Physics Opportunities with e+A Collisions at an Electron Ion ColliderA New Experimental Quest

to Study the Glue That Binds us All Thomas Ullrich, BNL

Hall C Meeting, JLAB

August 9, 2007

2

mnmnm

mm

m gg aaa

aQCD GGAqTqgqmiqL41

)()( ---ﾶ=

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

Requires fundamental investigation via experiment

3

What Do We Know About Glue in Matter?

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

Deep Inelastic Scattering : d 2s epﾮeX

dxdQ2=

4pa e .m .2

xQ41- y +

y 2

2

�

� �

�

� �F2 (x,Q2 ) -

y 2

2FL (x,Q2 )

�

� �

�

� �

Gluons dominate low-x wave function

)201( ﾠxG

)201( ﾠxS

vxu

vxd

ﾠ

4

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

5

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

6

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 protonﾠ

(QsA )2 ﾻcQ0

2 Ax

�

� �

�

� �

1/3

ﾠ

Qs2 ~

a s xG(x ,Qs2 )

pRA2

HERA : xG ~1

x 0.3 A dependence : xGA ~ A

7

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)

ﾠ

Note :

Q2 > Qs2 a s = a s(Q

2)

Q2 < Qs2 a s = a s(Qs

2)

8

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

9

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

10

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

?

“Research is what I'm doing when I don't know what I'm doing. “

Wernher von Braun

11

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

12

eA Landscape and a New Electron Ion Collider

Well mapped in e+pNot so for ℓ+A (nA)

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

Different EIC Concepts: eRHIC ELIC

Terra incognita: small-x, Q Qs

high-x, large Q2

13

Electron Ion Collider Concepts eRHIC (BNL): Add Energy

Recovery Linac to RHICEe = 10 (20) GeVEA = 100 GeV (up to U)seN = 63 (90) GeVLeAu (peak)/n ~ 2.9·1033 cm-2 s-1

PHENIX

STAR

e-cooling (RHIC

II)

Four e-beam passes

Main ERL (2 GeV per pass)

Electron Cooling

Snake

Snake

IR

IReRHIC(Linac-Ring)

ELIC

ELIC (JLAB): Add hadron beam facility to existing electron facility CEBAFEe = 9 GeVEA = 90 GeV (up to Au)seN = 57 GeVLeAu (peak)/n ~ 1.6·1035 cm-2 s-1

Both allow for polarized e+p collisions !

14

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

15

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

ﾠ

d2s epﾮ eX

dxdQ2=

4pa 2

xQ41 - y +

y2

2

�

� �

�

� �F2 (x,Q2 ) -

y2

2FL (x ,Q2 )

�

� �

�

� �

16

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

ﾠ

d2s epﾮ eX

dxdQ2=

4pa 2

xQ41 - y +

y2

2

�

� �

�

� �F2 (x,Q2 ) -

y2

2FL (x,Q2 )

�

� �

�

� �

17

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

18

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

?

19

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/xIPﾠ

d4s ehﾮ eXh

dxdQ2dbdt=

4pa e.m .2

b 2Q41 - y +

y2

2

�

� �

�

� �F2D -

y 2

2FL

D �

� �

�

� �

20

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/nEnergy transfer in lab rest frameEIC: 10 < n < 1600 GeV HERMES: 2-25 GeVEIC: can measure heavy flavor energy loss

21

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.

22

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

23

Experimental Aspects

Concepts: Focus on the rear/forward acceptance and thus on low-x / high-x physics

compact system of tracking and central electromagnetic calorimetry inside a magnetic dipole field and calorimetric end-walls outside

Focus on a wide acceptance detector system similar to HERA experiments allow for the maximum possible Q2 range.

I. Abt, A. Caldwell, X. Liu, J. Sutiak, hep-ex 0407053

J. Pasukonis, B.Surrow, physics/0608290

24

EIC Timeline & Status NSAC Long Range Plan 2007

Recommendation: $6M/year for 5 years for machine and detector R&D

Goal for Next Long Range Plan 2012 High-level (top) recommendation for construction

EIC Roadmap (Technology Driven) Finalize Detector Requirements from Physics 2008 Revised/Initial Cost Estimates for eRHIC/ELIC 2008 Investigate Potential Cost Reductions 2009 Establish process for EIC design decision 2010 Conceptual detector designs 2010 R&D to guide EIC design decision 2011 EIC design decision 2011 High priority in Long Range Plan 2012

25

SummaryThe EIC presents a unique opportunity in high energy nuclear

physics and precision QCD physics e+A

Study the Physics of Strong Color Fields Establish (or not) the existence of the saturation regime Explore non-linear QCD Measure momentum & space-time of glue

Study the nature of color singlet excitations (Pomerons) Test and study the limits of universality (eA vs. pA)

e+p (polarized) Precisely image the sea-quarks and gluons to determine the spin,

flavor and spatial structure of the nucleon

For more see: http://web.mit.edu/eicc/

26

The EIC Collaboration17C. 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, 3M. Lamont, 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 Institut 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

96 Scientists, 28 Institutions, 9 countries

27

Additional Slides

28

Regimes of QCD Wave Function in 3D

29ﾠ

dg1

d log(Q2)ﾵ- Dg(x,Q2 )

DG = Dg(x,Q2 )dxx= 0

x= 1

Superb sensitivity to g at small x!

Spin Physics at the EIC - The Quest for G

Spin Structure of the Proton

½ = ½ S + G + Lq + Lg

quark contribution ΔΣ ≈ 0.3 gluon contribution ΔG ≈ 1 ± 1 ?

G: a “quotable” property of the proton (like mass, charge)

Measure through scaling violation:

30

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

31

What Do We Know About Glue in Matter?

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

Deep Inelastic Scattering : d 2s epﾮeX

dxdQ2=

4pa e .m .2

xQ41- y +

y 2

2

�

� �

�

� �F2 (x,Q2 ) -

y 2

2FL (x,Q2 )

�

� �

�

� �

Gluons dominate low-x wave function

)201( ﾠxG

)201( ﾠxS

vxu

vxd

ﾠ

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 )( mm kkqQ ﾠ--=-=

pq

Qx

2

2

=