elena bruna for the star collaboration yale university
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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
cel
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
ctua
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
/are
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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