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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
0.1 1
0.1
En
erg
y
Den
sity
(G
eV
/fm
3)
10Time (fm)
10
100
General Theoretical Picture (?)
L. McLerran
(modified by R.S.)
Simulation and model by K. Geiger, ….
Stages of the Collision
0.1 1 10
do I believe this?
0.1 1
0.1
En
erg
y
Densi
ty
(GeV
/fm
3)
10Time (fm)
10
100
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
0.1 1
0.1
En
erg
y
Den
sity
(G
eV
/fm
3)
10Time (fm)
10
100
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
0.1 1
0.1
En
erg
y
Densi
ty
(GeV
/fm
3)
10Time (fm)
10
100
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
0.1 1
0.1
En
erg
y
Densi
ty
(GeV
/fm
3)
10Time (fm)
10
100
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
0.1 1
0.1
En
erg
y
Densi
ty
(GeV
/fm
3)
10Time (fm)
10
100
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
0.1 1
0.1
En
erg
y
Densi
ty
(GeV
/fm
3)
10Time (fm)
10
100
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)