Systematic Study of Di-jet Correlation in Heavy Ion Collisions

N. GrauOral Defense 11/21/05

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Systematic Study of Di-jet Correlation in Heavy Ion

Collisions

OutlineQCD and the phase transitionEvidence for new matter at RHIC Jet studies in d+Au compared to p+p

• Limit on initial state effects/cold nuclear matter

Jet studies in Au+Au compared to p+p and d+Au• Hot,dense medium modification

What do we learn about the matter at RHIC?


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QCD – Theory of Strong Interactions (I)

Quantum Chromodynamics• Describes forces between

hadrons: protons, neutrons

• ~ 5% of the universe Asymptotic Freedom –

s0 as Q2 increases.• Allows for perturbative

calculations (Feynman Calculus)

• Hard processes, high Q2, high outgoing energy (Jets) Q (GeV)

S(Q

)


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QCD – Theory of Strong Interactions (II)

QCD potential energy between quark pairs

Linear at large separations• Force is constant!

Confinement of partons• Not free at large

separations


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Deconfining the Partons

Free energy of heavy quark pairs as a function of separation

At high temperature Tc ~ 170 MeV ~1012K

Potential not linear but flat at large separation• F = 0! Deconfinement!


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Quark-Gluon Plasma (QGP)

Above Tc partons contribute to degrees of freedom

Increase energy density, entropy

Energy density required ~ 1 GeV/fm3

Dilute, non-interacting gas


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QCD Phase Diagram

AGS root-s ~ 5 GeV/nucleon

SPS root-s ~ 20 GeV/nucleon

RHIC root-s ~ 200 GeV/nucleon

LHS root-s ~ 7000 GeV/nucleon


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Relativistic Heavy Ion Collider (RHIC)

Began Operation Summer 2000

Can collide most any system (p+p, d+Au, Au+Au, Cu+Cu, Cu+Au, etc.)

Maximum Beam energy for nuclei= 100 GeV/nucleon

RHIC from Space


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RHIC Runs to Date

Run Year Species s1/2 [GeV ] Ldt NTot p-p Equivalent

01 2000 Au+Au 130 1 b-1 10M 0.04 pb-1

02 2001/2002 Au+Au 200 24 b-1 170M 1.0 pb-1

p+p 200 0.15 pb-1 3.7G 0.15 pb-1

03 2002/2003 d+Au 200 2.74 nb-1 5.5G 1.1 pb-1

p+p 200 0.35 pb-1 6.6G 0.35 pb-1

04 2003/2004 Au+Au 200 241 b-1 1.5G 10.0 pb-1 Au+Au 62 9 b-1 58M 0.36 pb-1

05 2004/2005 Cu+Cu 200 3 nb-1 8.6G 11.9 pb-1 Cu+Cu 62 0.19 nb-1 0.4G 0.8 pb-1 Cu+Cu 22.5 2.7 b-1 9M 0.01 pb-1 p+p 200 3.8 pb-1 85B 3.8 pb-1

Vacuum and cold nuclear medium effects

Hot, dense nuclear medium effects


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PHENIX Participation


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PHENIX Detector

Each central arm 90o in azimuth, 70-100o in Charged particle tracking outside of the fieldPhotons with highly segmented EmCal


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Evidence for New Matter:Energy Density

time for system to thermalize (0 ~ 1 fm/c)~6.5 fm

dy

dE

RT

Bj0

2

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PRL 87, 052301 (2001)

dydz 0

Bjorken Energy Density

Bj >> 15 GeV/fm3 >> necessary

R2


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Evidence for New Matter:Thermal Radiation

Potentially first evidence of radiation from a hot source

Red is PHENIX data on photons not from decay

Dashed blue pQCD expectation

Black line prediction from pQCD and photons from a source at 500 MeV >> 170 MeV!

Need p+p data to compare data with data.

PHENIX preliminary


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Evidence for New Matter:Pressure

Non-overlapping (peripheral) collisions: almond shape overlap region

Results in differential pressure gradient in-plane vs. out-of-plane

v2 sensitive to the pressure

x

yz

2cosv212 2N

d

dN


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Evidence for New Matter: Thermal Equilibrium

All particles flow!Flow reproduced by

hydrodynamic (fluid) model assuming an EOS and thermal equilibrium

First time ever!• Not at AGS or at SPS

At pT > 2 GeV/c, not just bulk properties but hard scattering (jets)


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Evidence for New Matter: Strongly-Interacting QGP

Hydrodynamical models confirm• Large pressures• Substantial collective

motion• Viscocity = 0

Heavy quarks flow• Expect much less flow

since heavy particles rescatter less

Strongly interacting, perfect fluid!

Greco,Ko,Rapp: PLB595(2004)202

Non-photonic electrons, dominantly decays of heavy quarks: c and b.


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Single Particles in Peripheral Collision

0 production in p+p

Compared with peripheral Au+Au on right

# n+n collisions in peripheral Au+Au


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Single Particlesin Central Collisions

p+p production scaled to central, head-on collisions

Suppression seen in Au+Au compared to the scaled p+p

New phenomenon at RHIC!


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Nuclear Modification Factor

Single 0 and are suppressed similarly• Partonic

phenomenon

Direct-photons not suppressed• Strong interaction

phenomenon

ppColl

AATAA YieldN

Yield)(pR


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Parton Energy Loss

Gluon bremsstrahlung• Like QED

bremsstrahlung• Radiated gluons

interact with the medium as well

Single particle suppression related to the gluon density

“Jet Quenching”


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Jets in Heavy Ion Collisions

Jet highly collimated spray of hadrons associated with fragmentation of a fast parton

LO diagram in QCD is 22

Easily seen in p+p at root-s 200 GeV 2 jet event in STAR

Find the jet in

Beam View


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Statistical Determination of Jets:Correlation Functions

Exploit property that jets are highly collimated, correlated in azimuth

Correlate the hadrons• ~ 0 intrajet pairs• ~ interjet pairs

X.-N. Wang Phys. Rev. D 47, 2754 (1993)

CP

P P( )

( , )

( ) ( )

1 2

1 2


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Away-side (Interjet) Suppression

Hadron pair (jet) correlations in Au+Au and p+p collisions

Near-side unmodified

Far-side suppressed


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Broadening of the Away-side Distribution

Incoherent multiple scattering increases the acoplanarity of di-jet pairs

Not measured in data because of the suppression of the distribution

QGP rest frame


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Suppression Summary as of 2002

Single particles are strongly suppressedAway-side correlations are suppressedExplained in terms of parton energy loss

• Color (gluon) density dN/dy ~ 1000Expected broadening of the away side

• Not measured due to the strong suppression


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Contribution of this Thesis

First systematic study of jet correlations in nuclear collisions

Study the cold nuclear medium modification in d+Au compared to p+p

Study the hot, dense nuclear medium modification• Extend the pT reach of previous analyses

(more jet-like)• Extract other distributions


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Importance of a Systematic Study

d+Au

= cold medium

Initial State Effects Only

Au+Au

= hot and dense medium

Initial + Final

State Effects


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Single particle Spectrum in d+Au

Single particle spectrum is enhanced and not suppressed!

Contrast with the final state suppression of single particles from Au+Au

PRL 91, 072303 (2003)

ppColl

AudTdA YieldN

Yield)(pR


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Cronin Effect: Parton Transverse Momentum

Multiple elastic scattering increase parton transverse momentum, kT.

Smears single particle spectrum

Measure kT from final state pair pT

• Drell-Yan • Pair pT of leptons is

pair pT of quarks• Same for di-jets

But the jets fragment

kT due to• Confinement• Hard gluon radiation,

i.e. 23 diagrams

qq


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Direct Measure of kT Broadening

in Nuclear Collisions

Factor of 2 increase from p+p to p+Pb Increases as A1/3 – length of the medium Test to see if this increase is as dramatic at RHIC

+A and +A offset due to an effect from jet reconstruction

After offset all data agree point-to-point


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Correlations in p+p and d+Au

Trigger 0 from 5-10 GeV/c

Increase in di-jet distribution should be seen in away-side


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Jet Acoplanarity Difference Between p+p and d+Au

J. Qiu, I. Vitev PLB 570 161 (2003)

Multiple scattering model not inconsistent with the data.

0 – crosses +/- – circles


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Why No Significant Difference Between p+p and d+Au?

“Intrinsic” kT larger than the kT due to multiple scattering.• More hard gluon radiation at higher root-s

Could it be recombination (Hwa and Yang)?• Shower (fragmented) partons recombining

with the soft (radiated) background• Initially predicted factor of 2 increase in the

near angle yield (R. Hwa, C.B. Yang: PRC 70,054902 (2004))


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Associated Particle YieldsR

a ti o

to

p+

p

Hwa and Tan, nucl-th/0503060


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Conclusions About Jet Correlations in d+Au Collisions

Two-particle correlations: d+Au like p+p• Lack of acoplanarity increase

Yields and acoplanarity are consistent with multiple scattering picture

And consistent with recombination picture.

Conclusion: Need different observables


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Away-Side Jet at High pT in Au+Au

Away-side jet seen in correlations with high trigger and associated pT

Distribution seen to measure broadening!

STAR Preliminary

D. Magestro QM2005

Trigger h 8-15 Associated h > 2

0-20%


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Extracted Away-Side Jet Widths at High pT

Far angle widths unchanged from p+p through central Au+Au!

8 < pT(trig) < 15 GeV/c

5-7 GeV/c(open), 7-10 GeV/c


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pout: Alternative Observable for Broadening

pout,F directly related to kTy

From gluon radiation from energy loss/suppression expect kT broadening

Correlation widths reveal lack of broadening…

trigg.

assoc.

Pout.F

sin,assocTout pp

Component of associated pT transverse to trigger


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pout Distributions

pout Distribution from jet only• Removed flow and

background

Broadening in kT would be seen in broadening of the pout distribution

Distribution is approximately Gaussian: dominated by intrinsic kT

5-7 GeV/c 0 1-10 GeV/c h


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Broadening of pout

Fit distribution with Gaussian

RMS of Gaussian is unchanged with centrality

Lack of broadening is not seen in pout

Bands are error due to assumed v2 value


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No broadening but Suppression

At lower pT suppression of yields in away-side measured

Suppression is also evident from STAR data

8 < pT(trig) < 15 GeV/c


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Measurement of Yields: dN/dxE

Measure the yield of particles per trigger as a function of xE

Compare central to peripheral distribution, ICP

If trigger emitted at the surface ICP ~ RCP

If trigger near center ICP ~ 1

Sensitive to differential energy loss between the particles assoc.

trigg.

trig

assoc

trigT

assocTtrigTE z

z

p

ppx

2,

,,


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xE Distributions

xE distribution due to jets only5-7 GeV/c 0 1-10 GeV/c h


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Comparison of Central to Peripheral

Ratio of central 0-20% to peripheral 40-92%

Weighted average of the data shows suppression!

11% normalization error band at 1

Point-to-point band error due to v2

5-7 GeV/c 0 1-10 GeV/c h


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Centrality and pT Dependence of ICP

Trigger pT dependence of average ICP

20-40%/40-92% (open) 0-20%/40-92% (closed) Suppression increases

with centrality, independent of pTtrig.

Possible evidence for differential energy loss• ICP between RCP and 1

~RCP

~RCP


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Conclusions from Au+Au Di-jets Correlations

Away-side jet is seen at high pT in central Au+Au

Away-side distribution is not broadened• p+p, d+Au, and Au+Au widths are similar

Away-side yield is suppressed when comparing central and peripheral


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Implications for Energy Loss Models in Au+Au

Energy loss picture before Aug 2005• Yield of single particles and away-side pairs from

gluon radiation• Broadening is the natural consequence of

incoherent multiple scattering Data shows

• Single particles and away-side pairs are suppressed• Distribution is not broadened

What does this imply for energy loss?• Coherent gluon radiation? A cone of radiation?• What property of the medium are the correlations

sensitive to? Color density, something else???


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Backup Slides


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Constructing Correlation Functions

ddN

ddNC

d

dN

N mix

realpairs

trig /

/1

Real events: events with two particle that are correlated

Mixed events: two particles from different events

Mixed events represent the pair efficiency and acceptance in .

Trigger: 0

Associated: h+/-


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Sources of Correlations

Hard scattering Resonance decays

(rad)

Elliptic flowTotal

C(

)

Documents

Systematic Study of Di-jet Correlation in Heavy Ion Collisions