Physics Opportunities with an Electron Ion Collider

1
Physics Opportunities with an Electron Ion
Collider

• Thomas Ullrich, BNL
• ISMD 2007, LBL Berkeley
• August 6, 2007

2
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 gt 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)

4
The Issue With Our Current Understanding

• Established 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
• Whats 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

Regimes of QCD Wave Function
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eA Studying Non-Linear Effects

• Scattering of electrons off nuclei
• Probes interact over distances L (2mN x)-1
• For L gt 2 RA A1/3 probe cannot distinguish
  between nucleons in front or back of nucleon
• Probe interacts coherently with all nucleons

Nuclear Oomph Factor Pocket Formula
Enhancement of QS with A ? non-linear QCD regime
reached at significantly lower energy in A than
in proton
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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 94022002,
Kowalski, Teaney, PRD 68114005)
8
Universality Geometric Scaling

• Crucial 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 gt Qs

x lt 0.01
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Qs a Scale that Binds them All
Geometrical scaling
Nuclear shadowing
proton ? 5
nuclei
Freund et al., hep-ph/0210139
Are hadrons and nuclei wave function universal at
low-x ?
10
Understanding Glue in Matter

• Understanding 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?
• ep and eA
• Exploration of saturation regime better in eA
  (Oomph Factor)
• What is the space-time distributions of gluons in
  matter?
• ep and eA
• Unknown in eA
• How do fast probes interact with the gluonic
  medium?
• Strength of eA
• Do strong gluon fields effect the role of color
  neutral excitations (Pomerons)?
• ep and eA
• Unknown in eA

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eA Landscape and a New Electron Ion Collider

• Well mapped in ep
• Not so for lA (nA)
• Electron Ion Collider (EIC)
• L(EIC) gt 100 ? L(HERA)
• Different EIC Concepts
• eRHIC
• ELIC

Terra incognita small-x, Q ? Qs high-x,
large Q2
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Electron Ion Collider Concepts

• eRHIC (BNL) Add Energy Recovery Linac to RHIC
• Ee 10 (20) GeV
• EA 100 GeV (up to U)
• ?seN 63 (90) GeV
• LeAu (peak)/n 2.91033 cm-2 s-1

• ELIC (JLAB) Add hadron beam facility to existing
  electron facility CEBAF
• Ee 9 GeV
• EA 90 GeV (up to Au)
• ?seN 57 GeV
• LeAu (peak)/n 1.61035 cm-2 s-1

Both allow for polarized ep collisions !
13
What is the Momentum Distribution of Gluons?

• Gluon distribution G(x,Q2)
• Shown here
• Scaling violation in F2 dF2/dlnQ2
• FL as G(x,Q2)
• Other Methods
• 21 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
15
FL at EIC Measuring the Glue Directly
FL requires ?s scan Q2/xs y Here ?Ldt 5/A
fb-1 (10100) GeV 5/A fb-1 (1050) GeV
2/A fb-1 (550) GeV statistical error only
? G(x,Q2) with great precision
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 r, J/y, DVCS)
• color transparency ? color opacity
• Deep Virtual Compton Scattering (DVCS)
• Integrated DVCS cross-section sDVCS A4/3
• Measurement of structure functions for various
  mass numbers A (shadowing, EMC effect) and its
  impact parameter dependence

17
Hadronization and Energy Loss

• nDIS
• 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
Energy transfer in lab rest frame EIC 10 lt n lt
1600 GeV HERMES 2-25 GeV EIC can measure
heavy flavor energy loss
zh Eh/n
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Connection to pA Physics
F. Schilling, hex-ex/0209001

• eA and pA 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/unive
  rsality ?
• Issues
• pA lacks the direct access to x, Q2

Breakdown of factorization (ep HERA versus pp
Tevatron) seen for diffractive final states.
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Connection to RHIC LHC Physics

• Matter at RHIC
• thermalizes fast (t0 0.6 fm/c)
• We dont 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

• 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.

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EIC Timeline Status

• NSAC Long Range Plan 2007
• Recommendation 6M/year for 5 years for machine
  and detector RD
• 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
• RD to guide EIC design decision 2011
• EIC design decision 2011
• MOUs with foreign countries? 2012

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Summary

• The EIC presents a unique opportunity in high
  energy nuclear
• physics and precision QCD physics
• eA
• 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)
• ep (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/

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The EIC Collaboration

• 17C. 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

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Additional Slides
25
Regimes of QCD Wave Function in 3D
26
Charm at EIC in eA
Based on HVQDIS model, J. Smith

• EIC allows multi-differential measurements of
  heavy flavor
• covers and extend energy range of SLAC, EMC,
  HERA, and JLAB allowing study of wide range of
  formation lengths

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Diffractive Physics in eA
Standard DIS event
Diffractive event
?
Detector activity in proton direction

• HERA/ep 15 of all events are hard diffractive
• Diffractive cross-section sdiff/stot in eA ?
• 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
xIP momentum fraction of the pomeron w.r.t the
hadron

• Distinguish between linear evolution and
  saturation models
• Insight into the nature of pomeron
• Search for exotic objects (Odderon)

Curves Kugeratski, Goncalves, Navarra, EPJ C46,
413
29
A Truly Universal Regime ?

• 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
A.H. Mueller, hep-ph/0301109

• 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

30
Spin Physics at the EIC - The Quest for ?G

• Spin Structure of the Proton
• ½ ½ ?? ?G Lq Lg
• quark contribution ?S 0.3
• gluon contribution ?G 1 1 ?

?G a quotable property of the proton (like
mass, charge) Measure through scaling violation
Superb sensitivity to ?g at small x!
31
What Do We Know About Glue in Matter?

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