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New Approaches to Hadronic Final State Reconstruction

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 .

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New Approaches to Hadronic Final State Reconstruction

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  1. New Approaches to Hadronic Final State Reconstruction David W. Miller SLAC ATLAS SLAC Annual Program Review

  2. 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. 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 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 SLAC Annual Program Review

  3. Extending jet algorithms for first data New 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 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 (DWM, SLAC) SLAC Annual Program Review

  4. Advances in jet resolution using tracking information Jet response improved using the charged energy fraction ftrk ftrkprovides 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 ATLAS: use tracks to provide additional information on jet composition Account for jet-to-jet fluctuations in charged particle content (Marshall, Columbia U.) Jet structure Jet response depends on ftrk Improvements with ftrk correction SLAC Annual Program Review

  5. Jets & tracks: improvements and implementation ftrk response corrections Improvements 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/Wjjγγ analysis • Jet resolution • Corrections applied in bins of ftrk • Jet-by-jet fluctuations are reduced • Improvement in overall jet energy resolution (Marshall, Columbia U.) SLAC Annual Program Review

  6. Missing energy applications Improvements to jet-energy has implications for missing energy measurements Using track information to identify a systematic effect in missing energy allows us to correct for it (Schwartzman, SLAC) Artificial missing energy from pT imbalance is corrected following jet energy corrections SLAC Annual Program Review

  7. Tracks-jets as complementary to calorimeter jets • New approach in ATLAS • Build “jets” from tracks in Inner Detector tracker • Inherently 3D objects (Z, η, φ) unlike calo jets Calorimeter jets can incorporate particles from multiple interactions Using track-jets with Z information, can separate interactions (Schwartzman, SLAC) These track-jets are completely independent from the calorimeter Good angular resolution, used in b-tagging trigger (see I. Aracena’s talk) SLAC Annual Program Review

  8. Improved jet-finding efficiency with track-jets (Schwartzman, SLAC) Δ Limited by tracker acceptance Improved efficiency for low-pT ttbar events η The use of tracks provides complementary information to calorimeters Track-jets allow for the measurement of jet reconstruction efficiency Energy losses in the inner detector and cryostat decrease calo efficiency but do not affect track-jets SLAC Annual Program Review

  9. 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 Radius (m) Gap at η=0.7-0.8 Beam axis (m) (Majewski, BNL) Missing calorimeter jet SLAC Annual Program Review

  10. Jet-vertex association (JVF) 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: use tracking + calorimeter + vertex information to account for jet origin • 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 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) SLAC Annual Program Review

  11. New approach to jet identification in ATLAS Using JVF to select hard-scatter jets in events with pile-up • 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 ttbar events: 2x1033 cm-2s-1 (DWM, SLAC) Using JVF we recover the flat jet-multiplicity distribution without raising the jet pTthreshold In official ATLAS software and available to all ATLAS users for first data Fully documented in internal ATLAS note: ATL-COM-PHYS-2008-008 SLAC Annual Program Review

  12. Jet-by-jet energy corrections for pile-up contributions Jet energy response vs. JVF: 1033 & 2x1033 cm-2s-1 (ttbar) Jet energy response vs. pTand JVF (ttbar) (DWM, SLAC) Using JVF to correct jet energies in a luminosity independent manner Derive jet-energy correction similar to that already done for ftrk to correct jets for pileup contributions on a per jet basis (new technique at hadron colliders) SLAC Annual Program Review

  13. 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 Muons andJVF tagged correct PV • 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 jet Jet & μ-tag Default μ jet jet (DWM, SLAC) SLAC Annual Program Review

  14. Semileptonic b-jet corrections Correcting for missing neutrino energy b-jet response in ttbar events • 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.) SLAC Annual Program Review

  15. Use of semileptonic b-jet correction in ATLAS 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 Data-driven methods for estimating corrections under way (Mateos, Columbia U.) SLAC Annual Program Review

  16. 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 Missing energy significance QCD Events (fake missing ET) W+jets (real missing ET) (Perez, Columbia U., Butler, SLAC) SLAC Annual Program Review

  17. 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 (gluonbbbar) 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

  18. 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 andare already proving useful in both standard model studies as well as physics searches. SLAC Annual Program Review

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