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Physics Opportunities with an Electron Ion Collider
Thomas Ullrich, BNL
ISMD 2007, LBL Berkeley
August 6, 2007
2
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aQCD GGAqTqgqmiqL4
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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
3
What Do We Know About Glue in Matter?
Scaling violation: dF2/dlnQ2 and linear DGLAP Evolution G(x,Q2)
Deep Inelastic Scattering : d2 ep eX
dxdQ2 4 e.m.
2
xQ4 1 y y 2
2
F2(x,Q2)
y 2
2FL (x,Q2)
Gluons dominate low-x wave function
)201( xG
)201( xS
vxu
vxd
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 proton
(QsA )2 cQ0
2 A
x
1/3
Qs2 ~
s xG(x,Qs2)
RA2 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)
Note :
Q2 Qs2 s s(Q
2)
Q2 Qs2 s 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
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
11
eA Landscape and a New Electron Ion Collider
Well mapped in e+pNot so for ℓ+A (A)
Electron Ion Collider (EIC):L(EIC) > 100 L(HERA)
Different EIC Concepts: eRHIC ELIC
Terra incognita: small-x, Q Qs
high-x, large Q2
12
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 !
13
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
14
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
d2 ep eX
dxdQ2
4 2
xQ41 y
y 2
2
F2(x,Q2)
y 2
2FL (x,Q2)
15
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
d2 ep eX
dxdQ2
4 2
xQ41 y
y 2
2
F2(x,Q2)
y 2
2FL (x,Q2)
16
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
17
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/Energy transfer in lab rest frameEIC: 10 < < 1600 GeV HERMES: 2-25 GeVEIC: can measure heavy flavor energy loss
18
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.
19
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
20
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
21
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 “MOU’s” with foreign countries? 2012
22
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/
23
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 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
96 Scientists, 28 Institutions, 9 countries
26
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
27
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
?
28
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
d4 eh eXh
dxdQ2ddt
4 e.m.2
2Q41 y
y 2
2
F2
D y 2
2FL
D
29
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
30
dg1
d log(Q2) g(x,Q2)
G g(x,Q2)dxx0
x1
Superb sensitivity to g at small x!
Spin Physics at the EIC - The Quest for G
Spin Structure of the Proton
½ = ½ + 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:
31
What Do We Know About Glue in Matter?
Scaling violation: dF2/dlnQ2 and linear DGLAP Evolution G(x,Q2)
Deep Inelastic Scattering : d2 ep eX
dxdQ2 4 e.m.
2
xQ4 1 y y 2
2
F2(x,Q2)
y 2
2FL (x,Q2)
Gluons dominate low-x wave function
)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 )( kkqQ
pq
Qx
2
2