new approaches to hadronic final state reconstruction
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New Approaches to Hadronic Final State Reconstruction. David W. Miller SLAC ATLAS. Overview. Standard model measurements and searches for new physics will both rely heavily on the understanding of the jet energy scale, missing energy measurements and the identification of b -jets . - PowerPoint PPT PresentationTRANSCRIPT
July 7, 2008 SLAC Annual Program Review Page 1
New Approaches to Hadronic Final State Reconstruction
David W. Miller
SLAC ATLAS
July 7, 2008 SLAC Annual Program Review Page 2
Overview
• Extending jet algorithm functionality and flexibility for first data
• New approaches to jet reconstruction and missing energy through use of tracking information
• Jet-vertex association for high luminosity jet identification, jet-energy corrections and primary vertex selection
• Semileptonic b-jet energy corrections
• Missing ET significance
Standard model measurements and searches for new physics will both rely heavily on the understanding of the jet energy scale, missing energy measurements and the identification of b-jets.
Through the Jet/MET/bTag Working Group (formed and led by Ariel Schwartzman) we have developed and applied new techniques for understanding and improving jets, missing transverse energy and heavy flavor in ATLAS and in collaboration with several other institutes.
Standard model measurements and searches for new physics will both rely heavily on the understanding of the jet energy scale, missing energy measurements and the identification of b-jets.
Through the Jet/MET/bTag Working Group (formed and led by Ariel Schwartzman) we have developed and applied new techniques for understanding and improving jets, missing transverse energy and heavy flavor in ATLAS and in collaboration with several other institutes.
Extending jet algorithms for first data
Legacy jet algorithms
• Tower algorithm (TowerJets)– Built from geometrical
arrangements of calorimeter cells– No noise suppression is applied
• Topological clusters (TopoJets)– Built from topological clusters of
calorimeter cells– Noise suppression is applied– Clusters size can grow non-
linearly in presence of pile-up
New approach in ATLASNew approach in ATLAS• Use the most desirable aspects
of each algorithm– Towers are finite, well-defined
objects– Clusters efficiently select cells
above a dynamic noise threshold
• Build towers using only cells used in topological clusters– Well-behaved at high-
luminosity– More easily understood in first
data
July 7, 2008 SLAC Annual Program Review Page 3
(DWM, SLAC)
Advances in jet resolution using tracking information
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Jet structureJet structure Jet response depends on ftrkJet response depends on ftrk Improvements with ftrk correctionImprovements with ftrk correction
Jet response improved using the charged energy fraction ftrk
ftrk provides information on jet fragmentation, particle content and topology and permits jet energy corrections after all other jet-energy scale corrections are applied
Jet response improved using the charged energy fraction ftrk
ftrk provides information on jet fragmentation, particle content and topology and permits jet energy corrections after all other jet-energy scale corrections are applied
New approach in ATLASNew approach in ATLAS: use tracks to provide additional information on jet composition
Account for jet-to-jet fluctuations in charged particle content
New approach in ATLASNew approach in ATLAS: use tracks to provide additional information on jet composition
Account for jet-to-jet fluctuations in charged particle content
(Marshall, Columbia U.)
Jets & tracks: improvements and implementation
ftrk response corrections
• Jet resolution– Corrections applied in bins of ftrk – Jet-by-jet fluctuations are reduced– Improvement in overall jet energy
resolution
• Jet resolution– Corrections applied in bins of ftrk – Jet-by-jet fluctuations are reduced– Improvement in overall jet energy
resolution
Improvements to jet-resolution: 10% at 40 GeVImprovements to jet-resolution: 10% at 40 GeV
• Official ATLAS software package integrated into full reconstruction
• Available to all ATLAS users for first data
• Documentation in ATLAS internal note– ATL-COM-PHYS-2008-074
• Used in Higgs Z/Wjjγγ analysis
July 7, 2008 SLAC Annual Program Review Page 5
(Marshall, Columbia U.)
Missing energy applications
July 7, 2008 SLAC Annual Program Review Page 6
Improvements to jet-energy has implications for missing energy measurementsUsing track information to identify a systematic effect in missing energy
allows us to correct for it
Improvements to jet-energy has implications for missing energy measurementsUsing track information to identify a systematic effect in missing energy
allows us to correct for it
Artificial missing energy from pT imbalance is corrected following jet energy corrections
(Schwartzman, SLAC)
Tracks-jets as complementary to calorimeter jets
Calorimeter jets can incorporate particles from multiple interactions
Using track-jets with Z information, can separate interactions
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New approach in ATLASNew approach in ATLASBuild “jets” from tracks in Inner Detector tracker
– Inherently 3D objects (Z, η, φ) unlike calo jets
New approach in ATLASNew approach in ATLASBuild “jets” from tracks in Inner Detector tracker
– Inherently 3D objects (Z, η, φ) unlike calo jets(Schwartzman,
SLAC)
These track-jets are completely independent from the calorimeter
Good angular resolution, used in b-tagging trigger (see I. Aracena’s talk)
Improved jet-finding efficiency with track-jets
Limited by tracker acceptance Improved efficiency for low-pT
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The use of tracks provides complementary information to calorimetersTrack-jets allow for the measurement of jet reconstruction efficiencyEnergy losses in the inner detector and cryostat decrease calo efficiency but do not affect track-jets
The use of tracks provides complementary information to calorimetersTrack-jets allow for the measurement of jet reconstruction efficiencyEnergy losses in the inner detector and cryostat decrease calo efficiency but do not affect track-jets
(Schwartzman, SLAC)
η
Δ
ttbar events
Identification of fake missing energy with track-jets
Unmeasured calorimeter jets results in artificial missing energy
• Using track-jets to pin-point un-reconstructed jets, events with fake missing energy can be removed
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(Majewski, BNL)
Gap atη=0.7-0.8
Missing calorimeter jet
Rad
ius
(m)
Beam axis (m)
Jet-vertex association (JVF)
• We can use the vertex detector to reconstruct these additional vertices
• Associate standard calorimeter jets to primary vertices using tracks
• Obtain jet-by-jet energy correction for pile-up and jet-selection criterion for calo jets
• Improve jet-energy, missing ET resolution and primary vertex (PV) selection
July 7, 2008 SLAC Annual Program Review Page 10
JVF measures the fraction of charged particle transverse (track) momentum in each jet from each identified primary vertex in the event.
JVF measures the fraction of charged particle transverse (track) momentum in each jet from each identified primary vertex in the event.
ATLAS detector with pileup(23 interactions per beam xing)
Expect >23 multiple simultaneous proton-proton interactions (extra jets and vertices).
Tevatron techniques to account for this discount large fluctuations in the form of jets
New approach:New approach: use tracking + calorimeter + vertex information to account for jet origin use tracking + calorimeter + vertex information to account for jet origin
Expect >23 multiple simultaneous proton-proton interactions (extra jets and vertices).
Tevatron techniques to account for this discount large fluctuations in the form of jets
New approach:New approach: use tracking + calorimeter + vertex information to account for jet origin use tracking + calorimeter + vertex information to account for jet origin
New approach to jet identification in ATLAS
Using JVF to select hard-scatter jets in events with pile-up
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Using JVF we recover the flat jet-multiplicity distribution
without raising the jet pT thresholdIn official ATLAS software and available to all ATLAS users for first dataFully documented in internal ATLAS note: ATL-COM-PHYS-2008-008
Using JVF we recover the flat jet-multiplicity distribution
without raising the jet pT thresholdIn official ATLAS software and available to all ATLAS users for first dataFully documented in internal ATLAS note: ATL-COM-PHYS-2008-008
ttbar events: 2x1033 cm-2s-1
(DWM, SLAC)
• Without any QCD pile-up, expect certain jet multiplicity
• As instantaneous luminosity increases, additional interactions contribute jets and energy to event
• Using JVF we can reliably select jets from the signal vertex
Jet-by-jet energy corrections for pile-up contributionsJet energy response vs. JVF: 1033 & 2x1033 cm-2s-1 (ttbar) Jet energy response vs. pT and JVF (ttbar)
July 7, 2008 SLAC Annual Program Review Page 12
Using JVF to correct jet energies in a luminosity independent mannerDerive jet-energy correction similar to that already done for ftrk to correct jets for pileup contributions on a per jet basis per jet basis (new technique at hadron colliders)(new technique at hadron colliders)
Using JVF to correct jet energies in a luminosity independent mannerDerive jet-energy correction similar to that already done for ftrk to correct jets for pileup contributions on a per jet basis per jet basis (new technique at hadron colliders)(new technique at hadron colliders)
(DWM, SLAC)
Using jet-vertex association to select primary vertices
Improving primary vertex selection in events with multiple interactions by using information from reconstructed objects in the event
• The high luminosity environment in ATLAS will present challenges to correct hard-scatter vertex identification
• Extremely important for b-tagging• New approach in ATLAS• Measure efficiencies in real data
with high-pT lepton tagging
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Muons and JVF tagged correct PV
Jet & μ-tagJet & μ-tag
DefaultDefaultμ
jet
jet
jet
(DWM, SLAC)
Semileptonic b-jet corrections
b-jet response in ttbar events
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Correcting for missing neutrino energy• Jets with constituent ν’s have energies
systematically underestimated by 10%– Add neutrino energy back into jet energy
• In official ATLAS jet reconstruction software
• Available to all users for first data
• Documented in internal ATLAS note: – ATL-COM-PHYS-2008-086
(Mateos, Columbia U.)
Use of semileptonic b-jet correction in ATLAS
Data-driven methods for estimating corrections under way
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Not only will the jet energy scale of b-jets occupy an integral role in many standard model measurements in early data (such as ttbar) but the preponderance of b-jet channels in SUSY and Higgs searches renders it even more important
Not only will the jet energy scale of b-jets occupy an integral role in many standard model measurements in early data (such as ttbar) but the preponderance of b-jet channels in SUSY and Higgs searches renders it even more important
Technique applied in Higgs Working Group H(120 GeV)bbbar analysis
(Mateos, Columbia U.)
Missing energy significance
• Define a likelihood ratio given the particular event topology (jet resolutions, jet energy and missing ET) to reject events with fake missing ET
– Separate true missing ET from fake missing ET
– Not simply MET/σ
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QCD Events (fake missing ET) W+jets
(real missing ET)
(Perez, Columbia U.,
Butler, SLAC)
Uncovered topics in this talk
• Several active areas of research and techniques not covered due to time constraint!– Quark/gluon tagging: SUSY jet+MET background
reduction– b/c quark-jet separation: b-jet background reduction– Gluon splitting (gluonbbbar) removal: large source of
background for SUSY b-jet+MET– Di-jet resolution measurements in first data
July 7, 2008 SLAC Annual Program Review Page 17
Summary
• A. Schwartzman’s Jet/MET/bTag WG and the collaborating is providing a coherent, wide range of contributions to ATLAS Jet/MET performance and physics groups– New approaches to combined detector performance using tracking and
calorimetry which maximize jet-energy precision for standard model processes & new physics searches
• This work has provided many important tools and techniques for understanding the first physics data from ATLAS– Roadmap for jet-energy scale being planned with this work at the forefront
and will be heavily involved in the data-driven approaches to measuring the jet-energy scale and applying these new techniques
• The tools and methodologies developed by Ariel’s Jet/MET/bTag group improve jets, missing ET and b-jets and are already proving useful in both standard model studies as well as physics searches.
July 7, 2008 SLAC Annual Program Review Page 18