Light h 0 decay in SUSY cascade Jet Reconstruction & B tagging
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02/06/2009 High-pT at LHC '09 1 Details of Jet Reconstruction and Their Impact on Jet Quenching Measurements Nathan Grau On Behalf of the ATLAS Collaboration
Transcript of Details of Jet Reconstruction and Their Impact on Jet ...ncgrau/physics/talks/Grau_JetReco… ·...
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
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With More To Come…
ATLASNG (QM’08)
CMSC. Roland(High pT at LHC ‘06)
ALICEJ. Harris(WWND‘07)
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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)
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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)
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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)
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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]
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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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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 ET
• Fake jetshave a highconcentration at lowET
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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)
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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)
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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)
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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
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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
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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?
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Backup Slides
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Example of Fake Jets
Real PYTHIA Jet Fake HIJING Jet
57 GeV 57 GeV
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Recast σΣjT
• Separationincreases withincreasing jetET
• Fake rate dropsfaster thanexpected jetrate
Real jet mean < Fake jet mean
