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02/06/2009High-pT at LHC '09
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Details of Jet Reconstructionand Their Impact on Jet
Quenching MeasurementsNathan Grau
On Behalf of the ATLASCollaboration
02/06/2009High-pT at LHC '09
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Jet Results Are Here
J. Putschke (this conference)
Y.S. Lei (DNP’08)
PHENIX
02/06/2009High-pT at LHC '09
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With More To Come…
ATLASNG (QM’08)
CMSC. Roland(High pT at LHC ‘06)
ALICEJ. Harris(WWND‘07)
02/06/2009High-pT at LHC '09
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Outline
• Nuts and bolts talk about what I’ve learned inpreparation for ATLAS HI jet reconstruction– Biggest lesson: Cannot just run “standard” algorithms
“out of the box”• Two specific issues will be discussed
– Fake jets from cone jet reconstruction• Summary and extension of arXiv:0810.1219 [nucl-ex]
accepted for publication in EPJC.– Losing peripheral jet energy in HI kT jet reconstruction
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Cone Jets in ATLAS
• Cone jet reconstruction requires subtraction of theunderlying event before jet finding
-0.5<φ<-1.5
Calorimeter energyin 0.1x0.1 towers
NG (QM’08)
02/06/2009High-pT at LHC '09
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Cone Jets in ATLAS
• Subtract <ET> at the cell level– but there are upward fluctuations that exist
-0.5<φ<-1.5
Calorimeter energyin 0.1x0.1 towers
NG (QM’08)
02/06/2009High-pT at LHC '09
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Cone Jets in ATLAS
• Embedded jets are found– but so are other cone jet
-0.5<φ<-1.5
Calorimeter energyin 0.1x0.1 towers
NG (QM’08)
02/06/2009High-pT at LHC '09
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Cone Jets in ATLAS
• Embedded jets are found– but so are other cone jet
Calorimeter energyin 0.1x0.1 towers
-0.5<φ<-1.5
NG (QM’08)
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Evaluation of the Fake Rate
• Run the jet reconstruction without embedding jets• Suppress jet production in HIJING
– Do not sample Q2>100 GeV2 hard scattering– Suppress longitudinal string fragmentation
• produces 10-2 jets per event with ET>50 GeV
• Any reconstructed jets with ET >> 10 GeV in thissample are FAKE.
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The Fake Rate Is Not Small!
• The fake rate islarge– Fake rate =
expected rateat 30 GeV
– Fake rate = 1/2expected rateat 50 GeV
• Need to reject(>10) toimprove purityof jet sample
Expected Jet Rate
arXiv:0810.1219 [nucl-ex]
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The Character of Fake Jets
• Fake jets tend to be asingle tower abovethreshold (5 GeV) atlarge angles w.r.t. thejet axis
• Need a discriminationvariable whichemphasizes largeangle energy
Fake HIJING Jet
57 GeV
arXiv:0810.1219 [nucl-ex]
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A Rejection Variable: ΣjT
• Define
• Has a trivial jet ETdependence
• whereFake jets have a much wider energy profile (factor of ~2 at 50 GeV) than real jets
arXiv:0810.1219 [nucl-ex]
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Modified Variable: σΣjT
• Recast to anET-dependent# of fake jetsigma fromthe fake jetmean:
Real jet mean < Fake jet meanarXiv:0810.1219 [nucl-ex]
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Modified Variable: σΣjT
• Recast to anET-dependent# of fake jetsigma fromthe fake jetmean:
Rejection means efficiency loss
Reject
arXiv:0810.1219 [nucl-ex]
02/06/2009High-pT at LHC '09
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Trade-off:Efficiency vs. Rejection
• Strong anti-correlationbetween jetreconstructionefficiency andfake jetrejection
• Need rejectionof >10– 50%
efficiency at50 GeV
arXiv:0810.1219 [nucl-ex]
02/06/2009High-pT at LHC '09
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Trade-off:Efficiency vs. Rejection
• Strong anti-correlationbetween jetreconstructionefficiency andfake jetrejection
• Need rejectionof >10– 50%
efficiency at50 GeV
arXiv:0810.1219 [nucl-ex]
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The Final Spectrum
• With fakerejection,good S/B forreconstructedjet down to10 GeV.
arXiv:0810.1219 [nucl-ex]
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The Outlook:Data-Driven Rejection
• Azimuthal jetcorrelationsare sensitive tofake jets
• Fake jets arerandomlycorrelatedwith real jets
All Pairs
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Highest ET Di-jet Pairs:Pure Fake Jet Sample
• Correlatetrigger jets(A) with thenext highest-ET jet (B)
• Only jetpairs with|Δφ|>2π/3are real
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Associated Jet σΣjT
• Clearly thereal andfake jetsare wellseparatedin thisvariable.
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Associated Jet σΣjT
Reject
• Clearly thereal andfake jetsare wellseparatedin thisvariable.
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The Outlook: Quenched Jets
• <ΣjT> vs. ET(see slide 10)
• 40% differencebetweenHIJING andPYTHIA
• No statisticallysignificantdifferencebetweenPYQUEN andPYTHIA
arXiv:0810.1219 [nucl-ex]
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kT Jet Reconstruction
• Apply Fast-kT directly on the calorimeter towersi.e. no subtraction first
Calorimeter energyin 0.1x0.1 towers
A Angerami,W. Holzmann (QM’08)
02/06/2009High-pT at LHC '09
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kT Jet Reconstruction
• Apply Fast-kT directly on the calorimeter towersi.e. no subtraction first
Calorimeter energyin 0.1x0.1 towers
All towers are included in a jet.Most jets not “real”Embedded jets are circled
A Angerami,W. Holzmann (QM’08)
02/06/2009High-pT at LHC '09
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kT Jet Reconstruction
• Apply Fast-kT directly on the calorimeter towersi.e. no subtraction first
Discriminate between realand background jetsEmbedded jets are found inthis variable
A Angerami,W. Holzmann (QM’08)
02/06/2009High-pT at LHC '09
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A Wrinkle: Jet Periphery
• Many more towers in the embedded PYTHIA jet
PYTHIA ONLY
MERGED ONLY
BOTH
Single jet 0.1x0.1 Calo TowersA Angerami, W. Holzmann
02/06/2009High-pT at LHC '09
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A Wrinkle: Jet Periphery
• PYTHIA onlytowers primarilyin the jetperiphery
• Really effectsthe energy scale
• Possiblesolution: modifythe energy scalecorrection
A Angerami, W. Holzmann
02/06/2009High-pT at LHC '09
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Summary
• There are subtleties in the jet reconstruction algorithm inheavy ion collisions– Fake rate from correlate background fluctuations– Modification of kT jet shape
• These issues can and need to be overcome by careful studyfrom data for jet reconstruction to become standard in heavyion collisions.
• Understanding of the underlying event is crucial tounderstanding and performing jet reconstructionmeasurements in heavy ion collisions.– Can we measure, non-jet, correlated background fluctuations?