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

02/06/2009High-pT at LHC '09

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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)

02/06/2009High-pT at LHC '09

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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.

02/06/2009High-pT at LHC '09

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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]

02/06/2009High-pT at LHC '09

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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]

02/06/2009High-pT at LHC '09

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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]

02/06/2009High-pT at LHC '09

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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]

02/06/2009High-pT at LHC '09

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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]

02/06/2009High-pT at LHC '09

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The Final Spectrum

• With fakerejection,good S/B forreconstructedjet down to10 GeV.

arXiv:0810.1219 [nucl-ex]

02/06/2009High-pT at LHC '09

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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]

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

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?

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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