The First 3 Years at RHIC an Overview and (careful) Assessment

Richard SetoUCRXXXIV International Symposium on Multiparticle Dynamics Sonoma State University, California, USA July 26-Aug 1 , 2004

Quantum ChromoDynamicsQCD : established theory of the strong interactionquark confinement the non-perturbative

structure of the vacuumvacuumvacuum structure modified at high temperatures quarks and gluons deconfined at high

______temperatures

QCD is a fundamental theory of QCD is a fundamental theory of nature nature

containing a phase transition that containing a phase transition that is is

accessible to experimental accessible to experimental investigationinvestigation

Lattice Predictions

Lattice: TC~170 MeV (C ~ 1 GeV/fm3) Phase transition- fast cross over to experimentalist: 1st order

difference between S-B and lattice

interactionsinteractions(in hindsight)

Relativistic Heavy IonsRHIC

“Fundamental Questions”

from the Long Range Plan for Nuclear PhysicsEvolution of system

thermal equilibrium? initial temperature or energy density ?

confinement signatures of deconfinement seen? origin of confinement?

the QCD vacuum connections to the masses of the hadrons? origin of chiral symmetry breaking?

properties of matter at high energy density?quark and gluon description correct?

Where are we? the RHIC White Papers

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General Theoretical Picture (?)

L. McLerran

(modified by R.S.)

Simulation and model by K. Geiger, ….

Stages of the Collision

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do I believe this?

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Initial State – a CGCFormationFormation

t~1/Qsat~0.2 fmt~1/Qsat~0.2 fm~15 GeV/fm~15 GeV/fm33

Hard processes – PQCDHard processes – PQCDhard probes hard probes

CGC-Saturated gluon fieldsCGC-Saturated gluon fieldsEvidence for CGCEvidence for CGC

Multiplicity distributionsMultiplicity distributionsdA forward suppressiondA forward suppression

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Equilibration – Elliptic flow startsEquilibrationEquilibration

t~0.6 fmt~0.6 fm~3-15 GeV/fm3~3-15 GeV/fm3Strong coupling Strong coupling Flow (elliptic) developsFlow (elliptic) developsEarly ThermalizationEarly Thermalization

EvidenceEvidenceElliptic flow strengthElliptic flow strength

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The strongly-interacting QGP (?)Hard EOS

sQGPsQGPviscocity ~0viscocity ~0Hard EOS Hard EOS Parton energy lossParton energy loss

Evidence: Evidence: strong couplingstrong coupling

Elliptic flowElliptic flow partonic energy losspartonic energy loss

jet suppressionjet suppression

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Mixed Phase-Latent heat-recombinationSoft EOSCross over(wanna be1st order)

Mixed PhaseMixed PhasePhase Transition Phase Transition

(Cross over)(Cross over)Latent Heat Latent Heat Soft EOS – flow stallsSoft EOS – flow stalls

hydro model comparisonhydro model comparisonHadronization – recombinationHadronization – recombination

baryon to meson ratiobaryon to meson ratiov2v2

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Hadronic phase Hard EOS

Hadronic PhaseHadronic Phase Hard EOS againHard EOS again radial flow developsradial flow develops chemical freezeoutchemical freezeout Tch ~ 160 MeVTch ~ 160 MeV

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Freezeout

Kinetic FreezeoutKinetic FreezeoutThermalThermal

T~130 MeVT~130 MeV

and on to the detector…and on to the detector…

First - Some general thoughts

Data is in good shape Redundancy in Experiments is Good

BRAHMS, PHENIX, PHOBOS,STAR

Problem is often in theoretical interpretation

transv

ers

e m

om

entu

m p

t

time

Models:

from Sid Bass

initial state

hadronic phaseand freeze-out

pre-equilibriumQGP and

hydrodynamic expansion hadronization

coloredglasscondensat

e

pQCDjet

production

hydrodynamics(partonic)

jetquenching

?!

partonrecombination

Cooper-Frye

fragmentation

hydrodynamics(hadronic)

Chemical models

HBT

Important to ask whether basic essence of these steps is correcte.g. CGC, recombination, parton energy loss, hydroOnce we get the right pieces then we go after the details

Sof

tH

ar

d

Centrality~impact parameter

“Spectators”

“Spectators”

“Participants”

Impact Parameter

Soft interactions ~ Nparticipants

Hard interactions ~ Ncollisions

A simple Glauber model gives Ncollisions and Nparticipants

Centrality 0% head on 90% glancing

0 4000

1000

Au-Au

Npart

Nco

ll

Initial State - A colored Glass Condensate?

Elena Ferreiro (Santiago): Saturation: CGC and color sourcesEnrico Cattaruzza (Trieste) Cronin effect and energy conservation constraints in high energy proton-nucleus collisionsKirill Tuchin (BNL): Manifestation of the Color Glass Condensate in particle production at RHIC

R. Seto

Nucleus as amplifier of gluon densityNucleus as amplifier of gluon density

xG(x)

x

proton

Nucleus

10-4(p)10-2(Au)1

~( ) partS S

dNN

dy Q

R. Seto

Npart

CGC- initial state - Saturation in CGC- initial state - Saturation in MultiplicityMultiplicity

dN/d

/ .5N

part

1~

( ) partS S

dNN

dy Q

~ 18 GeV/fm3

Kharzeev, NardiPLB 507, 121 (2001)

AuAu 130 GeV

R. Seto

Other energiesOther energiesData now available from 200, 130 and 19 GeVOnly CGC (Kharzeev, Nardi, Levin) provides

consistent description (?!?)saturation

at s=20GeV

?

R. Seto

x

Q (GeV)

1

10-1

10-3

10-2

10-4CGC

CQF

1 10 100

Y~0

Y~2

Regions Regions of a of a

nucleusnucleus(Au)(Au)

pQCDPHENIX/STAR

central detectorsBU

LK

BU

LK

pQCD

QS

look for suppressionat higher y in pA

y~log(1/x)

QCD

R. Seto

Rcp in d+AU BrahmsRcp in d+AU Brahms

)(/)(

)(/)(

peripheralNperipheralYield

centralNcentralYieldR

coll

collCP

Suppression at high yas expected in CGC model caveat – data includes protons

Kharzeev, Kovchegov, Tuchinhep-ph/0405045

Equilibration? –Elliptic flow starts? Thermalized Energy density?

Tetsufumi Hirano (BNL): Hydrodynamic approaches RHISergei Voloshin (Wayne State): Dense matter flow

elliptic flow –viscosity-strong coupling

Elliptic flow? space anisotropy

• pressure gradients momentum anisotropy

conversion efficiency depends on medium Strong coupling zero MFP viscosity() ~0 (perfect fluid) can use non-dissipative hydrodynamics

Strong elliptic flow How do we know this?

Hydro models Laboratory analogies Li6 AdS/CFT i.e. string theory CFT dual Can do calculations in a strongly coupled theory

_______ calculate. Find ~ small

Science 298, 2179 (2002)

RHIC data

1/S dN/dy (S=area)

Definitions v2 ~ 2nd fourier coefficient

of momentum anisotropy ~spatial anisotropy

RHIC data: nearing hydro prediction early thermalization

• Hydrodynamics assumes a thermal system

strong coupling• mfp~0

more central

weakly coupled weakly coupled QGP? QGP?

Why early thermalization?

If system free streams spatial anisotropy is lost v2 is not developed

More detailed hydrodynamic calculations (QGP+mixed+RG)

therm ~ 0.6 -1.0 fm/c ~15-25GeV/fm3

(ref: cold matter 0.16 GeV/fm3)

PHENIXHuovinen et al

(Teany et al, Huovinen et al)

bjorken energy density (to check)

dET/d = 600 GeV (PHENIX)take from hydro arguments ~0.6 fmR~6fm for Au~ 9GeV/fm3 @ thermalization (!?)

Note: the CGC model predicts ET 2-3 times observations of PHENIX pdV work??? hirano et al adds hydro afterburner – OK (ref)

ET(initial)=2-3 times ET(final) ~ 20 GeV/fm3

20

1 1 TdE

R dy

strongly interacting QGP??? partonic energy loss

Ivan Vitev (Iowa State): Jets in nuclear collisionsMike Miller (MIT): Jet measurements at RHIC

the Validity of binary scaling

Particle production via rare processes should scale with Ncoll, the number of underlying binary nucleon-nucleon collisions

Assuming no “suppression effects”

d b

Assuming no “collective” effects

Test this on various rare processes

check via hard processes

R. Seto

1/ T AB

1/ T AB

1/ T AB

1/T

ABE

dN/d

p3 [m

b

GeV

-2]

0 peripheral AA

charm via single edAu, AuAu

direct AuAu

0.906 < < 1.042

dN/dy = A (Ncoll)

charm

charm notsuppressed?“Dead coneeffect”

hard processes scale with Ncoll

R. Seto

parton energy loss – energy density parton energy loss – energy density (2)(2)

5 10 pT (GeV/c)

peripheral Au+Au

Calibrating the probes- pp reference data-agrees with NLO pQCD

Peripheral Collisions-Scale with Ncoll

Central CollisionsDO NOT SCALE!

Is it» Suppression of

low-x gluons in the initial state?

» Energy loss in a new state of matter?

Central 0-10%

p+p → 0 + X

PHENIX

R. Seto

dA – the null experiment

Au + Au Experiment d + Au Control Experiment

Preliminary DataFinal Data

2

2

/( )

/

AAT

AA T ppColl T

d N dp dR p

N d N dp d

Nuclear Modification Factor:

Its a final state thing!The combined data from Runs 1-3 at RHIC on p+p, Au+Au

and d+Au collisions establish that a new effect (a new state of matter?) is produced in central Au-Au collisions

PeripheralCentral

R. Seto

More details: Looking at the associated More details: Looking at the associated jetjet

Leading hadrons

Medium

near sideaway sideLook for away side Jet

» Trigger particle 4<pT<6 GeV

» Look for associated particles pT>2 GeV

Away side jet disappears in central Au-Au

R. Seto

Assoc. particles:0.15 < pT < 4 GeV/c

STAR PRELIMINARY

Angular correlations- the missing Angular correlations- the missing particlesparticles

Central Au+Au-balancing hadrons » Broader in angle ~ cos() » More » Softer in pt

Away side not jet-like!away-side products

approach equilibration

Closed 4 < pTtrig < 6 GeV/c

Open 6 < pTtrig < 10 GeV/c

STAR PRELIMINARY

STAR PRELIMINARY

more central

Assoc

Mu

lt

near

away

near side away side

R. Seto

High Energy Densities?High Energy Densities?

A Calculation of energy loss» Au+Au suppression (I. Vitev and M. Gyulassy,

hep-ph/0208108)» d+Au enhancement (I. Vitev, nucl-th/0302002 )

understood in an approach that combines multiple scattering with absorption in a dense partonic medium

dNg/dy ~ 1100

=15 GeV/fm3

Evaluation ofdifferent calculations? (Wang, detailed balance?

Au+Au

d+Au

Mixed Phase?? Latent heat??? Elliptic flow- the softest point

Tetsufumi Hirano (BNL): Hydrodynamic approaches RHISergei Voloshin (Wayne State): Dense matter flow

R. Seto

Models of both types get pions rightModels w/o mixed phase give too much flow to protonsFlow develops in hadronic stage as well as QGP stage

Elliptic flow and the softest point - Elliptic flow and the softest point - modelsmodels

Hydro Models:» w/ QGP mixed

phase – latent heat-Solid lines

» w/o QGP mixed phase – dotted lines

pion v2

protonv2

PHENIX

PHENIX

R. Seto

Soft Sector: Evidence for Soft Sector: Evidence for Thermalization and EOS with Soft Thermalization and EOS with Soft

Point?Point?

Hydro calculations: w/ mixed phaseKolb, Heinz and Huovinen

Hadronization recombination and the strange case of the p/ ratio

Julia Velkovska (Vanderbilt): Production and flow of identified hadrons at RHICRudy Hwa (Oregon): High pT physics at RHIC and evidence of recombination

R. Seto

Source of high pt particles?» hard scattering +

fragmentation universal fragmentation functions baryons ~ 20% at high pT

p/p/ (200 GeV) (200 GeV)

NONO

nominal p/~0.2-0.4

hydro ????» reproduces increasing

p/» BUT not applicable for

pT> 2 GeV/c (T>>TC )???

2

but p/ ~ 1

R. Seto

h/h/ (200) at high p (200) at high pTT

h/ ~ p/ +1 (+K/)

at pt > 5 GeV – back to “normal”

0 5

(h+ +

h- )/

20

0

2

pT (GeV)

1

3

0

R. Seto

Recombination modelsRecombination models

3 implementations [Duke, TAMU, Oregon]» Duke - Fries, Nonaka, Muller,Bass: PRC 68, 044902 » TAMU – Greco, Ko, LEvai PRL 20,202302

» Oregon – Hwa, Yang: nucl:th/0401001

All use thermal flowing constituent quarks plus hard quarks» Duke

– only thermal quarks recombine– hard component - fragmentation

– apply only to pT>2 GeV/c

» Oregon, TAMU– also include thermal+fragmentation quark

0

Tantalizing since this could be an indication of the

DOF of the system – i.e. quarks

R. Seto

Compare p/Compare p/ to to modelsmodels

The recombination models can nicely explain the p/ ratio » baryons with 3 valence quarks are boosted to a higher pT than

mesons. » for exponential spectrum recombination dominates over

fragmentation at moderate pT.

The simplest of the models (Duke) fails on getting the detailed shape at low pT correctly.

•All models return to baseline- nice test

1

1

0

2

3

Duke

nominal~1.4

R. Seto

• Hydrodynamic radial expansion at low pT in central collisions• At pT > 1.5 – 2 GeV/c all curves of baryon yield per collision merge

Accident?

OR

High pT baryons arise from hard processes with no energy loss?

0Protons? binary scaling?Protons? binary scaling?

R. Seto

RRCPCP

protons scale with NColl - Accident or not?

~mass OR ~ number of valence quarks ?

» Check hypothesis of partonic stage flow + recombination using particle with the same number of valence quarks as the pion, but the mass of the proton.

m=mproton and nvalence=n

valence

» Look at RCP()

)(/)(

)(/)(

peripheralNperipheralYield

centralNcentralYieldR

coll

collCP

0

R. Seto

Rcp (data)Rcp (data)Proton RCP~1

» protons ~ NColl

RCP< 0.5 » Suppression

Phi RCP

» more like than protons difference dictated by number

of valence quarks, not mass lends support to

recombination models

0

shapes of and proton same!Hydro or not?

can this rule out hydro?

hydro doesn’t work for peripheral collisions

look at slopes

Hydro still OK?

R. Seto

Recombination Extended : a revisit to Recombination Extended : a revisit to elliptic flowelliptic flow

The complicated observed flow pattern in v2(pT) for hadrons d2n/dpTd ~ 1 + 2 v2(pT) cos (2 )

is predicted to be simple at the quark level under pT → pT / n , v2 → v2 / n , n = (2, 3) for (meson, baryon)

if the flow pattern is established at the quark levelCompilation courtesy of H. Huang

R. Seto

FurtherFurther Extending Recombination Extending Recombination

New PHENIX Run-2 result on v2 of 0’s:New STAR Run-2 result on v2 for ’s:ALL (non-pion) hadrons measured to date obey quark

recombination systematics(!)

PHENIX Preliminary

0

STAR Preliminary

R. Seto

The models- associated jet particlesThe models- associated jet particles

Thermal quarksTemperature + Flow

Hard quarkspQCD calculation

recombination of thermal quarksshould not give “jettiness”

fragmentation of hard quarks should give jettiness

recombination of thermal +fragmentation quarks should give jettiness

Protons are mostly fromthermal recombination

DukePID Trigger particlepT=2.5-4 GeV

Associated particle“jettiness”

idea

0

R. Seto

Baryons have jettiness!

Duke mesonsDuke baryons

2Note: this is a preliminary plot. An updated plot will be in the white paper

PHENIXpreliminary

Recombination of thermal quarks only is ruled outRequires recombination of thermal+hard quarks

Recombination??

Successes: Accounts for pT dependence of baryon/meson yields Unifies description of v2(pT) for baryons and mesons

Challenged by “Associated emission” at high pT

Can the simple appeal of Thermal-Thermal correlations survive extension to Jet-Thermal ?

General Issues Entropy ? Chiral masses? Any possible way to put it on stronger footing?

If recombination ultimately works - what does it tell us about the “real” DOF??

Summary (courtesy B. Zajc)

Evidence for bulk behavior (flow, thermalization): unequivocal Evidence for high densities (high pT suppression): unequivocal

(Control measurement of d+Au essential supporting piece of evidence) Density far in excess of lattice transition density

Mounting evidence for CGC description of the initial state

Evidence for a strongly coupled system Empirical : strongly suggestive of recombination at work

scaling of v2 based on quark content pT dependence of meson/baryon ratios

What remains? more robust quantitative understanding Quantitative understanding of “failures” : Notably HBT Direct evidence for deconfiment/Degrees of freedom(?) Chiral Symmetry restoration

END

END

Do central Au+Au collisions produce a dense state of matter?

Answer: Yes Evidence 1: Jet suppressionJet suppression in Au+Au collision

• Suppression of high pT particles in Au+Au• Jet origin of high pT particles demonstrated by two particle

correlation and xT scaling• Absence of suppression in d+Au • TAB scaling of direct photon and charm yield in Au+Au

These observations provide a direct evidence that densedense matter formed in the final state is the cause of the suppression.

Evidence 2: Strong elliptic flowelliptic flow• Scaling of v2 with eccentricity shows that a high degree of

collectivity built up at a very early stage of collision.• Success of hydrodynamic models in reproducing the elliptic flow

shows that the state can be well described as fluid – a dense state of matter.state of matter.

Is it conceivable that the state of matter consisted only of hadrons?

Answer: No dEdETT/d/d : The lower limit of the energy densities derived are:

≥ 15 GeV/fm3 at formation - ≤ 0.35 fm/c

≥ 2.8 GeV/fm3 at thermalisation - ≤ 2 fm/c Elliptic FlowElliptic Flow: hydro-models require early thermalization (therm<1fm/c) and high

initial energy density > 10 GeV/fm3

Jet suppressionJet suppression: Initial gluon density dng/dy~1000 and initial energy density ~15 GeV/fm3 are obtained from GLV model. A similarly high initial energy density is obtained by others.

All these estimates of energy density are well in excess of ~1 GeV/fm3 obtained in lattice QCD as the energy density needed to form a deconfined phase.

Is the state of matter a QGP?or a strongly interacting Quark Gluon Plasma?

Answer: We don’t know.+ We know that the matter is extremely dense and it

thermalizes very rapidly. The estimated energy densities are all well in excess of the density needed for a QGP.

But We currently do not have a consistent model of Au+Au

collision that can describe spectra, v2, and HBT. This prevents us from drawing quantitative conclusions on the properties of the matter such as the equation of state and the presence of a mixed phase.

In addition There is no obvious and common definition of QGP. – no

evidence for QUARKSQUARKS and GLUONSGLUONS as the DEGREES DEGREES OF FREEDOMOF FREEDOM

What now?

Key Scientific Questions for the future

Deconfinement?Deconfinement?Degrees of freedom?Degrees of freedom?Chiral symmetry Chiral symmetry

restoration?restoration?

political note: we probably don’t need all ofthese to declare “victory”

R. Seto

Spare: gluon shadowing in nucleiSpare: gluon shadowing in nuclei

L. Frankfurt, M. StrikmanEur. Phys. J A5, 293 (99)

Q = 2 GeVQ = 5 GeV

Q = 10 GeV

Gousset, PirnerPhys. Lett B 375, 349 (96)