Physics Opportunities with an Electron Ion ColliderPart I: e+A (Thomas Ullrich)Part II: e+p (Rolf Ent)

Rolf Ent and Thomas Ullrich

for the EIC Working Group

NSAC Long Range Plan Meeting

Galveston, TX

April 30 – May 4, 2007

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EIC in the 2002 Long Range Plan

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EIC White Papers 2007 The Electron Ion Collider (EIC)

White Paper The GPD/DVCS White Paper Position Paper: e+A Physics at an

Electron Ion Collider The eRHIC machine: Accelerator

Position Paper ELIC ZDR Draft

Available at:• NSAC LRP2007 home page• Rutgers Town Meeting page• http://www.bnl.gov/eic

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The EIC Working Group17C. Aidala, 28E. Aschenauer, 10J. Annand, 1J. Arrington, 26R. Averbeck, 3M. Baker, 26K. Boyle, 28W. Brooks, 28A. Bruell, 19A. Caldwell, 28J.P. Chen, 2R. Choudhury, 10E. Christy, 8B. Cole, 4D. De Florian, 3R. Debbe, 26,24-1A. Deshpande, 18K. Dow, 26A. Drees, 3J. Dunlop, 2D. Dutta, 7F. Ellinghaus, 28R. Ent, 18R. Fatemi, 18W. Franklin, 28D. Gaskell, 16G. Garvey, 12,24-

1M. Grosse-Perdekamp, 1K. Hafidi, 18D. Hasell, 26T. Hemmick, 1R. Holt, 8E. Hughes, 22C. Hyde-Wright, 5G. Igo, 14K. Imai, 10D. Ireland, 26B. Jacak, 15P. Jacobs, 28M. Jones, 10R. Kaiser, 17D. Kawall, 11C. Keppel, 7E. Kinney, 18M. Kohl, 9H. Kowalski, 17K. Kumar, 2V. Kumar, 21G. Kyle, 13J. Lajoie, 16M. Leitch, 27A. Levy, 27J. Lichtenstadt, 10K. Livingstone, 20W. Lorenzon, 145. Matis, 12N. Makins, 6G. Mallot, 18M. Miller, 18R. Milner, 2A. Mohanty, 3D. Morrison, 26Y. Ning, 15G. Odyniec, 13C. Ogilvie, 2L. Pant, 26V. Pantuyev, 21S. Pate, 26P. Paul, 12J.-C. Peng, 18R. Redwine, 1P. Reimer, 15H.-G. Ritter, 10G. Rosner, 25A. Sandacz, 7J. Seele, 12R. Seidl, 10B. Seitz, 2P. Shukla, 15E. Sichtermann, 18F. Simon, 3P. Sorensen, 3P. Steinberg, 24M. Stratmann, 22M. Strikman, 18B. Surrow, 18E. Tsentalovich, 11V. Tvaskis, 3T. Ullrich, 3R. Venugopalan, 3W. Vogelsang, 28C. Weiss, 15H. Wieman,15N. Xu,3Z. Xu, 8W. Zajc.

1Argonne National Laboratory, Argonne, IL; 2Bhabha Atomic Research Centre, Mumbai, India; 3Brookhaven National Laboratory, Upton, NY; 4University of Buenos Aires, Argentina; 5University of California, Los Angeles, CA; 6CERN, Geneva, Switzerland; 7University of Colorado, Boulder,CO; 8Columbia University, New York, NY; 9DESY, Hamburg, Germany; 10University of Glasgow, Scotland, United Kingdom; 11Hampton University, Hampton, VA; 12University of Illinois, Urbana-Champaign, IL; 13Iowa State University, Ames, IA; 14University of Kyoto, Japan; 15Lawrence Berkeley National Laboratory, Berkeley, CA; 16Los Alamos National Laboratory, Los Alamos, NM; 17University of Massachusetts, Amherst, MA; 18MIT, Cambridge, MA; 19Max Planck Institüt für Physik, Munich, Germany; 20University of Michigan Ann Arbor, MI; 21New Mexico State University, Las Cruces, NM; 22Old Dominion University, Norfolk, VA; 23Penn State University, PA; 24RIKEN, Wako, Japan; 24-1RIKEN-BNL Research Center, BNL, Upton, NY; 25Soltan Institute for Nuclear Studies, Warsaw, Poland; 26SUNY, Stony Brook, NY; 27Tel Aviv University, Israel; 28Thomas Jefferson National Accelerator Facility, Newport News, VA

95 Scientists, 28 Institutions, 9 countries

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μνμνμ

μμ

μ γγ aaa

aQCD GGAqTqgqmiqL4

1)()( −−−∂=

Theory of Strong Interactions: QCD

“Emergent” Phenomena not evident from Lagrangian Asymptotic Freedom & Color Confinement In large part due to non-perturbative structure of QCD vacuum

Gluons: mediator of the strong interactions Determine essential features of strong interactions Dominate structure of QCD vacuum (fluctuations in gluon fields) Responsible for > 98% of the visible mass in universe

Hard to “see” the glue in the low-energy world Gluon degrees of freedom “missing” in hadronic spectrum

but drive the structure of baryonic matter at low-x are crucial players at RHIC and LHC

QCD requires fundamental investigation via experiment

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Goals & Key Questions …

Explore the new QCD frontier: strong color fields in nuclei How do the gluons contribute to the structure of the nucleus? What are the properties of high density gluon matter? How do fast quarks or gluons interact as they traverse nuclear

matter?

Precisely image the sea-quarks and gluons in the nucleon How do the gluons and sea-quarks contribute to the spin structure

of the nucleon? What is the spatial distribution of the gluons and sea quarks in the

nucleon? How do hadronic final-states form in QCD?

Common goals from LRP White Papers of Hadron Phases of QCD Matter & QCD and Hadron Physics:

How do we understand the visible matter in our universe in terms of the fundamental quarks and gluons of QCD?

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Gluons dominate low-x wave function

What Do We Know About Glue in Matter?

Scaling violation: dF2/dlnQ2 and linear DGLAP Evolution G(x,Q2)

)201( ×xG

)201( ×xS

vxu

vxd

Deep Inelastic Scattering:

e

ee

E

EEy

'−=

Measure of momentum fraction of struck quark

Measure of inelasticity

“Perfect” Tomography

Measure of resolution power:~1/wavelength2

222 )( μμ kkqQ ′−−=−=

pq

Qx

2

2

=

€

Deep Inelastic Scattering : d2σ ep →eX

dxdQ2=

4πα e.m.2

xQ41− y +

y 2

2

⎛

⎝ ⎜

⎞

⎠ ⎟F2(x,Q2) −

y 2

2FL (x,Q2)

⎡

⎣ ⎢

⎤

⎦ ⎥

€

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The Issue With Our Current UnderstandingEstablished Model:

Linear DGLAP evolution scheme Weird behavior of xG and FL from

HERA at small x and Q2 Could signal saturation, higher twist

effects, need for more/better data? Unexpectedly large diffractive cross-

section

more severe:

Linear Evolution has a built in high energy “catastrophe” xG rapid rise for decreasing x and

violation of (Froissart) unitary bound must saturate

What’s the underlying dynamics?

Need new approach

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Non-Linear QCD - Saturation

BFKL Evolution in x linear explosion of color field?

New: BK/JIMWLK based models

introduce non-linear effects saturation characterized by a scale Qs(x,A) arises naturally in the Color

Glass Condensate (CGC) framework

proton

N partons new partons emitted as energy increasescould be emitted off any of the N partons

proton

N partons any 2 partons can recombine into one

Regimes of QCD Wave Function

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Scattering of electrons off nuclei: Probes interact over distances L ~ (2mN x)-1

For L > 2 RA ~ A1/3 probe cannot distinguish between nucleons in front or back of nucleon

Probe interacts coherently with all nucleons

e+A: Studying Non-Linear Effects

Nuclear “Oomph” FactorPocket Formula:

Enhancement of QS with A non-linear QCD regime reached at significantly lower energy in A than in proton€

(QsA )2 ≈ cQ0

2 A

x

⎛

⎝ ⎜

⎞

⎠ ⎟

1/3

€

Qs2 ~

α s xG(x,Qs2)

πRA2

HERA : xG ~1

x 0.3 A dependence : xGA ~ A

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Nuclear “Oomph” Factor

More sophisticated analyses more detailed picture even exceeding the Oomph from the pocket formula (e.g. Armesto et al., PRL 94:022002, Kowalski, Teaney, PRD 68:114005)

€

Note :

Q2 > Qs2 ⇒ α s = α s(Q

2)

Q2 < Qs2 ⇒ α s = α s(Qs

2)

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Universality & Geometric ScalingCrucial consequence of non-linear evolution towards saturation: Physics invariant along trajectories

parallel to saturation regime (lines of constant gluon occupancy)

Scale with Q2/Q2s(x) instead of x and

Q2 separately

Geometric Scaling Consequence of saturation which

manifests itself up to kT > Qs

x < 0.01

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Qs a Scale that Binds them All

Freund et al., hep-ph/0210139

Nuclear shadowing: Geometrical scaling

Are hadrons and nuclei wave function universal at low-x ?

proton 5

nuclei

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A Truly Universal Regime ?

Radical View: Nuclei and all hadrons have a component of their wave function

with the same behavior This is a conjecture! Needs to be tested

A.H. Mueller, hep-ph/0301109

Small x QCD evolution predicts: QS approaches universal

behavior for all hadrons and nuclei

Not only functional form f(Qs) universal but even Qs becomes the same

?

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Understanding Glue in MatterUnderstanding the role of the glue in matter involves understanding its key properties which in turn define the required measurements:

What is the momentum distribution of the gluons in matter? e+p and e+A Exploration of saturation regime better in e+A (Oomph Factor)

What is the space-time distributions of gluons in matter? e+p and e+A Unknown in e+A

How do fast probes interact with the gluonic medium? Strength of e+A

Do strong gluon fields effect the role of color neutral excitations (Pomerons)? e+p and e+A Unknown in e+A

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eA Landscape and a New Electron Ion Collider

Well mapped in e+pNot so for ℓ+A (νA)

Electron Ion Collider (EIC):L(EIC) > 100 L(HERA)

eRHIC (e+Au):Ee = 10 (20) GeVEA = 100 GeVseN = 63 (90) GeVLeAu (peak)/n ~ 2.9·1033 cm-2 s-1

ELIC (e+Au):Ee = 9 GeVEA = 90 GeVseN = 57 GeVLeAu (peak)/n ~ 1.6·1035 cm-2 s-1

Terra incognita: small-x, Q Qs

high-x, large Q2

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What is the Momentum Distribution of Gluons?Gluon distribution G(x,Q2) Shown here:

Scaling violation in F2: F2/lnQ2

FL ~ s G(x,Q2)

Other Methods: 2+1 jet rates (needs jet algorithm and modeling of hadronization

for inelastic hadron final states) inelastic vector meson production (e.g. J/) diffractive vector meson production ~ [G(x,Q2)]2

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F2 : Sea (Anti)Quarks Generated by Glue at Low x F2 will be one of the first

measurements at EIC

nDS, EKS, FGS:

pQCD based models with different amounts of shadowing

Syst. studies of F2(A,x,Q2): G(x,Q2) with precision distinguish between models

€

d2σ ep →eX

dxdQ2=

4πα 2

xQ41− y +

y 2

2

⎛

⎝ ⎜

⎞

⎠ ⎟F2(x,Q2) −

y 2

2FL (x,Q2)

⎡

⎣ ⎢

⎤

⎦ ⎥

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FL at EIC: Measuring the Glue Directly FL requires s scan

Q2/xs = y

Here: Ldt = 5/A fb-1 (10+100) GeV

= 5/A fb-1 (10+50) GeV= 2/A fb-1 (5+50) GeV

statistical error only

G(x,Q2) with great precision

€

d2σ ep →eX

dxdQ2=

4πα 2

xQ41− y +

y 2

2

⎛

⎝ ⎜

⎞

⎠ ⎟F2(x,Q2) −

y 2

2FL (x,Q2)

⎡

⎣ ⎢

⎤

⎦ ⎥

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The Gluon Space-Time Distribution What we know is mostly the momentum distribution of glue?

How is the glue distributed spatially in nuclei? Gluon density profile: small clumps or uniform ?

Various techniques & methods: Exclusive final states (e.g. vector meson production , J/, DVCS)

color transparency color opacity Deep Virtual Compton Scattering (DVCS)

Integrated DVCS cross-section: DVCS ~ A4/3

Measurement of structure functions for various mass numbers A (shadowing, EMC effect) and its impact parameter dependence

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Hadronization and Energy LossnDIS: Suppression of high-pT hadrons analogous but weaker than at RHIC Clean measurement in ‘cold’ nuclear matter

Fundamental question: When do colored partons get neutralized?

Parton energy loss vs. (pre)hadron absorption

zh = Eh/νEnergy transfer in lab rest frameEIC: 10 < ν < 1600 GeV HERMES: 2-25 GeVEIC: can measure heavy flavor energy loss

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Charm at EIC in e+A

EIC: allows multi-differential measurements of heavy flavorcovers and extend energy range of SLAC, EMC, HERA, and JLAB

allowing study of wide range of formation lengths

Based on H

VQ

DIS

model, J. S

mith

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Diffractive Physics in e+A‘Standard DIS event’Diffractive event

Detector activity in proton direction

HERA/ep: 15% of all events are hard diffractive Diffractive cross-section diff/tot in e+A ?

Predictions: ~25-40%? Look inside the “Pomeron”

Diffractive structure functions Diffractive vector meson production ~ [G(x,Q2)]2

?

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Diffractive Structure Function F2D

at EIC

Distinguish between linear evolution and saturation models

Insight into the nature of pomeron Search for exotic objects (Odderon)

xIP = momentum fraction of the pomeron w.r.t the hadron

Curves: Kugeratski, Goncalves, Navarra, EPJ C46, 413

= x/xIP €

d4σ eh →eXh

dxdQ2dβdt=

4πα e.m.2

β 2Q41− y +

y 2

2

⎛

⎝ ⎜

⎞

⎠ ⎟F2

D −y 2

2FL

D ⎡

⎣ ⎢

⎤

⎦ ⎥

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Connection to p+A Physics e+A and p+A provide excellent

information on properties of gluons in the nuclear wave functions

Both are complementary and offer the opportunity to perform stringent checks of factorization/universality

Issues: p+A lacks the direct access to x, Q2

F. Schilling, hex-ex/0209001

Breakdown of factorization (e+p HERA versus p+p Tevatron) seen for diffractive final states.

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Matter at RHIC: thermalizes fast ( ~ 0.6 fm/c) We don’t know why and how? Initial conditions? G(x, Q2)

Role of saturation ? RHIC → forward region LHC → midrapidity

bulk (low-pT matter) & semi-hard jets

Jet Quenching: Need Refererence: E-loss in

cold matter No HERMES data for

charm energy loss in LHC energy range

Connection to RHIC & LHC Physics

RHICLHC

EIC provides new essential input:• Precise handle on x, Q2

• Means to study exclusive effects

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Summary (e+A)

e+A collisions at an EIC allow us to: Explore the Physics of Strong Color Fields

Measure properties (momentum & space-time) of glue Explore non-linear QCD Existence of universal saturation regime ?

Understand how fast partons interact as they traverse nuclear matter & new insight into fragmentation processes

Clarify the nature of color singlet excitations (Pomerons)