simulation session @ calor 2002, march. 27, 2002m. wielers, triumf1 performance of jets and missing...
TRANSCRIPT
Simulation Session @ Calor 2002, March. 27, 2002
M. Wielers, TRIUMF 1
Performance of Jets and missing ET in ATLAS
Monika WielersTRIUMF, Vancouver
on behalf of the ATLAS Collaboration
10th International Conference on Calorimetry in High Energy Physics
Pasadena – March 2002
Simulation Session @ Calor 2002, March. 27, 2002
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Contents
Physics Goals for Jet/ETmiss Studies in ATLASThe ATLAS Calorimeter SystemSimulation of Detector Response and Pile-UpJet Reconstruction in ATLASPhysics Examples for Jet ReconstructionET
miss Reconstruction
Conclusions
I won’t have time to discuss physics
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Physics Goals for Jet/ETmiss Studies
Jet PhysicsQCD Studies
Measure jet cross sectionReconstruction of resonances
QCD: W jet jet, Z bb, t b W Exotics: W’jet jetSUSY: A
Measure jet multiplicity in SUSY decayJet veto in central region to reject backgroundJet tagging in forward region
Missing transverse energy measurementSUSY and other New Physics signatures, e.g. HZZllReconstruct inv. mass in decays with ’s, e.g. A, tlb
Simulation Session @ Calor 2002, March. 27, 2002
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The ATLAS Calorimeter System
EM CalorimeterLArg technologyCoverage: ||<3.2High granularity up to ||=2.5
Hadronic CalorimeterFe-Scintillating tiles in ||<1.5LArg EC (||<4.9)
HEC, Cu/LAr : 1.5< ||<3.2FCAL: Cu/Tungsten/LAr 3.2<||<4.9
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Simulation of Detector Response
Geant 3 still “standard”simulation programGeant 4 to come see various talks
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Simulation of Pile-up Noise~23 minimum bias events per bunch crossing at L=1034 cm-2 s-1
LArg uses bipolar shaper for read-out
“history” from around 27 bunch crossingsNoise peaks around zero GeVTile uses mono polar shapers“history” from 4 bunch crossingsResponse ~ Gaussian with FWHM of 50 ns
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Simulation of Pile-up Noise~700 minimum bias events need to be added to signal events at high luminosity ( 30 BC’s)
For given cell type weigh energy according to shaper response for given timeRead minimum bias events via secondary stream
On the fly via direct access file which is kept in memoryAdd an additional simulation step (together with pile-up in all other detectors)
used to keep correlations between detectors Keep big number of min. bias events in
memoryCurrently done in Fortran, work ongoing to get it running in C++
Simulation Session @ Calor 2002, March. 27, 2002
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Factors influencing Jet Reconstruction
Experimental factorsElectronic (and pile-up) noiseDifferent response to charged and neutral particles (0)Non-linearitiesgranularityLateral shower sizeDead material and cracksLongitudinal leakageMagnetic field effects
Physics related factors
Initial and final state radiationFragmentationUnderlying eventDepending on luminosity: minimum bias events
No unique strategy for jet reconstruction, depends on
Jet reconstruction algorithm, luminosity, physics process
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Energy ResolutionCalorimeter resolution at ||=0.3/E = 52%/E 3.0 GeV/E 1.7% for R=0.7 at L=1033cm-2s-1
/E = 81%/E 3.9 GeV/E 1.7% for R=0.4 at L=1034cm-2s-1
High luminosity pile-up, + el noise
el. noise
||=0.3
R=0.4 R=0.7
El. Noise 1.9 GeV
3.3 GeV
Pile-Up (L=1034cm–2s-
1)
4.7 GeV
14 GeV
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Physics Effects
Particle level jet energy
Parton energy
Depends on physics process
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Dete
ctor
Level
Gen
eri
c(Sub) Detector Levelcell signal reconstruction
cell signal corrections
Combined Detectorstower findingcluster finding
Algorithmic flow of jet reconstruction in software (in C++)
Get detector dataCalibrate to EM scaleCreate towers and clusters (if input to jets)Prepare input (cell, cluster, tower, jet…) for jet finder
Do jet finding Reconstruct and calibrate jets
Do jet classification
Redo any step if desired!
Physics JetJet classification
An
aly
sis
Jet ReconstructionProtoJet building
Jet findingJet reconstruction
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Forward Jet Tagging and Jet Veto
Important for background reduction for heavy Higgs production via Vector-Boson-Fusion ((VBF) ~ 1/5 (gg))
H produced centrallyAccompanying jets in forward region
Small hadronic activity in central regionVeto low-pT jets rejection of tt background
90 (80) % for ||<4 at low (high) luminosity (tt)~10%
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Resonance ReconstructionSimple ResonanceWjj at L=1034cm-2s-1
ET(W) ~ 150 GeV
In resonance chain Hhhbbbb no electronic, no pile-up noise
Use m(h) as mass constraint to reduce combinatorial background
apply recalibration using m(h)=m(bb) requires b-jet taggingBias in jet direction
if jets overlap
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ETmiss Reconstruction
Important for new physics signatures (SUSY particle production and decay)Invariant mass reconstruction for channels involving
A, tlb, HZZllET
miss reconstructed from cell energies within ||<5Accurate calibration of calorimeters vital
L=1033cm-2s-1
L=1034cm-2s-1
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ETmiss Tails
ETmiss Reconstruction important for
H ZZ ll with m(H)~500-700GeVPotential problem: background from Z+jet(s) events with badly measured jet
pT(Z)>200GeV
Assume jet is undetected
Jet reconstructed
Requirement of rejection factor of 1000 for ET
miss > 200 GeV achieved!
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ConclusionsLot’s of interesting physics to be done using jets and ET
miss
Higgs, SUSY, the unknown,…
ATLAS calorimeter well designed for jet/ETmiss
studiesWe are ready to face the challenge
Let’s see what 2007 brings us, when first collisions are expected !