The Glue that binds us all

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The Glue that binds us all
Probing the nature of gluonic matter
with EIC the worlds first eA collider
Raju Venugopalan Brookhaven National
Laboratory
Phases of Matter Town Hall meeting, Jan. 12th,
2007
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Talk Outline

• Outstanding questions in QCD at high energies
• Lessons and open questions from HERA and RHIC
• How these are addressed by measurements with
  EIC
• The discovery potential of eA at EIC
• Summary

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QCD explains 99 of the mass of the visible
universe
hep-lat/0304004
Hadron mass spectrum vs quenched lattice results
Quenched QCD
full QCD

Quenched QCD (no dynamical quark-antiquark
pairs) explains hadron mass spectrum to 10
The dynamics of glue is central to our
understanding of the structure of matter
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The DIS Paradigm
Measure of resolution power
Measure of inelasticity
Measure of momentum fraction of struck quark
quarkanti-quark mom. dists.
gluon mom. dists
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Where is the glue ?
of partons per unit rapidity
momentum fraction of hadron
The proton is dominated for x lt 0.01 by glue-
which grows rapidly What happens when the
density of gluons becomes large ?
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Mechanism of gluon saturation in QCD
p, A
Saturation scale QS(x) - dynamical scale below
which non-linear QCD dynamics is dominant
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The Color Glass Condensate
In the saturation regime
Strongest fields in nature!

• CGC Classical effective theory of QCD
  describing
• dynamics of gluon fields in non-linear regime
• Novel renormalization group equations
  (JIMWLK/BK)
• describe how the QCD dynamics changes with
  energy
• A universal saturation scale QS arises
  naturally in the theory

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Saturation scale grows with energy
Typical gluon momenta are large
Typical gluon kT in hadron/nuclear wave function

• Bulk of high energy cross-sections
• obey dynamics of novel non-linear QCD regime
• Can be computed systematically in weak coupling

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Saturation scale grows with A
High energy compact (1/Q lt Rp) probes interact
coherently across nuclear size 2 RA - experience
large field strengths
Pocket formula
Enhancement of QS with A gt non-linear QCD
regime reached at significantly lower energy in
A than in proton
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New window on universal properties of the
matter in nuclear wavefunctions
Can we quantify the various regimes ?
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Evidence of non-linear saturation regime at
HERA ?
Linear pQCD describes inclusive observables
well-however hints of non-linear (higher twist)
at small x and Q2
partons per unit rapidity
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Saturation Models-excellent fits to HERA data
Kowalski et al., hep-ph/0606272
Also see Forshaw et al. hep-ph/0608161
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Caveat Saturation scale extracted from HERA
data inconsistent with model assumptions
Model assumes
Typical sat. scale is rather low... QS2 ltlt 1
GeV2
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Evidence of non-linear saturation regime at RHIC
?
Global multiplicity observables in AA described
in CGC models
Au-Au mult. at eta0
Kharzeev,Levin,Nardi
Krasnitz, RV
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DA
Kharzeev,Kovchegov,Tuchin Albacete,Armesto,Salgado
,Kovner,Wiedemann
D-Au pt spectra compared to CGC prediction
Hayashigaki, Dumitru, Jalilian-Marian
Talk by M. Leitch
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Estimates of the saturation scale from RHIC
A
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Outstanding questions in high energy QCD
(QCD Theory Workshop, DC, Dec. 15th-16th, 2006)

• What is the nature of glue at high density ?
  

• How do strong fields appear in hadronic
• or nuclear wavefunctions at high energies ?
• How do they respond to external probes or
  scattering ?
• What are the appropriate degrees of freedom ?
• Is this response universal ? (ep,pp,eA, pA, AA)

An Electron Ion Collider (EIC) can provide
definitive answers to these questions.
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The Electron Ion Collider
Quantitative QCD studies in largely terra
incognita small x-large Q2 regime

• Variable ep c.m energy up to 100 GeV and high
  luminosity
• (100 times HERA) unpolarized e-p scattering
  
• pol. e-pol. p - highest energies and collider
  mode
• for the first time (parallel Town Hall
  discussion tomorrow)
• First eA collider with wide range of nuclear
  beams
• and c.m. energy up to 63 (90) GeV/ nucleon
  
• Precision studies of QCD in nuclear media
• very high parton densities

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What are the measurements with EIC ?
See Thomas Ullrichs talk

• Momentum distributions of gluons and quarks
  in nuclei
• Space-time distributions of quarks and gluons
  in nuclei
• Extract space-time dist. of nuclear
  glue from exclusive final states
• Interaction of fast probes with nuclear media
• First semi-inclusive measurements charm
  and bottom dists.
• energy loss in nuclei
• Role of color neutral (Pomeron) excitations
  in scattering
• off nuclei
• Semi-hard (M QSA ) diffractive final
  states predicted to be gt 30 of
• cross-section

Gluon dists. measured for x lt 0.01 in nuclei for
first time
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Strong color fields are vastly more accessible
in eA at EIC relative to ep at HERA
Nuclear profile more uniform- study centrality
dependence
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The nuclear oomph factor
Saturation scale significantly enhanced in
nuclei 6 enhancement in central Au relative to
min-bias proton
Matches pocket formula to 10
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EIC can cleanly access cross-over region from
weak field to novel strong field QCD dynamics
Weak field regime Q2 gtgt QS2
Strong field regime Q2 ltlt QS2
Qualitative change in final states eg., 1/Q6
1/Q2 change in elastic vector meson production!
McDermott,Guzey,Frankfurt,Strikman
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p/D-A and AA are complementary probes to eA
Universality
Soft color exchange between proton and nucleus
breaks factorization at order 1 / Q4
Qiu,Sterman
RHIC DA and LHC AA/pA -significant discovery
potential Universality gt genuine discovery
will require complementary probes
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Summary

• EIC with variable energies, nuclear beams and
  high
• luminosity is a powerful tool to access and study
  
• universal properties of QCD at high parton
  densities
• These studies have profound ramifications for
  our
• understanding of QCD dynamics at the
  LHC-especially
• in heavy ion collisions
• The ability of EIC to distinguish between model
• predictions for measurements is discussed in the
• following talk by Thomas Ullrich.

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EXTRA SLIDES
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Inclusive measurements
Measure of resolution power
Measure of inelasticity
Measure of momentum fraction of struck quark
quarkanti-quark mom. dists.
gluon mom. dists
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Diffractive measurements
Color singlet multi-glue (Pomeron ) exchange
Very sensitive to glue mom. dists.
Extract spatial (impact parameter) dists. of
gluon fields Deg. of freedom classsical fields,
Pomeron interactions?
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DIS highlights

• Bjorken scaling the parton model.
• Scaling violations QCD- asymptotic freedom,
• renormalization group precision tests of
  pQCD.
• Rapid growth of gluon density at small x,
  significant
• hard diffraction.
• Measurement of polarized structure functions
  the spin crisis.
• QCD in nuclei EMC effect, shadowing, color
  transparency,

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II Extracting gluon distributions in pA relative
to eA
Direct photons
Open charm
Drell-Yan
As many channelsbut more convolutions, kinematic
constraints-limit precision and range.
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Dramatic breakdown of factorization between
ep and pp for diffractive final states
Alvero,Collins,Terron,Whitmore
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A dependence of saturation scale - estimates
from fits to HERA and NMC data
A dependence
0.33
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Space-time dist. of strong Color Fields!
Data from
Dipole Survival Probability
In pQCD, survival probability 1
A 0.3 fm qq dipole survives only 20 of the time
scattering off center of the proton!
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Dominant impact parameters in DIS
scattering off a proton
b (GeV-1)
Strong color fields are localized here
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Hubble is taking beautiful pictures of dark
matter binding Galaxies
Hubble
Can EIC obtain similar pictures of glue
binding visible matter ?