QCD@work 2007

Thomas PeitzmannThomas PeitzmannUtrecht University/NIKHEFUtrecht University/NIKHEF

Working under PressureWorking under Pressure––

QCD QCD Thermodynamics Thermodynamics at RHICat RHIC

QCD@work 2007

Outline

• The Making of Hadrons– Statistical Hadronization– Recombination,

Coalescence• The Perfect Liquid?

– Equilibration, EOS andViscosity from Elliptic Flow

• How Strong is Hot QCD?– Quarkonium Production

• “X-ray Scans”– Parton Energy Loss

• Density Estimates• Geometry Bias• Quarks vs. Gluons• Light vs. Heavy

• Induced Waves– Ridges and Mach Cones

• The Making of HadronsRevisited

QCD@work 2007

Statistical Statistical HadronizationHadronizationRecombination/CoalescenceRecombination/Coalescence

The Making of HadronsThe Making of Hadrons

QCD@work 2007

Statistical Model Fits at RHIC

• good description of hadronratios with

– other calculations lead tosimilar parameters

– resonances (K*, Λ*) not welldescribed!

• statistical hadronization– not necessarily thermal– phase space populated

statistically– “temperature” determined by

average energy per hadron

T =160 MeV, µB= 20 MeV

QCD@work 2007

Chemical Freeze-Out at the PhaseBoundary?

• Hadron abundances arefrozen with a certaincharacteristic energy perparticle.

• At SPS and RHIC this“coincides” with criticaltemperature.

• If the system isthermalized, this impliesthat hadrons are made ina stage with

Tchem

=Tc

QCD@work 2007

Baryon Enhancement at RHIC

• strong baryon enhancementin AA vs. ppat intermediate pT (2-4 GeV/c)– not expected for fragmentation

products• some enhancement expected

from collective flow– relevant up to 4 GeV/c?

• new production mechanismproposed:recombination or coalescence

T. Chujo, QM2006

L. Ruan, QM2006

Au+Au 0-10%

p+p

QCD@work 2007

Fragmentation vs. Recombination

• mesons easier to createthan baryons

• expectation in ppcollisions

• new mechanism for highparton densities

• baryon enhancement atgiven momentum

• also effects on elliptic flow

phadron ! z " pparton

z <1phadron ! nq " pparton

nq = 2(mesons), 3(baryons)

QCD@work 2007

Recombination Models

• reasonable description ofbaryon/meson ratios

• naïve conclusion:not only total abundances,but alsomomentum distributions atintermediate pTcarry memory of thepartonic phase

• models need refinement!

S. Blyth, QM2006

QCD@work 2007

The Perfect LiquidThe Perfect Liquid

Equilibration, EOS and Viscosity from Elliptic FlowEquilibration, EOS and Viscosity from Elliptic Flow

Elliptic Flow• anisotropy of the reaction

zone in non-central events– different pressure gradient– preferred emission in the

reaction plane• results in asymmetric

particle emission– observed at SPS and RHIC

• hint for thermal pressure– hydrodynamic expansion– early thermalization

dN12

d !"12( )

= N 1+ 2v2

2cos !"

12( )#$ %&

QCD@work 2007

D. Teaney PRC 68 (2003) 034913

Ideal Fluid Behaviour

• Strong elliptic flowobserved at RHIC– early equilibration

• Viscosity needs to besmall

– estimates in pQCD:– better predictions from

AdS/CFT?– Microscopic picture

responsible for large v2 stillnot understood

!s=4

3

"

# + p

!s"0# 0.2

QCD@work 2007

Testing the Equation of State

• elliptic flow for different species as a function of pT• description by hydrodynamic calculations

– best described by QGP (soft!!) EoS!?

QCD@work 2007

Elliptic Flow

• anisotropy saturates atdifferent values forbaryons and mesons– importance of number of

constituent quarks• hadron flow determined by

quark flow?– production through

coalescence?

200 GeVpreliminary

QCD@work 2007

Scaling of Elliptic Flow

scaling with• number of constituent quarks• transverse kinetic energy mT-m0

Y. Bai, QM 2006

Star Preliminary

QCD@work 2007

Status of Elliptic Flow Measurements

• strong elliptic flow– ideal fluid behaviour

• low viscosity– early equilibration

• constituent quark scalingat intermediate pT– recombination/coalescence

• anisotropy persisting atvery high pT– due to anisotropy in energy

loss (see below)

• many more results (andsome open questions)– higher moments (v4)– flow fluctuations– influence of non-flow effects

QCD@work 2007

How Strong is Hot QCD?How Strong is Hot QCD?

Quarkonium Quarkonium ProductionProduction

QCD@work 2007

Quarkonia• 20 years ago: Matsui & Satz

– color screening in deconfined matter→ J/ψ suppression = “smoking gun”

• experimental & theoretical progresssince then– (anti)shadowing, saturation– “normal” absorption in cold matter– suppression via comovers– feed down from χc, ψ’ and sequential

screening• first: χc, ψ’, J/ψ only well above Tc

– regeneration via statistical hadronizationor charm coalescence?

• relevant for “large” charm yield, i.e. RHICand LHC

QCD@work 2007

NA50 at SPS (0<y<1)PHENIX at RHIC (|y|<0.35)

Bar: uncorrelated errorBracket : correlated errorGlobal error = 12% is not shown

R. Averbeck, QM 2006

J/y suppression: SPS vs. RHIC• similarity of J/ψ suppression pattern for

– Pb+Pb from NA50 (0<y<1)– Au+Au from PHENIX (|y|<0.35)

• accidental?– compensation of higher density, smaller

cold nuclear matter effects andpotentially recombination at RHIC?

• ‘sequential dissociation’ at SPS andRHIC?– dissociation of ψ’ and χc

• feed down constrained?– J/ψ survives well above Tc

• awaiting more data from RHIC

QCD@work 2007

X-Ray ScansX-Ray Scans

Medium Opacity from Medium Opacity from Parton Parton Energy LossEnergy Loss

QCD@work 2007

J. Adams et al, STARPRL 91 (2003) 072304 Inclusive Suppression

• studied with nuclearmodification factor

• established probe of final statesuppression– sensitive to density of the

medium• very close to maximum

suppression• surface bias in high pT hadron

emission?

RAA (pT ) =d2N

AA/ dpT d!

TAAd2" NN

/ dpT d!

QCD@work 2007

RAA vs. Reaction Plane - L-dependence

In Plane

Out ofPlane

Lε

• study RAA vs. azimuthalangle relative to reactionplane– equivalent to v2 analysis +

global RAA

• scaling with lengthparameter Lε

• no significant suppressionfor Lε < 2 fm– weighting with survival

probability?– formation time?

• V. Pantuev, hep-ph/0506095

50-60%

0-10%

QCD@work 2007

Constraints on Medium Density• statistical analysis of RAA

– folds geometry, energy lossand fragmentation

• extract medium density– GLV (I. Vitev)

– WHDG (W. Horowitz)

– PQM (C. Loizides)

1000 < dNg dy < 2000

600 < dNg dy <1600

6 < q̂ < 24 GeV2fm

q̂!kT2

Lmed

QCD@work 2007

Energy Loss: Quarks vs. Gluons?

• expectation– larger gluon component in

antiproton vs. proton– stronger suppression for

gluons vs. quarks• colour factor

• but: no sign of strongergluon energy loss fromparticle ratios– medium modifications of

fragmentation functions?• X.N. Wang and X.F. Guo,

NPA 696, 788 (2001)

• W. Liu, C.M. Ko, B.W. Zhang,nucl-th/0607047

STAR - B. Mohanty, QM 2006,Curves: X-N. Wang et alPRC70(2004) 031901

QCD@work 2007

• describing the suppression is difficult formodels– naïve expectation: smaller energy loss for

heavy quarks– “ordinary” radiative energy loss not sufficient– collisional energy loss?– dissociation of heavy hadrons?

• RAA of electrons fromheavy flavor decays– rough agreement

between PHENIX andSTAR in ratios

– beware: generaldisagreement inabsolute cross section!

A. Suaide, QM 2006

Djordjevic et al., PLB 632(2006)81Armesto et al., PLB 637(2006)362Wicks et al., nucl-th/0512076van Hees & Rapp, PRC 73(2006)034913Adil & Vitev, hep-ph/0611109

Energy Loss: Heavy Quarks

QCD@work 2007

Jet Quenching Studies with Dihadrons• clear near- and away-side peak

even in central Au+Au for hightrigger pT– jet signature– background reduced for higher

associated pT

• little modification of near-sideyield

• suppression of away-side yieldapparent in central Au+Au

• shape of distributions ofassociated particles very similarfor all systems– vacuum fragmentation?

8 < pT(trig) < 15 GeV/c

STAR, PRL 97 (2006) 162301

QCD@work 2007

Dihadron Suppression at High pT

H. Z. Zhang, QM2006

T. Renk and K. Eskola, hep-ph/0610059

Emission points

• different surface biascompared to single hadrons– potentially better differential

probe– study with modified dihadron

fragmentation function

– suppression on away sidesimilar to RAA

– comparison to theory ongoing

Dh1h2 zT , pT

trig( ) = pTtrigd! AA

h1h2 dpTtrigdpT

d! AA

h1 dpTtrig

zT "pTassoc

pTtrig

QCD@work 2007

Induced WavesInduced Waves

Ridges and Mach ConesRidges and Mach Cones

QCD@work 2007

A Closer Look at Dihadron Correlations -Near Side

• Study dihadron correlationin Δη and Δφ– two distinct components on

near side in Au+Au– jet peak

• properties similar to pp(vacuum fragmentation?)

– ridge• jet related• momentum spectrum

similar to bulk• almost uniform in Δη

3 < pt,trigger < 4 GeVpt,assoc. > 2 GeV

STAR - J. Putschke, QM2006

Near-side jet peak

Near-side ridgeAway-side (and v2)

associatedΔϕ

trigger

QCD@work 2007

STAR - M. Horner, QM2006

zT = pT,assoc/pT,trig

Subtracting the Ridge

• strong near side increaseat low pT,assoc– seen by STAR and

PHENIX.

QCD@work 2007

STAR - M. Horner, QM2006

zT = pT,assoc/pT,trig

Subtracting the Ridge

• Subtraction of Δη-independent ‘ridge-yield’recovers centrality-independent jet yield– vacuum fragmentation after

energy loss?

• strong near side increaseat low pT,assoc– seen by STAR and

PHENIX.

QCD@work 2007

• ridge momentum spectra similar toinclusive (“bulk-like”)– approximately independent of

trigger pT– jet-like spectra harder than inclusive

• flatter for higher trigger pT

• other properties:– ridge yield increases with centrality– ridge persists out to pT,trig = 9 GeV/c– observed for all trigger particle types– energy content: a few GeV

STAR - J. Putschke QM2006Ridge Properties

QCD@work 2007

Interpretation of the Ridge• coupling of high energy parton

to longitudinal flow (Armesto etal, nucl-ex/0405301)– expect broadening but not

plateau• correlation from radial flow

(Voloshin nucl-th/0312065)– not expected at high pT

• thermal recombination + localheating from energy loss (Chiu& Hwa, Phys. Rev. C72034903, 2005)– qualitatively consistent

• more general:– ridge is jet-related structure

with properties similar tobulk

– early coupling: beforelongitudinal expansion

– local energy (heating) ormomentum transfer(collective flow)

• Does ridge measure theamount of energytransferred to the bulk?

QCD@work 2007

A Closer Look at Dihadron Correlations -Away Side

• clear evolution of away-sideyield with pT,trig and pT,assoc– high pT,trig and pT,assoc :

suppression– low pT,trig and pT,assoc :

enhancement• energy loss requires higher Q2

process for same pT,trig in Au+Au• attention: non-trivial changes

in shape of the correlation

STAR - M. Horner, QM2006

8< pTtrig < 15 GeV, PRL 95, 152301

Preliminary

|Δϕ| > 0.9

QCD@work 2007

Dihadrons: Away-Side ShapePHENIX, nucl-ex/0611019

• clear evolution of away-side peakperipheral → central– widening– flattening– emerging dip at Δφ = π

• enhanced yield not inpeak at Δφ = πbut in shoulder

• conical emission?

QCD@work 2007

M. Horner, QM2006

Away-Side Shape: pT,trig Dependence

• Similar dip-structureobserved in STAR.

• Shoulder of broad distributionstays unchanged forhigher pT,trig.

• Dip disappears forhigher pT,trig.– Filled by stronger “punch-

through” jet?• two-component picture?

– jet peak at Δφ = π– medium response at shoulder

QCD@work 2007

Interpretation of Away-Side Broadening

• different mechanisms– supersonic shock wave/

Mach cone• Stöcker, Casalderrey-

Solana et al., Renk &Ruppert

– Cherenkov gluon radiation• Koch, Majumder, Wang

– in-medium gluon radiation• Polosa & Salgado, Vitev

– jet deflection (large kT)• Fries, Armesto et al., Hwa

• need more input toconclude

• additional information fromthree-particle correlations– first results (STAR,

PHENIX) consistent withconical emission

– not yet conclusive• small signal, model

dependent bkg.subtraction, …

QCD@work 2007

The The Making Making of of Hadrons RevisitedHadrons Revisited

QCD@work 2007

Recombination and DihadronCorrelations

• large baryon yield atintermediate pT explainedby recombination models

• difference in correlationstructure expected– naïve expectation from pure

thermal recombination: nocorrelation

– more realistic models: somecorrelation due torecombination of thermalquarks with jet quarks(R. Hwa)

• study correlations withstrange baryons– Λ and Ξ have some

contribution from valencequark jets

– Ω should come from purelythermal recombination:expect no (or weaker) jet-like correlation structure

QCD@work 2007

STAR - J. Bielcikova, QM2006

Λ,Ξ,Ω - Hadron Correlation

• near-side yield similar for all triggers– inconsistent with original recombination prediction (R. C. Hwa et al., nucl-th/0602024)

– possible modifications in model have to be tested against data

QCD@work 2007

Summary• open question in energy loss

– no hint of quark/gluondifference

– mechanism for heavy quarksuppression?

• jet-medium interaction– ridge-like correlation on near

side• quantifies energy loss?• careful in Δφ-only analysis!

– broadening and softening onaway side

• 2-part. correlations consistentwith Mach cone

• conclusion needs more input• puzzles

– consistency of recombinationmodels with observation of jet-like correlation?

– …

• hadrons produced at phasetransition?– statistical model fits– baryon/meson ratios

• strong elliptic flow– early equilibration– low viscosity– constituent quark scaling

• J/ψ suppression– RHIC similar to SPS

• interpretation?• jet quenching established

– final state parton energy loss– needs high gluon density– vacuum fragmentation after

energy loss?– use different surface bias in

dihadron measurements

QCD@work 2007

Testing Thermalization with v4

STAR - Y. Bai, QM 2006

QCD@work 2007

Charm Cross Section: STAR vs PHENIX

C. Zhong • factor ≈ 2 difference– shape similar

• PHENIX– agrees (marginally) with

FONLL– better S/B for electrons

• STAR– disagrees with FONLL– direct charm measurement