Experimental Status of Parton Saturation at RHIC

1
Experimental Status ofParton Saturation at RHIC

• Peter SteinbergBrookhaven National Laboratory
• ISMD2003, Krakow, Poland
• 5-11 September 2003

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What we do _at_ RHIC
HydrodynamicPhase (QGP?)
EnergyDeposition
Colliding Nuclei
We measure the final state, we are most
interested in the intermediate state, so
clearly we need to understand the initial state
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Nuclear Geometry
Glauber Model
BinaryCollisions
b
Participant
ParticipantsLong distance,Coherent
Collisions Short distance,Incoherent
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Particle Density at 90o in pp AA
PHOBOS _at_ RHIC (PRL 2001)
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dN/dh Theory vs. Experiment
Why is the multiplicity so low? Where is the
dramatic risein hard processes expectedat RHIC
energies?
Eskola, QM2001
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Color Glass Condensate
Lipatov, Levin, Ryskin, McLerran, Venugopalan,
Mueller, Iancu, Jalilian-Marian, Dumitru, etc.

• Implementation of low-x QCD
• Color
• Integrates (freezes) out the hard scales (time
  dilation)
• Glass
• Coherent multi-gluon state
• Condensate
• Universal
• Same for all hadrons

Multiplicity
Particle Spectra
Soft Physics controlled by scale Qs2
Geometry Energy
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Geometric Scaling
Saturation predictsthat a single scale
dominates low-x gluon structure Predicts
geometrical scaling
Stasto, Golec-Biernat, Kwiecinski (2001)
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Geometrical Scaling _at_ RHIC

• RHIC data can be said to also show geometric
  scaling
• NB Corrections are needed
• Strangeness x 2
• Baryons / 2

Schaffner-Bielich, McLerran,Venugopalan,
Kharzeev (2001)
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Saturation Phenomenology

• Qs reflects density of partons in transverse
  plane
• Golec-Biernat-Wusthoff energy scaling of gp
  cross section
• Rapidity
• Centrality Npart scaling (sources) modified by
  thickness
• McLerran-Venugopalan?Mueller?Kharzeev/Nardi

HERAG-BW
Geometry
QCD
Initial?Final
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Centrality Dependence
Accardi Gyulassy (2003)
Many modelscan incorporatenuclear thickness
Two-componentHard Soft
One-componentCGC DGLAP(Kharzeev Nardi)
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Saturation vs. Real Data

• Basic CGC process 2?1 scattering

Antoni Szczurek, Sunday

• Overall scale
• Jacobian
• Quark counting

(LPHD)
Energy, Rapidity, Centrality
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BRAHMS dN/dy
BRAHMSrapiditydistribution
BRAHMS Preliminary 2003 Central AuAu
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Limiting Fragmentation in AA
PHOBOS AuAu
200 GeV
130 GeV
19.6 GeV

• Away from y0, low-xgluons scatter from high-x
• Free forward structure

Limiting Fragmentation
h? h - ybeam
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Limiting Fragmentation in pp
UA5 inelastic
?
3
Data from pbar p alsoshows limiting
fragmentationHow essential is partonsaturation
to this effect?
2
1
0
-2
0
-4
-6
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Limiting Behavior in ee-
DELPHI, PLB459 (1999)
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Saturation vs. pp data
Can saturation describe elementary collisions?
Kharzeev, Levin, Nardi (2002)
PHOBOS vs. UA5
Success in AuAu is helped by similar shape with
pp
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Is Soft Physics Universal?
PHOBOS(submitted to PRL)
ee- AA despite different Q (Qs vs. ?s) pT,
flow, etc. Simple but a puzzle for CGC ? is
ee- a dense state?
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Soft Scaling in AuAu
PHOBOS
Total charged multiplicityreflects soft
scaling(i.e. participants) ? not much room
forHard Soft
Slight modificationof original questions
Why is the multiplicity so low,and why is it so
close to ee-? Where is the contributionfrom
hard processes expectedin RHIC central
collisions?
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Violation of Ncoll scaling
b
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Soft Scaling of Hard Processes
PHOBOS studied thisin detail. Npart scaling
seen at low and high pT
After first showing ofthis effect in July
2002,Kharzeev, Levin, McLerranoffered a
theoreticaldescription
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A New Phase Diagram?
ln 1/x
Quantum evolutionretains correlationscharacteris
tic ofsoft physics
QuantumColor Fluid(ExtendedScaling)
CGC
PartonGas
NPQCD
D. Kharzeev
ln Q2
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A Control Experiment
To rule out saturation scenario, RHIC devoteda
large fraction of Run 3 to dAu collisions
Non-saturateddeuteronwave function
d
Cronin
Saturatednuclearwave function
A
Suppressed
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First RHIC dAu Results
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Search for suppression in dAu
STAR
PHENIX
Striking absence of suppression claimed by
allexperiments, especially relative to central
AuAu Dominant physics seems to be Cronin
Effect (Rgt1)
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Centrality Dependence
h1
PHOBOS
70-100
0-20
Centrality dependence rulesout an onset of
saturationin central dAu
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Is CGC _at_ RHIC Dead?

• This has been a major set-back for CGC-based
  phenomenology
• Lessons from AuAu not applicable to dAu
• Was success in AuAu fortuitous?
• However, we seem to observe dominance of soft
  degrees of freedom
• Saturation provides a natural framework
• A problematic model should not invalidate a
  compelling theory

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Npart Scaling in dA?
STAR Data(PAS Representation)
Can perform sameanalysis for AA dA Is
this a similar structurewith different
parameters?
AuAu
dAu
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Summary Conclusions

• Saturation physics offers a compelling
  perspective on nuclear collisions
• Dominance of soft degrees of freedom due to
  initial state gluon coherence
• A single scale controlling various physics
• Diminished importance of final state effects
• Regularities in data supportive of CGC
• Multiplicities, limiting fragmentation, mT
  scaling, Npart scaling at high pT
• However, not unique to saturation (or even heavy
  ions)
• dAu failure may not be the end of the story

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Extra Slides
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Update to mT scaling
With new PHENIX data,scaling plot is
somewhatmodified

1. Weak corrections toprotons
2. Scaling is somewhatdifferent (20 vs. 100)

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mT scaling in pp
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Is Saturation Unique?
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130 GeV
200 GeV
19.6 GeV
PHOBOS Preliminary
dN/dh
Most Central
Npart
h
h
h

• AuAu collisions at ?s19.6, 130, 200 GeV
• dN/dh for hlt5.4 over full azimuth
• Centrality from paddles (130/200) Nhits (19.6)
• Top 50 of total cross section (Npart65-360)

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Centrality Dependence of dN/dh?
CentralityDependence
Interpretation
Location
h -5 Rises Saturation? 2-component?
h -1.5 Stable Scaling
h 1.5 Falls Cascading in spectators?

• Are these effects related?
• Long-range correlations?
• Energy conservation?
• Stopping?
• Other collision systems?

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Mid-rapidity Revisited
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Geometric Scaling Revisited
Amusing repeat of hard?soft duality seen in
geometrical scaling
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KLN in y and h
PHOBOS Data 200 GeV Central AuAuKLN
Implementation by P.A.S.