-34 cm-2s-1 • Possibilities of having more than one interaction region

The EIC accelerator design requirements will push the machine designs to the forefront of accelerator technology, requiring significant R&D. The cooling of the hadron beam is essential to attain the luminosities demanded by the science. The development of coherent electron cooling (CEC) is now underway at BNL, while conventional electron cooling techniques are being pushed in the JLab design.


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Figure 8: Left: The schematic of eRHIC at BNL: will require construction of the electron beam facility (indicated in RED) to collide with the RHIC blue beam at three interaction points. Right: The schematic of the ELIC at JLab: will require construction of the ELIC complex (red and black in the center) and its injector (shown in green, top right) next to the 12 GeV CEBAF

The success of high-power Energy Recovery LINACs (ERLs) is essential for the realization of the EIC. The eRHIC design at BNL also requires a polarized electron source producing beam intensities an order of magnitude beyond the current state of the art, while the ELIC design at JLab, utilizes a novel figure-8 storage ring design for electrons and ions and relies on attaining unprecedentedly small hadron beam bunch dimensions and avoid proton beam depolarization. The physics driven requirements on the EIC accelerator parameters and extreme demands on the kinematic coverage for measurements, makes integration of the detector into the accelerator a particularly challenging feature of the design. Lessons learnt from past experience at HERA have been considered while designing the EIC interaction region. Driven by the demand for high precision on particle detection and identification of final state particles in both e-p and e-A programs, modern particle detector systems will be at the heart of the EIC. In order to keep the detector costs manageable, R&D efforts are under way on various novel ideas for, compact (fiber sampling & crystal) calorimetry, tracking (NaI coated GEMs, GEM size & geometries), particle identification (compact DIRC, dual radiator RICH & novel TPC) and high radiation tolerance for electronics. Meeting these R&D challenges will keep the U.S. nuclear science community at the cutting edge in both accelerator and detector technology.

Physics potential of the Stage I of EIC A staged realization of the EIC is being planned for both eRHIC and ELIC designs. The first stage is anticipated to have up to ~60-100 GeV in center-of-mass-energy, with polarized nucleon and electron beams, a wide range of heavy ion beams for nuclear DIS, and a luminosity for electron-proton collisions approaching 1034 cm-2s-1. With such a facility, the EIC physics program would have an excellent start toward addressing (but not limited to) the following fundamental questions with key measurements:

• The proton spin: Within just a few months of operation, the EIC could enable decisive measurements on how much the intrinsic spin of quarks and gluons contribute to the proton spin, as shown in Fig. 2 (right), and clarify the role of the motion of quarks and gluons in determining the hadron properties. No other facility in the world could achieve this.

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• The confined motion of quarks and gluons in the proton: By choosing a particular final state particle in electron-proton and electron-nucleus scattering, the EIC with its collider energies and polarized beams will be able to selectively probe the correlation between the spin of a fast moving proton and the confined transverse motion of quarks and gluons inside the proton with precision. Figure 3 shows a model calculation of such dynamics. With the EIC one would be able to measure and compare data with such models. Currently no such pictures are possible, and nor are they expected before the EIC is realized.

• The tomographic images of the proton: By measuring the exclusive processes, the EIC with its unprecedented luminosity and detector coverage could image the proton in terms of the distributions of quarks and gluons from the center of the proton to a transverse distance as far as 1.5 femtometer, as shown in Fig. 4. Such measurements would reveal aspects of proton structure that are intimately connected with the dynamics of color confinement. The EIC could achieve such tomographic images of the proton with the precision and details that no other current or proposed facility could match.

• QCD matter at an extreme gluon density: By measuring the coherent diffractive cross sections as well as total DIS cross sections in electron-proton and electron-nucleus collisions as shown in Fig. 6, the EIC could provide the first unambiguous evidence for the discovery of the new and novel QCD matter of saturated gluons. The EIC could define the new field of QCD at high gluon density.

• Quark hadronization: By measuring pion and D0 meson production in both electron-proton and electron-nucleus collisions, the EIC could provide the first unambiguous measurement of the quark mass dependence of the hadronization as well as the response of nuclear matter to a fast moving quark.

Figure 9: illustrates the physics potential of the EIC as it a function of its center of mass and integrated luminosity. Both machine designs under consideration are intended to provide clear paths toward later increases in center-‐of-‐mass energy to extend the reach for gluon saturation studies and Standard Model tests. The Relativistic Heavy Ion Collider (RHIC) at BNL has revolutionized our understanding of hot and dense QCD matter through its discovery of the strongly coupled quark-gluon plasma that existed a few microseconds after the birth of the universe. Unprecedented studies of the nucleon

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12

and nuclear structure including the nucleon spin, and the nucleon's tomographic images in the valence quark region have been, and will be, possible with the high luminosity fixed target experiments at Jefferson Laboratory using the 6 and 12 GeV CEBAF, respectively. The EIC promises to propel both programs to the next QCD frontier, by unraveling the gluonic and sea quark 3-dimensional structure of the basic building blocks of matter and exploring dense QCD matter in confinement. The EIC will thus enable the US to continue its leadership role in the study of nuclear science through the quest for understanding the unique gluon-dominated nature of the visible matter in the universe. References: [1] J. A. Wheeler, Geometrodynamics, Academic, New York, 1962, p25; F. Wilczek, Mass Without Mass I, Physics Today, November 1999 [2] D. Boer et al., Gluons and sea quarks at high energy: distributions, polaraization and tomography, arXiv: 1108.1713; INT-PUB-11-034, BNL-96164-2011, JLAB-PHY-11-1373 [3] NSAC Long Range Plan 2007; website

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