Elena Bruna for the STAR CollaborationYale University

Quark Matter 09, Knoxville 03/29 -04/04 2009

Our approachInvestigate Jet Fragmentation Functions in AuAu

w.r.t. ppWe use di-jets in triggered events:

“Trigger” jet: large neutral energy in single tower jet comes from surface – will test this !

“Recoil” jet: away side of trigger jet jet travels thru the medium and suffers quenching – will test this!

Di-jet ratesIf we can fully reconstruct jets, the di-jet rate in AuAu

should not be suppressed – will test this!If unbiased jet population, quenching

modification of Fragmentation Function – will test this!

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“recoil” jet“recoil” jet

“trigger” jet“trigger” jet

• Trigger setup with the STAR e.m. calorimeter (EMC):

•High Tower Trigger (HT): tower 0.05x0.05 (ηxϕ) with Et> 5.4 GeV

• Data Set analyzed:

• pp (2006): HT trigger events

• AuAu (2007): HT trigger events, 0-20% central

• Jet Finder Algorithm: Anti-kT (from FastJet package)

• R=0.4 , |jet|<1-R

• charged particle pT (TPC), 0.1<pT<20 GeV/c

• neutral tower Et 0.05x0.05 (ηxϕ) (EMC)

• Hadronic correction

• Electron correction for double counting 3

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[M. Cacciari, G. Salam, G. Soyez 0802.1188]

Jet Finding in Heavy-Ion collisionsGOAL: Fully reconstruct jets in high-multiplicity

environmentHow to suppress background:

Reduce the jet area (in pp >80% of pT(Jet) in R<0.4)

Apply a pTcut,particle on tracks and towers before Jet Finding

4ϕη

pt p

er g

rid

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l [G

eV]

STAR preliminary~ 21 GeV

di-jet event

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pp √s=200 GeVSTAR Preliminary

Event Background in AuAu

Bac

kgro

und

flu

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tions

[G

ev]

Rc

• Event-by-event basis: • pT (Jet Measured) ~ pT (Jet) + A ± √A

• is the background energy per unit area• A is the jet area• , A estimated from FastJet algorithm• Background energy in R=0.4 ~ 45 GeV

• Substantial region-to-region background fluctuations • Comparable in magnitude from FastJet and naïve random cones⇒ significantly reduced by applying a pT cut,particle on tracks and towers

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STAR Preliminary

Multiplicity

(G

eV

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a)

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AuAu √s=200 GeVSTAR Preliminary

Background to di-jets in AuAuBackground di-jet rate = “Fake” + Additional Hard ScatteringFake jets:

background particles clustered as jets Additional hard scattering contribution in HI Collisions:

uncorrelated in w.r.t. Trigger jet (does not contribute in inclusive jet measurements)

Is estimated using “jet” spectrum at 90° to trigger jet Use “jet” spectrum at 90° to correct for “fake” di-jets

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Trigger jet pT > 10 GeV

pTcut,particle = 0.1 GeV pTcut,particle=2 GeVTrigger jet pT > 10 GeV

di-jet

Trigger jet

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STAR Preliminary STAR Preliminary

Au+Au HT 0-20%

Au+Au HT 0-20%

Towards Fragmentation FunctionsGOAL: get a good energy estimate for recoil jet in

AuAu

Two approaches:

1) Use trigger jet energy as proxy for recoil jet: Trigger jet found with pTcut,particle on tracks and towers small

background fluctuations Energy of trigger jet used for FF in recoil jet (gamma-jet like

approach)

2) The energy of recoil jet used Recoil jet found with no pTcut,particle large background

fluctuations Use recoil jet energy after correcting for background

fluctuations (unfolding) 7

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HT trigger

“recoil” jet“recoil” jet

“trigger” jet“trigger” jet

Fragmentation Functions

In AuAu:FF(Jet)=FF(Jet+Bkg)-FF(bkg)

Bkg estimated from charged particle spectra out of jets, rescaling to the area with R=0.7

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Charged particle FF: R(FF)=0.7

AuAu (Jet+Bkg)

AuAu (Bkg)

pT Jet rec(trigger)>20 GeV & pTcut,particle=2 GeV

STAR Preliminary

large uncertainties due to background (further systematic evaluation needed)

rec=ln( pT,Jet rec / pT,hadr)

low zhigh z

Assumption: trigger jet in AuAu is equivalent to pp vacuum fragmentation (no large nuclear effects)

Shapes of spectra and FF are similar in pp and AuAu

trigger jets not significantly modifiedTrigger jet energy can be used as a proxy for recoil jet

Trigger Jet Energy as a proxy?

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pT(trigger jet)>20 GeVPtcut=2 GeV

Ratio of FF: AuAu/pp

STAR Preliminary

Normalized spectra above 7 GeV for shape comparisonUncorrected spectra

STAR Preliminary

zrec=pT,hadr/pT,Jet rec (trigger) zrec=pT,hadr/pT,Jet rec (trigger)

pT Jet rec(trigger)>20 GeV pTcut,particle=2 GeV

large uncertainties due to background (further systematic evaluation needed)

Recoil Jet FF from 1st approach

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Energy of trigger jet used pT Jet rec(trigger)>20 GeV & pTcut,particle=2 GeV

pT Jet rec(recoil)>25 GeV & pTcutparticle=0.1 GeV

CAVEAT: nuclear kT effect not taken into account, expected to be of the order 2-3 GeV

No significant modification of FF of recoil jets with pTrec>25 GeV

STAR Preliminary

STAR Preliminary

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large uncertainties due to background (further systematic evaluation needed)

large uncertainties due to background (further systematic evaluation needed)

zrec=pT,hadr/pT,Jet rec (trigger)

zrec=pT,hadr/pT,Jet rec (trigger)

R=0.4

R=0.7

Trigger jet energy uncertainty

Large background fluctuations in AuAu w/o pTcut,particle

Parameterized by Gaussian smearing with =6 GeV in AuAu 0-20%

Solution: unfold background fluctuations and extract “true” spectrum allows to compare pp and AuAu

Data driven – model independent approach

2nd approach: “unfolding” methodE

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Pythia jetsPythia+AuAu MB jets

STAR Preliminary

Simulation:Effect of bkg fluctuations on true jet spectrum

di-jet spectra from unfolding

• Significant suppression seen Indicates:

• Energy shifts to larger cone radii (>0.4)• Some Jets “absorbed”

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STAR Preliminary

STAR Preliminary

• Biased to extreme path length of recoil jets

Recoil Jet FF from unfoldingE

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pTrec(trigger) > 10 GeV & pTcut,particle=2 GeVpTrec(recoil) > 25 GeV & pTcut,particle=0.1 GeV

Energy of recoil jet used

No significant modification of FF of recoil jets with pTrec>25 GeVDominated by non-interacting jets?

STAR Preliminary

R=0.4

R=0.7

pt,rec(AuAu)>25 GeV ⇒ < pt,rec(pp)> ~ 25 GeV

STAR Preliminary

Recoil Jet FF: Lower Jet pT

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Reducing the jet energy indication of modification of FF

pTrec(trigger) > 10 GeV & pTcut,particle=2 GeV20<pTrec(recoil)<25 GeV & pTcut,particle=0.1 GeVEnergy of recoil jet used

STAR Preliminary

R=0.4

R=0.7

20<pt,rec(AuAu)<25 GeV ⇒ < pt,rec(pp)> ~ 18 GeV

SummaryEvidence that di-jet rates are suppressed

A. Recover a fraction of the jet energy shift towards smaller energies

B. Do not reconstruct jet

Biased jet population selectedpTrec(recoil)>25 GeVNo strong modification of FF

(two approaches lead to a similar conclusion) High-energy recoil jets are biased (non interacting)

20<pTrec(recoil)<25 GeVdi-jet rates less suppressed

A. “Feed-down” from high-energy jets B. More complete jet energy recovered

Indication of modification of FF

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STAR Preliminary

STAR Preliminary

pt,rec(AuAu)>25 GeV

STAR Preliminary

20<pt,rec(AuAu)<25 GeV

Outlookdi-jets are a promising tool to study Jet Fragmentation

FunctionsExtreme selection: recoil jets have a longer in-medium path

Investigate further the systematicsCompare to quenching models (JEWEL, qPYTHIA, …)How can we recover an unbiased jet population?

Look at larger radiiLook at di-jets in AuAu Min Bias

Change path length bias?Investigate sub-jets / energy flow profile

Clustering/re-distribution of energy within the jet

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Extra slides

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Trigger jet: FF ratio

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