Peter SteinbergISMD2003 Experimental Status of Parton Saturation at RHIC Peter Steinberg Brookhaven National Laboratory ISMD2003, Krakow, Poland 5-11 September

Peter Steinberg ISMD2003

Experimental Status ofParton Saturation at RHIC

Peter SteinbergBrookhaven National Laboratory

ISMD2003, Krakow, Poland5-11 September 2003


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…


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


Particle Density at 90o in pp & AA

Evidence forcollective behavior?

PHOBOS @ RHIC (PRL 2001)


dN/d: Theory vs. Experiment

Eskola, QM2001

Why is the multiplicity so low?

Where is the dramatic risein hard processes expected

at RHIC energies?


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


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)


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)


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


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)


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


BRAHMS dN/dy

BRAHMS Preliminary 2003 Central Au+Au

BRAHMSrapidity

distribution


= - 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


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?


Limiting Behavior in e+e-

DELPHI, PLB459 (1999)


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


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)


“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


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


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


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


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”


First RHIC d+Au Results


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


Centrality Dependence

Centrality dependence rulesout an “onset” of saturation

in central d+Au

70-100%

0-20%

PHOBOS~1


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


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)


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


Extra Slides


Update to mT scaling

With new PHENIX data,scaling plot is somewhatmodified:

1. Weak corrections toprotons

2. Scaling is somewhatdifferent (20% vs. 100%)


mT scaling in p+p


Is Saturation Unique?


• 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


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


Mid-rapidity Revisited

(dN/dyT )


Geometric Scaling Revisited

Amusing repeat of hardsoft “duality” seen in geometrical scaling


KLN in y and

PHOBOS Data: 200 GeV Central Au+AuKLN Implementation by P.A.S.

Documents

Peter SteinbergISMD2003 Experimental Status of Parton Saturation at RHIC Peter Steinberg Brookhaven National Laboratory ISMD2003, Krakow, Poland 5-11 September