the electron ion collider: a next qcd frontier · 2019. 9. 30. · the white paper as of today: 146...

The Electron Ion Collider: A Next QCD Frontier Understanding the glue that binds us all Physics Opportunities @ An Electron Ion Collider Bloomington Indiana August 20, 2012 A. Deshpande, Z.-E. Meziani, J. Qiu for the EIC WP Writing Group

Monday, August 20, 12 The EIC White Paper 2

•  A brief background on the White Paper (WP)

•  An overview of the current WP draft

•  The executive summary (ES) and its time lines: Tribble-II subcommittee deliberations (Sept. 7)

•  WP and its time lines: Broader NP community’s townhall meetings at the DNP’12 and Tribble-II final report (December’12)

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Dear Zein-Eddine, It is important in preparing for the next Nuclear Physics Long Range Plan to produce a community-wide science White Paper for a non-site-specific EIC by the end of calendar year 2011. This schedule would give us time to get further community and funding agency input, and to iterate to a final version by Fall of 2012, when Town Meetings for the next LRP might start.

I am writing to see if you would be willing to serve with Abhay Deshpande and Jianwei Qiu as overall editors and overseers of the White Paper. I lay out the organization of the Steering Committee you would lead below. The committee membership has been endorsed by Tom Ludlam, Bob McKeown and Hugh Montgomery, and at this point all the individuals listed below have agreed to serve.

I think the “Yellow Book” that will soon result from the Fall 2010 INT program should serve as a resource for the White Paper, but it will be far too long and detailed for a non-expert audience. I think the White Paper should be no more than ~100 pages, and should be aimed at non-experts, including our potential “champions” within DOE (e.g., Tim Hallman) and the rest of the Nuclear Physics community that will be asked to endorse this vision. It should start with an ~5-page Introduction – basically the “elevator speech” we have discussed, slightly amplified – laying out the goals, importance and uniqueness of the facility, and answering the basic questions raised about EIC at the last LRP meeting in Galveston in 2007, all in clear, concise, compelling, jargon-free language. The Introduction should also indicate the features of the science goals that could be accomplished with a first-stage machine and detectors, and should include a few of the most compelling “money plots” envisioned. I expect writing of the Introduction would fall mostly to the three overall editors in the organization outlined below.


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There should then be ~10-page sections on specific science areas, laying out and fleshing out “golden experiment matrices” with simulated money plots. It is very important that these sections build on the excellent progress made at the INT workshop in selecting golden experiments to emphasize – the White Paper should not try to present all science that would be done at an EIC, but should concentrate on the measurement program that most compellingly illustrates the intellectual goals and performance requirements of the facility. These sections should get across the fundamental goals of the research and enough technical discussion to convince a reader of the basic feasibility and interpretability of the results, but should otherwise be relatively light on technical experimental and theoretical detail. There should be a section of ~10-15 pages on basic machine parameters, design options, technical challenges and ongoing R&D, which covers both design approaches (again without gory technical detail). A similar section of ~10 pages on detector design features and challenges would round out the document as I see it. I don’t think this White Paper needs to get into cost estimates, although we will clearly need that as well by the time of the LRP. We have tried to assemble a Steering Committee comprising experimentalist/theorist pairs, broadly representative of the interested institutions (and not too heavily BNL- and JLab-laden), aligned with an envisioned breakdown of the science and technical sections, as follows: Overall editors: A. Deshpande (Stony Brook), J. Qiu (BNL) and Z.-E. Meziani (Temple U.) Gluon saturation in e+A: T. Ullrich (BNL) and Y. Kovchegov (Ohio State U.) Nucleon spin structure (mostly inclusive e+N): E. Sichtermann (LBNL) and W. Vogelsang (Tubingen) GPD’s and exclusive reactions: F. Sabatie (Saclay) and M. Diehl (DESY) TMD’s, hadronization and SIDIS: H. Gao (Duke) and F. Yuan (LBNL) Electroweak physics: K. Kumar (U. Mass.) and M. Ramsey-Musolf (Wisconsin) Accelerator designs and challenges: T. Roser (BNL) and A. Hutton (JLab) Detector design and challenges: E. Aschenauer (BNL) and T. Horn (CUA) Senior advisors: R. Holt (ANL) and A. Mueller (Columbia)

If you are willing to serve as one of the overall editors, please let me know soon, and then get in touch with Abhay and Jianwei to discuss how to launch the efforts. I am hoping that considerable progress can be made on the White Paper over the coming summer.

Cheers, and thanks for a reasonably prompt response, Steve


Final committee: Overall editors:

A. Deshpande (Stony Brook), J. Qiu (BNL) and Z.-E. Meziani (Temple U.) Gluon saturation in e+A:

T. Ullrich (BNL) and Y. Kovchegov (Ohio State U.) Parton propagation and energy loss in nuclei: (added)

W. Brooks (UTF, Santa Maria) & J. Qiu (BNL) Nucleon spin structure (mostly inclusive e+N):

E. Sichtermann (LBNL) and W. Vogelsang (Tübingen) GPD’s and exclusive reactions:

F. Sabatiė (Saclay) and M. Diehl (DESY) TMD’s, hadronization and SIDIS:

H. Gao (Duke) and F. Yuan (LBNL) Electroweak physics:

K. Kumar (U. Mass.) and M. Ramsey-Musolf (Wisconsin) Accelerator designs and challenges:

T. Roser (BNL) and A. Hutton (JLab) Detector design and challenges:

E. Aschenauer (BNL) and T. Horn (CUA) Senior advisors:

R. Holt (ANL) and A. Mueller (Columbia)


The White Paper as of today: 146 pages •  Executive Summary (ES) à 12 pages to Tribble-II •  Spin and 3D structure of the nucleon:

•  Introduction (9 pages) •  Longitudinal Spin: (11 pages) •  Confined Motion of Partons in Nucleons (TMDs) (9 pages) •  Generalized Parton Distributions (14 pages)

•  The Nucleus: A QCD Laboratory •  Introduction (5 pages) •  Physics of extreme gluon density (23 pages) •  Quarks and gluons in nuclei (8 pages) •  Connections to A-A, p-A and Cosmic Rays (11 pages)

•  Opportunities at the extreme luminosity (6 pages) •  Accelerator design (12 pages) •  Detector design (12 pages) •  Acknowledgment (1 page à will certainly grow….) •  References (9 pages)


Comment on EIC WP Draft 146 page EIC White Paper (draft) Too long: not appropriate for Tribble-II deliberations. Submit the 12 Page Executive Summary to Tribble-II. The WP was and is intended for the broader NP community for the Long Range Planning discussions, and town-hall meetings.

Long Range Plan (When?) Town meetings being organized in DNP’12 (October 2012)

Need to open the EIC White Paper for Comments from BNL & Jlab User communities: More on this later.

Aim to prepare an almost final draft of the White Paper by October Town hall meetings, and submit the final White Paper to NSAC (and its subcommittees in November after input/comment from the DNP)


LETS FOCUS ON THE EXECUTIVE SUMMARY: We would appreciate comments from the POETIC participants!


Feedback needed: Executive Summary •  Written for a non-expert nuclear (+1 CM) physicists •  Emphasis on complete but concise •  August 28 submission aimed at Tribble-II 1st meeting: Sept. 7 •  Currently, 12 pages: approximately correct length •  The rest of the WP DRAFT will serve as a back-up document Feedback needed by August 24th: •  What is absolutely needed to strengthen the case? •  Are there statements made that are detrimental to the case? •  Relative emphases amongst different components correct? •  Any mistakes in the physics? (in the process of simplification) •  Is there anything crucial that is missing? •  Are there better figures? Could one improve cartoons?

•  Only YES does not help: suggest/contribute alternatives!


(Adapted from Steve V.’s recent discussion on RHIC WP)

Method to convey your comments on the Executive Summary (by August 24): Please Email: Subject: “Comments on EIC Executive Summary” • Abhay Deshpande ([email protected]) •  Zein-Eddine Meziani ([email protected]) •  Jianwei Qiu ([email protected])

We will convey the comments to the entire WP writing group.



Contents

1 Exploring the Glue that Binds Us All – The Executive Summary 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Science Highlights of the Electron Ion Collider . . . . . . . . . . . . . . . . 3

1.2.1 Nucleon Spin and its 3D Structure and Tomography . . . . . . . . . 31.2.2 The Nucleus, a Laboratory for QCD . . . . . . . . . . . . . . . . . . 61.2.3 Physics Possibilities at the Intensity Frontier . . . . . . . . . . . . . 10

1.3 The Electron Ion Collider and its Realization . . . . . . . . . . . . . . . . . 101.4 Physics Deliverables of the Stage I of EIC . . . . . . . . . . . . . . . . . . . 12

2 Spin and Three-Dimensional Structure of the Nucleon 132.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2 Longitudinal spin of the nucleon . . . . . . . . . . . . . . . . . . . . . . . . 22

2.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.2.2 Status and Near Term Prospects . . . . . . . . . . . . . . . . . . . . 232.2.3 Open questions and the role of an EIC . . . . . . . . . . . . . . . . . 26

2.3 Confined Motion of Partons in Nucleons: TMDs . . . . . . . . . . . . . . . 342.3.1 Opportunites for measurements of TMDs at the EIC . . . . . . . . . 36

Semi-inclusive Deep Inelastic Scattering (SIDIS) . . . . . . . . . . . 37Access to the Gluon TMDs . . . . . . . . . . . . . . . . . . . . . . . 39

2.3.2 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412.4 Spatial imaging of quarks and gluons . . . . . . . . . . . . . . . . . . . . . . 43

2.4.1 Physics motivations and measurement principle . . . . . . . . . . . . 432.4.2 Processes and observables . . . . . . . . . . . . . . . . . . . . . . . . 452.4.3 Parton imaging now and in the next decade . . . . . . . . . . . . . . 472.4.4 Accelerator and detector requirements . . . . . . . . . . . . . . . . . 482.4.5 Parton imaging with the EIC . . . . . . . . . . . . . . . . . . . . . . 512.4.6 Opportunities with nuclei . . . . . . . . . . . . . . . . . . . . . . . . 56

3 The Nucleus: A Laboratory for QCD 583.1 Physics of High Gluon Densities in Nuclei . . . . . . . . . . . . . . . . . . . 63

3.1.1 Gluon Saturation: a New Regime of QCD . . . . . . . . . . . . . . . 63Nonlinear Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . 63Classical Gluon Fields and the Nuclear “Oomph” Factor . . . . . . . 66Map of High Energy QCD and the Saturation Scale . . . . . . . . . 68Nuclear Structure Functions . . . . . . . . . . . . . . . . . . . . . . . 71Di�ractive Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

3.1.2 Key Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

3

Introduction: Relevance of studying the properties of fundamental structure of matter to nuclear science. What are the big questions? Why is EIC the ONLY appropriate facility to address them? What would be the deliverables for the 1st Stage? How would EIC position US towards continued leadership in nuclear physics (including accelerator science)

12 PAGES

Request your input by August 24. Will go to Tribble-II pannel by August 27/28

1.1 INTRODUCTION Three big questions in QCD: •  How are the sea quarks and gluons inside the nucleon distributed

in space, momentum, spin and flavor? •  Where does the saturation of gluon densities set in? •  How does the nuclear environment affect the distribution of gluons

and interactions of quarks in nuclei?

Why EIC? Why a collider is the right tool? Why is electron the right tool? Why is polarization of beams needed? How would nuclei help in addressing the gluon related questions? Why should heavy ion physicists care?



The scientific goals and the machine parameters for the EIC were delineated in delib-77

erations at a community wide program held at the Institute for Nuclear Theory (INT) [2].78

The physics goals were set by identifying critical questions in QCD that remain unanswered79

despite the significant experimental and theoretical progress made over the past decade. A80

White Paper being prepared for the broader nuclear science community presents a summary81

of those scientific goals, and a brief description of the golden measurement programs and82

accelerator and detector technology advances required to achieve them. This is an executive83

summary of that White Paper.84

1.2 Science Highlights of the Electron Ion Collider85

Figure 1.1: Evolution of our understanding of the nucleon spin: from a picture (1980s) in termsof constituent quarks alone, through incorporation (1990s/2000s) of contributions from gluonsand the orbital motion of quarks, to the present day picture, where many parton spins andtheir transverse motion contribute, resulting in a rich internal dynamics and 3D structure of thenucleon.

1.2.1 Nucleon Spin and its 3D Structure and Tomography86

Several decades of experiments on the deep inelastic scattering (DIS) of electron or muon87

beams o� nucleons have taught us a lot about how quarks and gluons (collectively called88

partons) share the momentum of a fast-moving nucleon. They have not, however, resolved89

the question of how partons share the nucleon’s spin. The earlier studies were also limited90

to provide a one-dimensional view of nucleon structure only. The EIC is designed to yield91

much greater insight into nucleon structure (Fig. 1.1, from left to right), by facilitating92

multi-dimensional maps of the distributions of partons in space, momentum (including93

momentum components transverse to the nucleon momentum), spin and flavor.94

The 12 GeV upgrade of CEBAF at JLab will start on such studies in the kinematic95

region of the valence quarks, but these will be dramatically extended at the EIC to explore96

the role of the gluons, dominant below the parton momentum fraction x � 0.2 or so, and97

sea quarks in determining the hadron structure and its properties, and to resolve crucial98

questions, such as whether the substantial “missing” portion of nucleon spin resides in the99

gluons. And by facilitating high-energy probes of transverse momentum, EIC should also100

illuminate the role of parton orbital motion within the nucleon.101

The Spin and Flavor Structure of the Nucleon:102

After an intensive and worldwide experimental program over the past two decades we have103

learned that the spin of quarks and antiquarks is only responsible for � 30% of the proton104

spin, while recent RHIC results indicate that the gluons spin contribution in the currently105

3

explored kinematic region is non-zero, but not yet su⌃cient to account for the missing 70%.106

With the unique capabilities to reach two orders of magnitude lower in parton momentum107

fraction x and to span a wider range of momentum exchange Q2 than fixed polarized target108

experiments, the EIC would o⇤er the most powerful tool to precisely quantify how the spin109

of gluons and that of quarks of various flavors contribute to the protons spin. The EIC could110

achieve this by colliding longitudinally polarized electrons and nucleons, with both inclu-111

sive DIS measurements, where only the scattered electron is detected, and semi-inclusive112

DIS (SIDIS) measurements, where, in addition, a hadron created in the collisions is to be113

detected and identified. Figure 1.2 (right) shows the reduction in uncertainties in the con-

x

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CERN DESY JLab SLAC

Current polarized BNL-RHIC pp data:

PHENIX π0 STAR 1-jet

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Figure 1.2: The range in parton momentum fraction x vs. resolution Q2 (GeV2) accessible tothe EIC for di�erent center-of-mass energies (

�s=45 and 140 GeV) in comparison with points

representing the kinematics of existing experiments probing nucleon spin structure. . Right: Theprojected reduction of uncertainties on the gluon (�g) and net quark (�⇥/2) contributions toproton spin with the addition of inclusive and semi-inclusive DIS measurements at EIC. Theblue region indicates the uncertainty calculated in a next-to-leading perturbative QCD analysisof all presently published data and the red region indicates what could be achieved with modestrunning times at the EIC

114

tributions to the nucleon spin from the spin of the gluons, and quark-antiquarks, evaluated115

between a parton momentum fraction x from 0.001 to 1.0, which could be achieved by the116

EIC as it turns on in its early phases of operation. At later stages of the EIC operation, this117

range can be extended down to x > 0.0001, which will significantly reduce the uncertainty118

on the contributions from the full interval 0 < x < 1. The uncertainties calculated here are119

based on the state-of-the art theoretical treatment of all available world data related to the120

nucleon spin puzzle. Clearly, the EIC will make a huge impact unmatched by any other121

existing or anticipated facility on our knowledge of these quantities. The reduced uncer-122

tainties would definitively resolve the question of whether parton spin preferences alone can123

account for the overall proton spin, or whether additional contributions are needed from124

the orbital angular momentum of partons in the nucleon.125

The Confined Motion of Partons inside the Nucleon:126

The SIDIS measurements have two natural momentum scales: the large momentum transfer127

from the electron beam needed to achieve the desired spatial resolution, and the momentum128

of the produced hadrons perpendicular to the direction of the momentum transfer, which has129

a small value sensitive to the motion of confined partons. Remarkable theoretical advances130

4

over the past decade have led to a rigorous framework where information on the confined131

motion of the partons inside a fast-moving nucleon is matched to transverse momentum132

dependent parton distributions (TMDs). In particular, TMDs are sensitive to correlations133

between the motion of partons and their spin, as well as the spin of the parent nucleon. These134

correlations can arise from spin-orbit coupling among the partons, about which very little135

is known to date. TMDs thus allow us to investigate the full three-dimensional dynamics136

of the proton, going well beyond the information about longitudional momentum contained137

in conventional parton distributions. With both electron and nucleon beams polarized at138

collider energies, the EIC will dramatically advance our knowledge of the motion of confined139

gluons and sea quarks in ways not achievable at any existing or proposed facility.140

Figure 3 (Left) shows the transverse-momentum distribution of up quarks in x and y141

(spatial) directions, inside a proton moving in the z direction (out of the drawing plane)142

with its spin polarized in the y direction. The anisotropy in transverse momentum induced143

by the proton polarization is described by the Sivers distribution function, whose very144

existence is a consequence of the color gauge invariance of QCD . While the figure is based145

on a preliminary extraction of this distribution from current experimental data, nothing146

is known about the spin and momentum correlations of the gluons and sea quarks. The147

achievable precision of the quark Sivers function from the EIC kinematics is also shown in148

Fig. 1.3 (right). Currently no data exist for extracting such a picture in the gluon-dominated149

region in the proton. The EIC would be crucial to initiate and realize such a program.

(GeV)xk

u quark

−0.5 0 0.5

yk

−0.5

0

0.5

Figure 1.3: Left: Transverse-momentum distribution of up quark in a transversely polarizedproton moving in z-direction, while polarized in y direction. Right: The transverse-momentumprofile of the Sivers function of valence up quark at five x values and corresponding uncertain-ties.

150

The Tomography of the Nucleon - Spatial Imaging of Gluons and Sea Quarks:151

By choosing particular final states in electron-proton scattering, the EIC would be able to152

selectively probe the transverse spatial distributions of sea quarks and gluons in the fast-153

moving proton as a function of the parton’s longitudinal momentum fraction x. This spatial154

imaging is a powerful way to go beyond normal DIS one-dimensional snapshots of nucleons,155

and is complementary to the extraction of TMDs in momentum space. Such tomographic156

information promises insight into the dynamics of confinement of partons within nucleons.157

With its broad range of collision energies, its high luminosity and nearly hermetic detectors,158

the EIC could image the proton with unprecedented detail and precision from small to159

5

large transverse distances. The accessible parton momentum fractions x extend from a160

region dominated by sea quarks and gluons to one where valence quarks become important,161

allowing the connection to the precise images expected from the 12 GeV upgrade at JLab.162

This is exemplified in Fig. 1.4, which shows the unprecedented precision expected for the163

spatial distribution of gluons as measured in the exclusive process: electron + proton �164

electron + J/� + proton.165

The tomographic images obtained from cross sections and polarization asymmetries for166

exclusive processes are encoded in generalized parton distributions (GPDs) that unify the167

concepts of parton densities and of elastic form factors. They contain detailed information168

about spin-orbit correlations and the angular momentum carried by partons, including their169

spin and their orbital motion. With the combined kinematic coverage of EIC and of the170

upgraded CEBAF, we shall obtain meaningful constraints on the quark orbital angular171

momentum contribution to the protons spin.172

bT (fm)

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Figure 1.4: Projected precision of the transverse spatial distribution of gluons as extractedfrom exclusive J/� production. bT is the distance of the gluon from the center of the proton,whereas the kinematic quantity xV determines the gluon momentum fraction. The collisionenergies assumed for stage-I and stage-II are Ee = 5 GeV, Ep = 100 GeV and Ee = 20 GeV,Ep = 250 GeV, respectively. The intermediate xV bin is accessible with either energy settingand gives almost identical uncertainty bands in both cases.

1.2.2 The Nucleus, a Laboratory for QCD173

The atomic nucleus is a “molecule” in QCD, made of nucleons, which, in turn, are174

bound states of quarks and gluons. Understanding the emergence of nuclei from QCD is175

an ultimate long-term goal of nuclear physics. With its wide kinematic reach, as shown in176

Fig. 5 (Left), the capability to probe a variety of nuclei in both inclusive and semi-inclusive177

DIS measurements, the EIC would be the first experimental facility capable of exploring the178

internal 3-dimensional sea quark and gluon structure of a fast-moving nucleus. Furthermore,179

the nucleus itself would be an unprecedented QCD laboratory for discovering the collective180

behavior of gluonic matter of high density, and for studying the propagation of a fast-moving181

color charge in a nuclear medium.182

6

Executive Summary: Cartoons and Figures

Figures from the ES

Physics with polarized nucleon


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Measurements with A ≥ 56 (Fe):

eA/μA DIS (E-139, E-665, EMC, NMC)

νA DIS (CCFR, CDHSW, CHORUS, NuTeV)

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DGLAP

BFKL

Figure 1.5: Left: The resolution (Q2) vs. the parton momentum fraction (x) landscape ofavailable data points for lepton-nucleus (DIS) and Drell-Yan (DY) experiments at fixed targetenergies, compared to the kinematic reach of the EIC at two di�erent center of mass energies.Right: Schematic map of high energy QCD in the resolution (Q2) vs. energy (Y=ln(1/x))plane. The high gluon density saturation region is shown.

QCD at Extreme Parton Density:183

In QCD, the large soft gluon density enables the non-linear process of gluon-gluon recom-184

bination to limit the density growth. Such a QCD self-regulation mechanism necessarily185

generates a dynamic scale from the interaction of high density massless gluons, known as186

the saturation scale, Qs, at which gluon splitting and recombination reach a balance. At187

this scale the density of gluons is expected to saturate, producing new and universal prop-188

erties of hadronic matter. The saturation scale Qs separates the condensed and saturated189

soft gluonic matter from the dilute but confined quarks and gluons in a hadron, as shown190

in Fig. 1.5 (Right).191

The existence of such a saturated soft gluon matter, often referred to as Color Glass192

Condensate, is a direct consequence of gluon self-interaction in QCD. The dynamic sat-193

uration scale Qs may be impossible to reach unambiguously in electron-proton scattering194

without a multi-TeV proton beam. However, heavy ion beams provide precocious access to195

the saturation regime because the virtual photon in forward lepton scattering probes matter196

coherently over a characteristic length proportional to 1/xp, which can exceed the diameter197

of a Lorentz-contracted nucleus. Then all the gluons at a given impact parameter of the198

nucleus, enhanced by the nuclear diameter proportional to A1/3 with the atomic weight199

A, contribute to the probed density, reaching saturation at far lower energies than would200

be needed in electron-proton collisions. While HERA, RHIC and LHC only found hints of201

saturated gluonic matter, the EIC would have the potential to seal the case, completing the202

process started at those facilities.203

Figure 1.6 illustrates some of the dramatic predicted e�ects of gluon density saturation in204

electron-nucleus vs. electron-proton collisions at an EIC. The left frame considers coherent205

di�ractive processes, defined to include all events in which the beam nucleus remains intact206

and there is a rapidity gap containing no produced particles. As shown in the figure, gluon207

saturation greatly enhances the fraction of the total cross section accounted for by such208

events. An early measurement of coherent di�raction in e+A collisions at the EIC would209

7

10-1 101

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x < 0.01

Figure 1.6: Left:The predicted double ratio of the coherent di�ractive and total cross sectionsin electron-gold (eA) and electron-proton (ep) collisions plotted as a function of the invariantmass M2

X of the produced particles, with and without incorporation of gluon saturation. Thebands indicate projected statistical and systematic measurement uncertainties with EIC. Right:Ratios of eA to ep cross-sections for J/⇥ (blue) and � (red) production with (square) and without(circle) gluon saturation. The statistical uncertainties possible with measurements at the EICare indicated by the error bars.

provide the first unambiguous evidence for gluon saturation.210

Figure 1.6 (Right) shows that gluon saturation is predicted to suppress vector meson211

production in e+A relative to e+p collisions at EIC. The vector mesons result from quark-212

antiquark pair fluctuations of the virtual photon, which hadronize upon exchange of gluons213

with the beam proton or nucleus. The magnitude of the suppression depends on the size (or214

color dipole moment) of the quark-antiquark pair, being significantly larger for produced �215

(red points) than for J/⇥ (blue) mesons. An EIC measurement of the processes in Fig. 1.6216

(Right) would provide a powerful probe to diagnose the characteristics of the saturated217

gluons.218

Propagation of a Color Charge in QCD Matter:219

One of the key pieces of evidence for the discovery of quark-gluon plasma (QGP) at RHIC220

is jet quenching, manifested as a strong suppression of fast-moving hadrons produced in221

the very hot matter created in relativistic heavy ion collisions. The suppression is believed222

to be due to the energy loss of partons traversing the QGP. It has been puzzling that the223

production is nearly as much suppressed for heavy as for light mesons, even though a heavy224

quark is much less likely to lose its energy via medium-induced radiation of gluons. Some of225

the remaining mysteries surrounding heavy vs. light quark interactions in hot matter can226

be illuminated by EIC studies of related phenomena in cold nuclear matter. For example,227

the variety of ion beams available for electron-nucleus collisions at the EIC would provide228

a femtometer filter to test and to help determine the correct mechanism by which quarks229

and gluons lose energy and hadronize in nuclear matter (see schematic in Fig. 1.7 (Left)).230

Figure 1.7 (Right) shows the ratio of number of produced mesons in electron-nucleus231

and electron-deuteron collisions for pion (light mesons) and D0-mesons (heavy mesons) at232

8

0.30

0.50

0.70

0.90

1.10

1.30

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zM

ultip

licity

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io

D0 mesons (lower energy)Pions (lower energy)D0 mesons (higher energy)Pions (higher energy)D0 (10% less energy loss)Wang, pions (lower energy)Wang, pions (higher energy)

0.01 < y < 0.85, x > 0.1, 10 fb-1

Higher energy: 25 GeV2< Q

2< 45 GeV

2, 140 GeV < < 150 GeV

Lower energy : 8 GeV 2< Q

2<12 GeV2, 32.5 GeV< < 37.5 GeV

1- D0systematic

uncertainty

1- pion systematicuncertainty

v

v

0.0 0.2 0.4 0.6 0.8 1.0

Figure 1.7: Left: Schematic drawings explaining interactions of the parton moving throughcold nuclear matter: the hadron is formed outside (top) or inside (bottom) the nucleus.Right:Ratio of semi-inclusive cross section for producing a pion (red), composed of light quarks, anda D0 meson (blue) composed of heavy quarks in electron-Lead collisions to the same producedin electron-deuteron as function of z, the momentum fraction of the exchanging photon carriedby the observed meson. Statistical uncertainties are shown in each case.

both low and high photon energy �, as a function of z - the momentum fraction of the233

virtual photon taken by the observed meson. The calculation of red lines and blue symbols234

assumes the mesons are formed outside of the nucleus, as shown in the top sketch of Fig. 1.7235

(Left), while the square symbols are simulated according to a model where a color neutral236

pre-hadron was formed inside the nucleus, like in the bottom sketch of Fig. 1.7 (Left).237

The di�erence between the red lines and the red square symbols would provide the first238

direct information on when the meson is formed. Unlike the suppression expected for pion239

production at all z, the ratio of heavy meson production could be larger than the unity240

due to very di�erent hadronization properties of heavy mesons. The discovery of such a241

dramatic di�erence in multiplicity ratios between light and heavy mesons at the EIC would242

shed light on the hadronization process and what governs the transition from quarks to243

hadrons.244

The Distribution of Quarks and Gluons in the Nucleus:245

The EMC experiment at CERN and experiments in the following two decades clearly re-246

vealed that the distribution of quarks in a fast-moving nucleus is not a simple superposition247

of their distributions within nucleons. Instead, the ratio of nuclear over nucleon structure248

functions follows a non-trivial function of Bjorken xB, deviating significantly from unity,249

with a suppression (often referred to as nuclear shadowing) as xB decreases. Amazingly,250

there is as of yet no knowledge whether the same holds true for gluons. With its much251

wider kinematic reach in both x and Q, the EIC could measure the suppression of the252

structure functions to a much lower value of x, approaching the region of gluon saturation.253

In addition, the EIC could for the first time reliably quantify the nuclear gluon distribution254

over a wide range of momentum fraction x. With its unprecedented luminosity, the EIC255

9

Figures from the ES

Physics with Nuclei


eSTAR

ePH

EN

IX

30 GeV

30 GeV

100 m

Lina

c 2.

45 G

eV

Linac 2.45 GeV

Coh

eren

t e-c

oole

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e-gun

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dump

0.60 GeV

5.50 GeV

10.4 GeV

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20.2 GeV

25.1 GeV

30.0 GeV

3.05 GeV

7.95 GeV

12.85 GeV

17.75 GeV

22.65 GeV

27.55 GeV

0.6 GeV

0.6 GeV

27.55 GeV

New detector

Figure 1.8: Top: The schematic of eRHIC at BNL: will require construction of the electronbeam facility (indicated in RED) to collide with the RHIC blue beam at three interaction points.Bottom: 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 GeVCEBAF.

the accelerator a particularly challenging feature of the design. Lessons learned from past ex-296

perience at HERA have been considered while designing the EIC interaction region. Driven297

by the demand for high precision on particle detection and identification of final state par-298

ticles in both e-p and e-A programs, modern particle detector systems will be at the heart299

of the EIC. In order to keep the detector costs manageable, R&D e�orts are under way300

on various novel ideas for: compact (fiber sampling & crystal) calorimetry, tracking (NaI301

coated GEMs, GEM size & geometries), particle identification (compact DIRC, dual ra-302

diator RICH & novel TPC) and high radiation tolerance for electronics. Meeting these303

R&D challenges will keep the U.S. nuclear science community at the cutting edge in both304

accelerator and detector technology.305

11

Figures from the ES

BEYOND AUGUST 24TH: FINALIZING THE WP Aim at a time line which prepares the final WP around the DNP Townhall meetings (last week of October)



Contents













3


Structure Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . 78Di-Hadron Correlations . . . . . . . . . . . . . . . . . . . . . . . . . 79Measurements of Di�ractive Events . . . . . . . . . . . . . . . . . . . 82

3.2 Quarks and gluons in the nucleus . . . . . . . . . . . . . . . . . . . . . . . . 863.2.1 The distributions of quarks and gluons in a nucleus . . . . . . . . . . 873.2.2 Propagation of a fast moving color charge in QCD matter . . . . . . 903.2.3 Strong color fluctuations inside a large nucleus . . . . . . . . . . . . 93

3.3 Connections to pA, AA and Cosmic Ray Physics . . . . . . . . . . . . . . . 943.3.1 Connections to pA Physics . . . . . . . . . . . . . . . . . . . . . . . 943.3.2 Connections to Ultrarelativistic Heavy-Ion Physics . . . . . . . . . . 97

Initial Conditions in AA collisions . . . . . . . . . . . . . . . . . . . 98Energy Loss and Hadronization . . . . . . . . . . . . . . . . . . . . . 102

3.3.3 Connections to Cosmic Ray Physics . . . . . . . . . . . . . . . . . . 104

4 Possibilities at the Luminosity Frontier: Physics Beyond the StandardModel 1064.1 Specific Opportunities in Electroweak Physics . . . . . . . . . . . . . . . . . 107

4.1.1 Charged Lepton Flavor Violation . . . . . . . . . . . . . . . . . . . . 1074.1.2 Precision Measurements of Weak Neutral Current Couplings . . . . 108

4.2 EIC Requirements for Electroweak Physics Measurements . . . . . . . . . . 111

5 The Accelerator Designs and Challenges 1125.1 eRHIC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

5.1.1 eRHIC design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1145.1.2 eRHIC interaction region . . . . . . . . . . . . . . . . . . . . . . . . 1165.1.3 eRHIC R&D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

5.2 MEIC: The Je�erson Lab Implementation . . . . . . . . . . . . . . . . . . . 1185.2.1 Je�erson Lab Staged Approach . . . . . . . . . . . . . . . . . . . . . 1185.2.2 Baseline Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

Ion Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120Collider Rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121Interaction Regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122Electron Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

6 The EIC Detector Requirements and Design Ideas 1256.1 Kinematic Coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

6.1.1 y Coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1256.1.2 Angle and momentum distributions . . . . . . . . . . . . . . . . . . 1266.1.3 Recoil Baryon angles and t resolution . . . . . . . . . . . . . . . . . 130

6.2 Detector and Interaction Region (IR) Layout . . . . . . . . . . . . . . . . . 1306.2.1 eRHIC Detector & IR Considerations and Technologies . . . . . . . 1306.2.2 Detector Design for MEIC/ELIC . . . . . . . . . . . . . . . . . . . . 133

Acknowledgment 137

References 138

4

Contents













3

















Acknowledgment 137

References 138

4
















Acknowledgment 137

References 138

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Acknowledgment 137

References 138

4

White Paper path forward: •  After ES is finalized and sent to the Tribble-II (end of August)

•  Present the WP (distribute the link of the WP draft) to the BNL and Jlab User communities, EIC collaboration lists

•  Comments gathered by a web based system that was used recently by Jefferson Laboratory for its 12-GeV White Paper.

•  Identical systems will be setup both at BNL and JLab

•  Approximately 2 weeks for the comments period •  Approximately 2 week for the conveners of sections to

accommodate them •  1 week for (JQ, Z-EM & AD) to finalize the WP beyond that

•  Aim for a final WP in the last week of October. To the BNL/Jlab managements in November


Feedback on the White Paper Keeping in mind the following things: Intended for the broader Nuclear Physics audience [High energy NP: Jlab, RHIC, but also, FRIB-community, symmetry violation seekers, theorists] who would influence the decision on EIC in the next Long Range Plan. Comprehensiveness less important than being concise and articulating the case most effectively. Length can not increase, can we reduce it to about 100 pages?


Feedback on the White Paper (Method and Dates on when, will be conveyed to you soon)

• What other things absolutely needed to strengthen the case for the EIC?

• Are there parts of current draft which are detrimental to making the case for the EIC?

• Are relative emphases and lengths amongst chapters & within chapters correct?

• Are there mistakes in the physics? • Can the presentation/articulation improve by simple changes

to the structure of the sections, inclusion of new things (replacing current parts)

• Are there better figures/cartoons which could be used?


Summary: • A good draft of the EIC White Paper is now ready, and was

released (for the first time) to THIS audience

• We anticipate your comments and suggestions on the Executive Summary (12 Pages) by August 24. We hope to send the improved draft to the Tribble-II panel by August 28. •  Comments by email to Abhay, Jianwei and Zein-Eddine

•  The comments on the rest of the White Paper: welcome but at a later date. We aim to finalize the WP by the end of October, November. Method of making the comments and the timelines will be distributed soon.


the electron ion collider: a next qcd frontier · 2019. 9. 30. · the white paper as of today: 146...

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