André Mischke

for the

STAR Collaboration

Rencontres de Moriond, La Thuile, Italy, March 18-25, 2006

Amsterdam

Recent STAR results on hard probes

Andre Mischke (Utrecht) 2

• Parton scattering in the early stage of the collision

• Traverse through the medium and interact strongly

• In-medium fragmentation

• Model expectations:- Collisional energy loss (small effect ?!)- Partonic energy loss via medium induced gluon Bremsstrahlung (jet-quenching)

- Jet broadening due to parton rescattering - Modification of the FF (softening)

• Energy loss sensitive to the medium density 2ˆ4

LqC

E RSmedium

modified

modified

q q

q

qhadronic

partonic

q ?

Probing QCD matter with jets

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• Two concentric rings

• 3.8 km circumference

• Counter-rotating ion beams

• 4 experiments

- Au+Au @ sNN = 19.6, 62.4, 130, 200 GeV

- Cu+Cu @ sNN = 200 GeV

- d+Au @ sNN = 200 GeV

- polarized p+p @ s = 200 GeV

STAR

PHENIX

PHOBOSBRAHMS

RHIC

The RHIC accelerator at BNLSTAR detector

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Most 200 GeV Au+Au results are from the FY04 run

RHIC performance

Ldt = 241 b-1

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20.7m

8.2m

7.9m

Solenoidal Tracker at RHICLarge acceptance magnetic spectrometer

The STAR detector

• Particle identification and tracking (TPC, ToF, SVT)• Energy measurement (EMC)

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p+p jet+jet Au+Au ???

?

sNN = 200 GeV, STAR-TPC profile

Jet measurements in Au+Au

Jet-like correlation Statistical reconstruction using angular correlations

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• Strong high-pT particle suppression

• Disappearance of the away-side jet

Jet quenching is a final state effect

Initial medium density is high

Results so far

NNinelastCollAA

TNN

AA

TAA

TAA

NT

ddpdT

ddpNdpR

/ where

/

/)(

2

2

factor 4-5

Inclusive hadron spectra di-hadron correlation

Nuclear modification factor:

Nucl. Phys. A757, 102 (2005)

4 < pTtrig < 6 GeV/c

pTasso > 2 GeV/c

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Outline

• Inclusive identified particle spectra (particle ratio)

- modification of jet fragmentation

• Low-pT recoil and three particle correlation

(Mach-like cones ?)

- jet-medium interplay

• Di-hadron correlation at higher pT

- reappearance of the away-side jet

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Identified particle ratios

Au+Au 0-10%

p+p

Au+Au 0-10%

p+p

/K

0 s

Au+Au 0-5%

p+p

vacuum fragmentation

• Significant baryon enhancement in central Au+Au collisions with a maximum around 2-3 GeV/c

• Additional effects besides jet fragmentation

• Vacuum fragmentation approach at pT > ~6 GeV/c

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4 < pTtrig < 6 GeV/c

pTasso > 0.15 GeV/c

Where does the deposited energy go?

cos()

pTasso > 2 GeV/c

• Suppression of high-pT hadrons enhancement of low-pT hadrons

• Broadening and softening of the recoil distribution momentum balance shape (cos )

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4 < pTtrig < 6 GeV/c

pTasso > 0.15 GeV/c

• Softening of the associate hadrons in the cone opposite to leading trigger hadron

• Indication for thermal equilibration with the bulk medium

Mean-pT of away-side asso. hadrons

central Au+Au

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Mediumaway

near

Deflected jets

1

2

1

2

• Casalderrey-Solana, Shuryak and Teaney, hep-ph/0411315• Stoecker, Nucl. Phys A750, 121 (2005)• Ruppert and Muller, PLB 618, 123 (2005)

away

near

Medium

Conical flow

Indication for Mach-like cones ?

• Does a shock wave form?

• Three-particle correlations, two scenarios:

- Conical flow: associated particles may appear on opposite sides of Δφ = π

- Deflected jets: associated particles on the same side of Δφ = π

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pTtrig = 3-4 GeV/c, pT

asso = 1-2 GeV/c2-particle corr., bg, v2 subtracted

φ2=φ2-φtrig d+Au min-bias

dN2/d

Δφ1dΔφ2/Ntrig

φ1=φ1-φtrig

φ2=φ2-φtrig Au+Au 10%

Mediumaway

near

Deflected jets

away

near

Medium

Conical flow

Three-particle correlations

• Elongated along diagonal: kT effect?

deflected jets?

• Distinctive features of conical flow are not seen in present data within these pT

windows.

Andre Mischke (Utrecht) 148 < pT

trig < 15 GeV/c

• Correlation peak visible at near- and away-side

• Away-side peak is - finite suppressed - not broadened

• Limits on parton energy loss

Di-hadron azimuthal correlation at higher pT

Large year-4 Au+Au dataset

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Width of the away-side peak8 < pT

trig < 15 GeV/c

• Away-side widths similar for central and peripheral collisions

• Fragmentation similar as in the vacuum?

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Away-side yields

• Away-side suppression similar to RAA measurement

• No zT dependencetrigT

assoT

T p

pz

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X-N Wang, PLB 595, 165 (2004)

STAR preliminary

Comparison to model prediction

• Model (recoil fragmentation) with modified di-hadron fragmentation function

• Suppression factor

shows no zT dependence

• Qualitative but not quantitative agreement with measurement

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Conclusion• Inclusive hadron spectra and di-hadron correlations - strong suppression in central Au+Au - final state modification of jets

• Baryon enhancement - non-perturbative effects dominates at intermediate pT

• Low-pT recoil - away-side correlation widely dispersed - strong coupling effects of jets to bulk matter

• Three particle correlation using intermediate trigger-pT particle - Mach-cone like patterns not observed in present data

• correlations at higher pT

- reappearance of the away-side jet - no modification of the fragmentation distribution (width and slope) - stringent constraints for model descriptions

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Back-up slides

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50 institutes from 13 countries, 545 participants

The STAR collaboration

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Experimental and theoreticalchallenges in the search for the QGP

• Strong evidence for a dense, opaque, non-viscous, pre-hadronic liquid state of matter at RHIC

• Open question: do the quarks and gluon reach thermal equilibration with each other in this matter before confinement

The STAR collaboration's critical assessment of the evidence from RHIC collisions, Nucl. Phys. A757, 102 (2005)

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Identified particle RCP

peripheralbinaryperipheral

centralbinarycentral

Nyield

Nyield

//

• Φ follows mesons at intermediate pT no mass dependence

• All particles consistent for pT larger than ~5 GeV/c

• Coalescence of valence quarks?

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• Parton energy loss depends on the path length traversed • Qualitatively consistent with jet-quenching picture:

in-plane

out-of-plane

4 < pTtrig < 6 GeV/c

2 GeV/c < pTasso < pT

trig 2ˆLqCE RSmedium

Azimuthal correlations wrt. to the reaction plane

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In vacuo fragmentation of a high momentum quark to producehadrons competes with the in medium recombination of lower momentum quarks to produce hadrons

Example:• Fragmentation: Dq→h(z) produces

a 6 GeV/c proton from a 10 GeV/c quark

• Recombination:

- produces a 6 GeV/c proton from two 3 GeV/c quarks

- produces a 6 GeV/c proton from three 2 GeV/c quarks

Recombination models

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STAR preliminary , ||<1

0-5% 5-10%

10-20% 20-40%

40-60% 60-80%

pTtrig = 2.5-4.0 GeV/c

pTasso = 1.0-2.5 GeV/c

Conical flow

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away

syst. error

pT spectra of associate hadrons