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Peter Steinberg ISMD2003
Experimental Status ofParton Saturation at RHIC
Peter SteinbergBrookhaven National Laboratory
ISMD2003, Krakow, Poland5-11 September 2003
Peter Steinberg ISMD2003
What we do @ RHIC
bang
HydrodynamicPhase (QGP?)
Freezeout intoknown hadrons
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…
Peter Steinberg ISMD2003
Nuclear Geometry
3/4
12
1part
N
icoll NNpart
1/3i A
b
Participant
BinaryCollisions
“Glauber Model”
Collisions:Short distance,
Incoherent
Participants:Long distance,
Coherent
Au+AuRHIC
Peter Steinberg ISMD2003
Particle Density at 90o in pp & AA
Evidence forcollective behavior?
PHOBOS @ RHIC (PRL 2001)
Peter Steinberg ISMD2003
dN/d: Theory vs. Experiment
Eskola, QM2001
Why is the multiplicity so low?
Where is the dramatic risein hard processes expected
at RHIC energies?
Peter Steinberg ISMD2003
Color Glass Condensate
• 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
Geometry & Energy
Lipatov, Levin, Ryskin, McLerran, Venugopalan, Mueller, Iancu, Jalilian-Marian, Dumitru, etc.
“Soft Physics” controlled by scale Qs2
Peter Steinberg ISMD2003
Geometric Scaling
2 2
2 20 0
~sx
Q x Q
Q Q
Saturation predictsthat a single scale dominates low-x gluon structure
Predicts “geometrical scaling”
Stasto, Golec-Biernat, Kwiecinski (2001)
Peter Steinberg ISMD2003
Geometrical Scaling @ RHIC
• RHIC data can be said to also show “geometric scaling”:
• NB: Corrections are needed• Strangeness x 2• Baryons / 2
3
3~ ~ 1T T
ss s
m md NE f
dp p p
n
Schaffner-Bielich, McLerran,Venugopalan, Kharzeev (2001)
Peter Steinberg ISMD2003
Saturation Phenomenology• Qs reflects density of partons in transverse plane
• Golec-Biernat-Wusthoff energy scaling of p cross section
• Rapidity
• Centrality – Npart scaling (sources) modified by thickness
• McLerran-VenugopalanMuellerKharzeev/Nardi
2 20
0s
xQ x Q
x
2
2
2, A s
part s
s s
S QdNcN xG x Q
dy Q
2sQ W
Geometry QCDInitialFinal
~ .25 3 20 ~ 2
@130 ( )
Q GeV
GeV RHIC
2 1/ 3~ / ~s s part coll partQ xG N N A
HERAG-BW
Peter Steinberg ISMD2003
Centrality Dependence
Many modelscan incorporate
nuclear thickness
“Two-component”:Hard + Soft
“One-component”:CGC + DGLAP
(Kharzeev & Nardi)
0
0
1 part
coll
n x NdN
d n xN
2
2ln s
partQCD
dN QcN
d
Accardi & Gyulassy (2003)
Peter Steinberg ISMD2003
Saturation vs. Real Data• Basic CGC process: 21 scattering
• Overall scale
• Jacobian
• Quark counting
gN cN
~dN dN
d dy
41 x
(LPHD)
Energy, Rapidity, Centrality
Antoni Szczurek, Sunday
Peter Steinberg ISMD2003
BRAHMS dN/dy
BRAHMS Preliminary 2003 Central Au+Au
BRAHMSrapidity
distribution
Peter Steinberg ISMD2003
= - ybeam
PHOBOS Au+Au
19.6 GeV
130 GeV
200 GeV
Limiting Fragmentation in A+A
' ~ ln ln /beam F T py x p M
Away from y=0, low-xgluons scatter from high-x“Free” forward structure
41 x
2part
dNdN
Limiting Fragmentation
Peter Steinberg ISMD2003
ddN
-2-4 0-6
beamy
1
2
3
UA5inelastic
0
Limiting Fragmentation in p+p
53 GeV
200 GeV
546 GeV
900 GeV
Data from pbar + p alsoshows limiting fragmentation
How essential is partonsaturation to this effect?
Peter Steinberg ISMD2003
Limiting Behavior in e+e-
DELPHI, PLB459 (1999)
Peter Steinberg ISMD2003
Saturation vs. pp data
Can saturation describe elementary collisions?
2 2~ 0.5GeVsQ
PHOBOS vs. UA5
Success in Au+Au is helped by similar shape with p+p
Kharzeev, Levin, Nardi (2002)
4 / 3AA pp
Peter Steinberg ISMD2003
Is Soft Physics Universal?
e+e- ~ A+A despite different Q (Qs vs. s): pT, flow, etc.
“Simple” but a puzzle for CGC is e+e- a “dense” state?
200 GeVs PHOBOS
(submitted to PRL)
Peter Steinberg ISMD2003
“Soft Scaling” in Au+Au
Why is the multiplicity so low,and why is it so close to e+e-?
Where is the contributionfrom hard processes expected
in RHIC central collisions?
Slight modificationof original questions:
Total charged multiplicityreflects “soft scaling”
(i.e. participants) not much room for
“Hard + Soft”
PHOBOS
Peter Steinberg ISMD2003
Violation of Ncoll scaling
1 T A AAA
coll T p p
N pR
N N p
Ncoll=1
collN
b
Participant
4i
Collisions
x4-5
PHENIX
Expectation ifall Ncoll contribute
at given pT
Peter Steinberg ISMD2003
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 theoretical
description
Peter Steinberg ISMD2003
A New Phase Diagram?
ln Q2
ln 1/x
PartonGas
QuantumColor Fluid
(ExtendedScaling)
CGC
NPQCD
D. Kharzeev
Quantum evolutionretains correlations
characteristic ofsoft physics
A
A
Peter Steinberg ISMD2003
A “Control” ExperimentTo rule out saturation scenario, RHIC devoted
a large fraction of Run 3 to d+Au collisions
d
A
Tp
Tp
Saturatednuclear
wave function
Non-saturateddeuteron
wave function
“Cronin”
“Suppressed”
Peter Steinberg ISMD2003
First RHIC d+Au Results
Peter Steinberg ISMD2003
Search for suppression in d+Au
/1
/T d A
dAT p p
dN dpR
T b d dp
Striking absence of suppression claimed by allexperiments, especially relative to central Au+Au
Dominant physics seems to be “Cronin Effect” (R>1)
STAR
PHENIX
Peter Steinberg ISMD2003
Centrality Dependence
Centrality dependence rulesout an “onset” of saturation
in central d+Au
70-100%
0-20%
PHOBOS~1
Peter Steinberg ISMD2003
Is CGC @ RHIC Dead?• This has been a major set-back for CGC-based
phenomenology• Lessons from Au+Au not applicable to d+Au• Was success in Au+Au 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
Peter Steinberg ISMD2003
Can perform sameanalysis for A+A & d+A
Is this a similar structurewith different parameters?
Npart Scaling in d+A?
/1
// 2partN Tcoll X A
XATpart p p
dN dpNR
T b d dpN
Au+Au
d+Au
STAR Data(PAS Representation)
Peter Steinberg ISMD2003
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…)
• d+Au failure may not be the end of the story
Peter Steinberg ISMD2003
Extra Slides
Peter Steinberg ISMD2003
Update to mT scaling
With new PHENIX data,scaling plot is somewhatmodified:
1. Weak corrections toprotons
2. Scaling is somewhatdifferent (20% vs. 100%)
Peter Steinberg ISMD2003
mT scaling in p+p
Peter Steinberg ISMD2003
Is Saturation Unique?
Peter Steinberg ISMD2003
• Au+Au collisions at s=19.6, 130, 200 GeV• dN/d for ||<5.4 over full azimuth
• Centrality from paddles (130/200) & Nhits (19.6)
• Top 50% of total cross section (Npart~65-360)
dN
/d
19.6 GeV 130 GeV 200 GeVPHOBOS Preliminary
Npart
Most Central
Peter Steinberg ISMD2003
Centrality Dependence of dN/d
• Are these effects related?• Long-range correlations?• Energy conservation?• Stopping?
• Other collision systems?
partpart
Nvsd
dN
N.
2
’ ~ -5 RisesSaturation?
2-component?
’ ~ -1.5 Stable Scaling
’~ 1.5 Falls Cascading in spectators?
CentralityDependence InterpretationLocation
200 GeV
130 GeV
19.6 GeV
dN
/d
/Np
art/2
= - ybeam
Peter Steinberg ISMD2003
Mid-rapidity Revisited
(dN/dyT )
Peter Steinberg ISMD2003
Geometric Scaling Revisited
Amusing repeat of hardsoft “duality” seen in geometrical scaling
Peter Steinberg ISMD2003
KLN in y and
PHOBOS Data: 200 GeV Central Au+AuKLN Implementation by P.A.S.