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Aug 23, 2009Aug 23, 2009

Introduction To Jet Reconstruction In ATLAS

Introduction To Jet Reconstruction In ATLAS

Peter LochPeter Loch

University of ArizonaUniversity of Arizona

Tucson, ArizonaTucson, Arizona

USAUSA

Peter LochPeter Loch

University of ArizonaUniversity of Arizona

Tucson, ArizonaTucson, Arizona

USAUSA

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2P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009My Road To Jets @ LHC

Started with calorimeter construction for H1/HERA (1987-1989)…Started with calorimeter construction for H1/HERA (1987-1989)… The “hardware years”…The “hardware years”…

……then Deep Inelastic Scattering with H1 (1989-1992,1993-1995)then Deep Inelastic Scattering with H1 (1989-1992,1993-1995) Electron, hadron, and jet reconstruction and calibration in a non-compensating calorimeter Electron, hadron, and jet reconstruction and calibration in a non-compensating calorimeter

Have always been attracted by complex response!Have always been attracted by complex response!

Experimental sensitivities to jet propertiesExperimental sensitivities to jet properties We did not call it sub-structure then!We did not call it sub-structure then!

Forward hadronic energy flow and jet productionForward hadronic energy flow and jet production E.g., DGLAP vs BFKL evolution at low Bjorken-E.g., DGLAP vs BFKL evolution at low Bjorken-xx

……with a short stop at DZero/Tevatron (1992)…with a short stop at DZero/Tevatron (1992)… Calorimeter response to minimum biasCalorimeter response to minimum bias

Mostly detector oriented noise, alignment and beam position studies Mostly detector oriented noise, alignment and beam position studies

……and at GEM/SSC (1992-1993)…and at GEM/SSC (1992-1993)… Design and response evaluation of the forward calorimeterDesign and response evaluation of the forward calorimeter

Detector prototyping and test beamsDetector prototyping and test beams

……and a long tenure in ATLAS/LHC (since 1995)and a long tenure in ATLAS/LHC (since 1995) Design and response evaluation of the forward calorimeterDesign and response evaluation of the forward calorimeter

Yes, it is the same design as for GEMYes, it is the same design as for GEM

Forward calorimeter detector construction managementForward calorimeter detector construction management My “Microsoft years”My “Microsoft years”

Focus on preparation for physics at LHC (since 2002)Focus on preparation for physics at LHC (since 2002) Initial interest jet physics and QCD…Initial interest jet physics and QCD…

Some leadershipSome leadership Calorimeter software coordination Calorimeter software coordination Jet and missing Et reconstruction performance coordinationJet and missing Et reconstruction performance coordination

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3P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009

Change of compositionChange of composition Radiation and decay inside Radiation and decay inside

detector volumedetector volume ““Randomization” of original Randomization” of original

particle contentparticle content Defocusing changes shape in Defocusing changes shape in

lab framelab frame Charged particles bend in Charged particles bend in

solenoid fieldsolenoid field Attenuation changes energyAttenuation changes energy

Total loss of soft charged Total loss of soft charged particles in magnetic fieldparticles in magnetic field

Partial and total energy loss of Partial and total energy loss of charged and neutral particles in charged and neutral particles in inactive upstream materialinactive upstream material

Hadronic and electromagnetic Hadronic and electromagnetic cacades in calorimeterscacades in calorimeters Distribute energy spatiallyDistribute energy spatially Lateral particle shower overlapLateral particle shower overlap

—

+

—

+

+—

+

100 MeV100 MeV

10 GeV10 GeV

1 GeV1 GeV

η

φ

Image of Jet in the Detector

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4P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009

+

—

+—

—

+

+

Change of compositionChange of composition Radiation and decay inside Radiation and decay inside

detector volumedetector volume ““Randomization” of original Randomization” of original

particle contentparticle content Defocusing changes shape in Defocusing changes shape in

lab framelab frame Charged particles bend in Charged particles bend in

solenoid fieldsolenoid field Attenuation changes energyAttenuation changes energy

Total loss of soft charged Total loss of soft charged particles in magnetic fieldparticles in magnetic field

Partial and total energy loss of Partial and total energy loss of charged and neutral particles in charged and neutral particles in inactive upstream materialinactive upstream material

Hadronic and electromagnetic Hadronic and electromagnetic cacades in calorimeterscacades in calorimeters Distribute energy spatiallyDistribute energy spatially Lateral particle shower overlapLateral particle shower overlap

100 MeV100 MeV

10 GeV10 GeV

1 GeV1 GeV

η

φ

Image of Jet in the Detector

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5P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009

+

—

+—

—

+

+

Change of compositionChange of composition Radiation and decay inside Radiation and decay inside

detector volumedetector volume ““Randomization” of original Randomization” of original

particle contentparticle content Defocusing changes shape in Defocusing changes shape in

lab framelab frame Charged particles bend in Charged particles bend in

solenoid fieldsolenoid field Attenuation changes energyAttenuation changes energy

Total loss of soft charged Total loss of soft charged particles in magnetic fieldparticles in magnetic field

Partial and total energy loss of Partial and total energy loss of charged and neutral particles in charged and neutral particles in inactive upstream materialinactive upstream material

Hadronic and electromagnetic Hadronic and electromagnetic cacades in calorimeterscacades in calorimeters Distribute energy spatiallyDistribute energy spatially Lateral particle shower overlapLateral particle shower overlap

100 MeV100 MeV

10 GeV10 GeV

1 GeV1 GeV

η

φ

Image of Jet in the Detector

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6P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009

+

—

+—

—

+

+

Change of compositionChange of composition Radiation and decay inside Radiation and decay inside

detector volumedetector volume ““Randomization” of original Randomization” of original

particle contentparticle content Defocusing changes shape in Defocusing changes shape in

lab framelab frame Charged particles bend in Charged particles bend in

solenoid fieldsolenoid field Attenuation changes energyAttenuation changes energy

Total loss of soft charged Total loss of soft charged particles in magnetic fieldparticles in magnetic field

Partial and total energy loss of Partial and total energy loss of charged and neutral particles in charged and neutral particles in inactive upstream materialinactive upstream material

Hadronic and electromagnetic Hadronic and electromagnetic cacades in calorimeterscacades in calorimeters Distribute energy spatiallyDistribute energy spatially Lateral particle shower overlapLateral particle shower overlap

+—

—

+

+

100 MeV100 MeV

10 GeV10 GeV

1 GeV1 GeV

η

φ

Image of Jet in the Detector

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7P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009

Change of compositionChange of composition Radiation and decay inside Radiation and decay inside

detector volumedetector volume ““Randomization” of original Randomization” of original

particle contentparticle content Defocusing changes shape in Defocusing changes shape in

lab framelab frame Charged particles bend in Charged particles bend in

solenoid fieldsolenoid field Attenuation changes energyAttenuation changes energy

Total loss of soft charged Total loss of soft charged particles in magnetic fieldparticles in magnetic field

Partial and total energy loss of Partial and total energy loss of charged and neutral particles in charged and neutral particles in inactive upstream materialinactive upstream material

Hadronic and electromagnetic Hadronic and electromagnetic cacades in calorimeterscacades in calorimeters Distribute energy spatiallyDistribute energy spatially Lateral particle shower overlapLateral particle shower overlap

+—

—

+

+

+—

—

+

+

c

c

c

c

c

c

c

c

c

c

c

c

100 MeV100 MeV

10 GeV10 GeV

1 GeV1 GeV

η

φ

Image of Jet in the Detector

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8P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009Jet Reconstruction Task

Jet Reconstruction ChallengesJet Reconstruction Challenges

physics reaction of interest (interaction or parton level)physics reaction of interest (interaction or parton level)

added particles from underlying eventadded particles from in-time (same trigger)

pile-up eventjet reconstruction algorithm efficiency

added particles from underlying eventadded particles from in-time (same trigger)

pile-up eventjet reconstruction algorithm efficiency

longitudinal energy leakagedetector signal inefficiencies (dead channels, HV…)

noise from (off- and in-time) pile-upelectronic noise

calo signal definition (noise suppression etc.)dead material losses (front, cracks, transitions…)

detector response characteristics (e/h ≠ 1)jet reconstruction algorithm efficiency

lost soft tracks due to magnetic field

longitudinal energy leakagedetector signal inefficiencies (dead channels, HV…)

noise from (off- and in-time) pile-upelectronic noise

calo signal definition (noise suppression etc.)dead material losses (front, cracks, transitions…)

detector response characteristics (e/h ≠ 1)jet reconstruction algorithm efficiency

lost soft tracks due to magnetic field

Experiment (“Nature”)

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9P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009ATLAS Jet Reconstruction

Move to better motivated jet algorithmsMove to better motivated jet algorithms ATLAS uses FastJet and SISCone librariesATLAS uses FastJet and SISCone libraries

Common implementation theory/CMS/ATLAS Common implementation theory/CMS/ATLAS

Narrow (0.4) AntiKt jets preferred for initial physics Narrow (0.4) AntiKt jets preferred for initial physics Most regularly shapedMost regularly shaped More straight forward handle on extra energy from pile-up (?)More straight forward handle on extra energy from pile-up (?)

Legacy (ATLAS) seeded fixed cone still availableLegacy (ATLAS) seeded fixed cone still available Usefulness basically limited to LO final statesUsefulness basically limited to LO final states Fast algorithm ok for trigger and event filterFast algorithm ok for trigger and event filter

Factorized reconstructionFactorized reconstruction Attempt to use calorimeter signal definitions to address most detector Attempt to use calorimeter signal definitions to address most detector

specific issuesspecific issues Use towers and clustersUse towers and clusters

Calibrations and corrections determined independentlyCalibrations and corrections determined independently Distinguish carefully between relative and absolute corrections by least Distinguish carefully between relative and absolute corrections by least

interdependencyinterdependency Jet energy scale evolution

Limit to factorization introduced by calorimeterLimit to factorization introduced by calorimeter E.g. detector couples response and fragmentation – corrections often only possible on

average

More details on Thursday!More details on Thursday!

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10P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009First Data Jet Calibration Priorities

calo signals

calot t

tjetcalo

t

p p

pR

p

1.6 2.4 GeVTp 1.6 2.4 GeVTp

single simulations

extracted from minbias

(plots from CERN-OPEN-2008-020)

Need flat jet response rather quicklyNeed flat jet response rather quickly Basis for first physics analysesBasis for first physics analyses

E.g., inclusive jet cross-sectionE.g., inclusive jet cross-section

Systematics can dominate total error Systematics can dominate total error already at 10-200 pbalready at 10-200 pb-1-1

Depending on final state, kinematic range Depending on final state, kinematic range considered, fiducial cuts,…considered, fiducial cuts,…

Requires understanding of hadronic Requires understanding of hadronic responseresponse Minimum biasMinimum bias

Single charged hadron response of calorimeterSingle charged hadron response of calorimeter Pile-up with tracks and clustersPile-up with tracks and clusters

Di-jetsDi-jets Underlying event with tracks and clustersUnderlying event with tracks and clusters Direction dependent response correctionsDirection dependent response corrections

Prompt photonsPrompt photons Absolute scale by balancing, missing Et Absolute scale by balancing, missing Et

projection fraction, etc.projection fraction, etc.

Initial focus on simple approachesInitial focus on simple approaches Least number of algorithmsLeast number of algorithms Least numerical complexityLeast numerical complexity Realistic systematic error evaluationRealistic systematic error evaluation

Conservative ok for first data!Conservative ok for first data!

But also try new or alternative But also try new or alternative approaches!approaches!

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11P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009Initial Jet Calibration Scheme

optional

data driven

MC

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12P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009Beyond First Physics

Initial tasks focus on jet energy scale (JES) Initial tasks focus on jet energy scale (JES) errorserrors Binds most of the manpower todayBinds most of the manpower today

Rather wide spectrum of signal and calibration choices Rather wide spectrum of signal and calibration choices But did not want to make hard decisions based on MC alone!But did not want to make hard decisions based on MC alone!

Future activity more focused on jet featuresFuture activity more focused on jet features Jet-by-jet corrections to reduced fluctuations of JESJet-by-jet corrections to reduced fluctuations of JES

Attempt to unfold residual fragmentation dependenciesAttempt to unfold residual fragmentation dependencies Useful observables include jet shapes Useful observables include jet shapes

Jet-by-jet pile-up suppression Jet-by-jet pile-up suppression Qualitative change from initial run scenariosQualitative change from initial run scenarios

Pile-up signal history (= noise in ATLAS) depends on instantaneous luminosity and bunch crossing frequency

Large source of signal fluctuations affects all jet final statesLarge source of signal fluctuations affects all jet final states Experimental resolution for jet masses and substructureExperimental resolution for jet masses and substructure

Discovery tool for heavy particle decaysDiscovery tool for heavy particle decays E.g., fully hadronic decay of boosted top (“3 in 1”)

Experimentally challenging but not impossibleExperimentally challenging but not impossible (more on Wednesday & Thursday)

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13P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009

Multiple interactions Multiple interactions between partons in other between partons in other protons in the same protons in the same bunch crossingbunch crossing Consequence of high rate Consequence of high rate

(luminosity) and high (luminosity) and high proton-proton total cross-proton-proton total cross-section (~75 mb)section (~75 mb)

Statistically independent Statistically independent of hard scatteringof hard scattering Similar models used for soft Similar models used for soft

physics as in underlying physics as in underlying eventevent

Signal history in Signal history in calorimeter increases calorimeter increases noisenoise Signal 10-20 times slower Signal 10-20 times slower

(ATLAS) than bunch (ATLAS) than bunch crossing rate (25 ns)crossing rate (25 ns)

Noise has coherent Noise has coherent charactercharacter Cell signals linked through Cell signals linked through

past shower developmentspast shower developments

Et ~ 58 GeV

Et ~ 81 GeV

without pile-up

Pile-Up

Prog.Part.Nucl.Phys.60:484-551,2008

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14P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009

Multiple interactions Multiple interactions between partons in other between partons in other protons in the same protons in the same bunch crossingbunch crossing Consequence of high rate Consequence of high rate

(luminosity) and high (luminosity) and high proton-proton total cross-proton-proton total cross-section (~75 mb)section (~75 mb)

Statistically independent Statistically independent of hard scatteringof hard scattering Similar models used for soft Similar models used for soft

physics as in underlying physics as in underlying eventevent

Signal history in Signal history in calorimeter increases calorimeter increases noisenoise Signal 10-20 times slower Signal 10-20 times slower

(ATLAS) than bunch (ATLAS) than bunch crossing rate (25 ns)crossing rate (25 ns)

Noise has coherent Noise has coherent charactercharacter Cell signals linked through Cell signals linked through

past shower developmentspast shower developments

Et ~ 58 GeV

Et ~ 81 GeV

with design luminosity pile-up

Pile-Up

Prog.Part.Nucl.Phys.60:484-551,2008

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15P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009

Multiple interactions Multiple interactions between partons in other between partons in other protons in the same protons in the same bunch crossingbunch crossing Consequence of high rate Consequence of high rate

(luminosity) and high (luminosity) and high proton-proton total cross-proton-proton total cross-section (~75 mb)section (~75 mb)

Statistically independent Statistically independent of hard scatteringof hard scattering Similar models used for soft Similar models used for soft

physics as in underlying physics as in underlying eventevent

Signal history in Signal history in calorimeter increases calorimeter increases noisenoise Signal 10-20 times slower Signal 10-20 times slower

(ATLAS) than bunch (ATLAS) than bunch crossing rate (25 ns)crossing rate (25 ns)

Noise has coherent Noise has coherent charactercharacter Cell signals linked through Cell signals linked through

past shower developmentspast shower developments

34 2 110 cm sL 34 2 110 cm sL

8 GeV

0.4R

0.7R

18 GeV

0.1 0.1 R

( ) (GeV)TRMS p

Pile-Up

Prog.Part.Nucl.Phys.60:484-551,2008

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16P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009ATLAS Status

ATLAS is up and runningATLAS is up and running Used the LHC delay for continued sub-detector Used the LHC delay for continued sub-detector

commissioning and repaircommissioning and repair Cosmics for long term signal stability estimates and detector Cosmics for long term signal stability estimates and detector

timing and alignmenttiming and alignment Understanding of calorimeter noise with random (“empty”) Understanding of calorimeter noise with random (“empty”)

eventsevents Millions of cosmics recordedMillions of cosmics recorded

Test of computing chainTest of computing chain Re-reconstruction of cosmics trains ATLAS world wide Re-reconstruction of cosmics trains ATLAS world wide

computingcomputing

Some initial impressions for jets and missing Some initial impressions for jets and missing EtEt Noise contributions to missing Et reconstruction in random Noise contributions to missing Et reconstruction in random

eventsevents Calorimeter signal definition evaluationCalorimeter signal definition evaluation

Jets in cosmicsJets in cosmics Helps develop in-time background suppression strategiesHelps develop in-time background suppression strategies

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17P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009Cosmics In ATLAS

Single cosmic muons traversing the Single cosmic muons traversing the experiment help understanding experiment help understanding mostly timing and detector mostly timing and detector sensitivitiessensitivities

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18P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009Cosmic Showers In ATLAS

Cosmic showers hitting the detector Cosmic showers hitting the detector help understanding fake jet rates help understanding fake jet rates from this background in collision from this background in collision physicsphysics

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19P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009Understanding Cosmics

20 GeV

ATLAS Cone 0.4

(electromagnetic energy scale)

jett

cone

E

R

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20P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009Understanding Noise

The scalar missing Et sum is a very powerful diagnostic variable in random events (stability with time, noise suppression, major detector faults…)

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21P. LochP. Loch

U of ArizonaU of Arizona

Aug 23, 2009Aug 23, 2009Conclusion & Outlook

ATLAS is ready for collisionsATLAS is ready for collisions The detector is operating since ~1 year The detector is operating since ~1 year

Major faults detected in 2008 are repairedMajor faults detected in 2008 are repaired Calorimeter noise is well understoodCalorimeter noise is well understood

Response to cosmics can be simulated reasonably wellResponse to cosmics can be simulated reasonably well First algorithms for suppression of cosmic background First algorithms for suppression of cosmic background

availableavailable New studies with overlaying simulated signal events with New studies with overlaying simulated signal events with

real cosmics underwayreal cosmics underway

More on ATLAS this weekMore on ATLAS this week Wednesday Wednesday

Experimental aspects of jet reconstruction in new physics Experimental aspects of jet reconstruction in new physics (informal)(informal) Jet masses, shapes, topologies, missing Et in ATLAS,…

ThursdayThursday Talk on “Jet Reconstruction in ATLAS” Talk on “Jet Reconstruction in ATLAS”

More details on calorimeter signals, algorithms, performance expectations, jet shapes,…