Transcript The Glue that binds us all Probing the nature of gluonic matter with EIC: the world’s first eA collider Raju Venugopalan Brookhaven National Laboratory Phases.

The Glue that binds us all
Probing the nature of gluonic matter
with EIC: the world’s first eA collider
Raju Venugopalan
Brookhaven National Laboratory
Phases of Matter Town Hall meeting, Jan. 12th, 2007
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
Q 2   q 2  ( k   k  ) 2
 
Q 2  4 Ee Ee sin2  e 
2
pq
Ee
2   e 
y
 1
cos  
pk
Ee
2
Q2 Q2
x

2 pq sy
quark+anti-quark
mom. dists.
Measure of
resolution
power
Measure of
inelasticity
Measure of
momentum
fraction of
struck
quark
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 < 0.01 by gluewhich grows rapidly…
What happens when the density of gluons
becomes large ?
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Mechanism of gluon saturation in QCD
Large x - bremsstrahlung
linear evolution (DGLAP/BFKL)
Small x -gluon recombination
non-linear evolution
(BK/JIMWLK)
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
o
Novel renormalization group equations (JIMWLK/BK)
describe how the QCD dynamics changes with energy
o
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:
a) obey dynamics of novel non-linear QCD regime
b) Can be computed systematically in weak coupling
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Saturation scale grows with A
High energy compact (1/Q < Rp) probes interact coherently across
nuclear size 2 RA - experience large field strengths
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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New window on universal properties of the
matter in nuclear wavefunctions
A
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 << 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 eta=0
Krasnitz, RV
Kharzeev,Levin,Nardi
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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 Ullrich’s talk

Momentum distributions of gluons and quarks in nuclei
Gluon dists. measured for x < 0.01 in nuclei for first time

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 > 30 % of
cross-section
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Strong color fields are vastly more accessible
in eA at EIC relative to ep at HERA
Nuclear profile more uniformstudy 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
Strong field
regime
Q2 >> QS2
Q2 << 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 =>
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
Q 2   q 2  ( k   k  ) 2
 
Q 2  4 Ee Ee sin2  e 
2
pq
Ee
2   e 
y
 1
cos  
pk
Ee
2
Q2 Q2
x

2 pq sy
quark+anti-quark
mom. dists.
Measure of
resolution
power
Measure of
inelasticity
Measure of
momentum
fraction of
struck
quark
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
QuickTime™ and a
OpenTIFF
charm
(LZW) decompressor
are needed to see this picture.
Drell-Yan
As many channels…but more convolutions, kinematic
constraints-limit precision and range.
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Dramatic breakdown of factorization between
ep and pp for diffractive final states
At the Tevatron:
Predictions obtained with HERA
diffractive pdfs overestimate CDF
data by a factor of about 10
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 ?
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