EIC White Paper Executive Summary
7/31/2012
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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EIC White Paper Executive Summary
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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 onedimensional 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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EIC White Paper Executive Summary
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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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EIC White Paper Executive Summary
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data. Right: The possible reduction in the measurement of the contribution from the gluon (G) vs. that
from quarks & antiquarks () 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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EIC White Paper Executive Summary
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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  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/ 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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EIC White Paper Executive Summary
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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.
2
non-perturbative region
Y = ln 1/x
saturation c Qs(Y)
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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 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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EIC White Paper Executive Summary
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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/(blue) and (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, MX2, 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 quarkantiquark 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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EIC White Paper Executive Summary
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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 socalled “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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EIC White Paper Executive Summary
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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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EIC White Paper Executive Summary
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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 electronproton 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.
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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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EIC White Paper Executive Summary
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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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