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B-Tagging and ttH, H → bb Analysis on Fully Simulated Events in the ATLAS Experiment

B-Tagging and ttH, H → bb Analysis on Fully Simulated Events in the ATLAS Experiment. A.H. Wildauer Universit ät Innsbruck CERN ATLAS Computing Group. Overview. Introduction The ttH, H → bb Channel topology, cross section, backgrounds B-Tagging Algorithms

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B-Tagging and ttH, H → bb Analysis on Fully Simulated Events in the ATLAS Experiment

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  1. B-Tagging and ttH, H → bb Analysison Fully Simulated Eventsin theATLAS Experiment A.H. Wildauer Universität Innsbruck CERN ATLAS Computing Group

  2. Overview • Introduction • The ttH, H→bb Channel • topology, cross section, backgrounds • B-Tagging Algorithms • impact parameter, weight, performance on ttH • Analysis of ttH • reconstruction, event selection, bkg rejection • comparison with fast simulation

  3. Introduction • I am a PhD student in the • Austrian Doctoral Student Program • at CERN. • Main Working Areas • start-up: work on e/gamma trigger efficiencies for the High Level Trigger TDR • work on Atlas reconstruction software (Athena) with focus on Inner Detector • development of vertex software and its Event Data Model • development and performance of b-tagging software and its EDM • analysis of the ttH, Hbb channel on AOD level

  4. W W ttHjjb lb bb • promising discovery channel for a light Standard Model Higgs Boson complex final state • 6 jets where 4 are b-jets (εb4!!) • 1 W has to decay leptonicaly (trigger!) • 1 neutrino: missing energy! • efficient b-tagging very important for signal reconstruction • channel has to be fully reconstructed to reduce combinatorics

  5. Signal and Background • fully simulated events with “initial” detector layout (2 pixel layers) • Signal: • ttH(120) → lb jjb bb (0.52 pb, H(120)→bb 70%) • mH chosen to be 120 Gev/c2 • 60k events from private production on the Grid • Background: • ttjj background (474 pb) – 250k events • ttbb (QCD) (gg: 8.1 pb, qq: 0.5 pb) – 50k events • ttbb (EW) none produced • large background! good rejection needed: efficient b-tagging very important to reduce background

  6. Lepton Jet-Axis Secondary Vertex a0 > 0 B Primary Vertex a0 < 0 B-Tagging • b-tagging: identify jets which come from a b-quark • How? By using the properties of B-hadrons: • longer lifetime • reconstructable 2nd vertex • semileptonic decay modes • “Dependencies”: • Tracking • Vertex reconstruction • Jet finding

  7. Impact Parameter Tagging • most common way to tag b-jets: the signed IP significance distribution • better than IP alone: give higher weight to well measured tracks! rφ Signed Impact Significance z Signed Impact Significance • knowledge of primary vertex important to calculate IP!

  8. Likelihood/Weight • Significance distributions are used as input pdfs to calculate a jet weight • or a normalized b-tag likelihood. • typical likelihood/weight plot for combined tagging in z and rphi:

  9. B-Tagging Performance • B-Tagging performance is given in 2 connected quantities: • light jet rejection Ru at a given b-jet selection efficiency εb: • numbers are without 2nd vertex tagger • !performance depends heavily on truth matching and jet cleaning! • more important: performance in an actual analysis (e.g. )

  10. H W  ℓ t b b b j j W b ttH Event Reconstruction • 2 jets out of 4 b-jets out of 6 reco jets need to be assigned to the Higgs … •  full reconstruction necessary to reconstruct Higgs Boson Event Selection • 1 (e) with pt > 20(25) GeV, |η|<2.5 • 6 jets with pt > 20 GeV, |η|<5. • 4 jets tagged as b-jets (cut defined at εb = 60%) • 2 reconstructed tops with |mtop|<20 GeV • this leaves 2 b-jets for the reco of the Higgs t

  11. Cut Flow Signal • comparison of my analysis (AOD) with fully simulated events to 2 analyses based on fast simulation and older detector layout (3 pixel layers) • numbers in () are relative to previous cut • problem with top reconstruction? (might be at W→l reco)

  12. Cut Flow ttjj Background • cut flow in the background with largest cross section: ttjj • selection efficiency and background rejection comparable • ttbb (QCD) background also comparable with earlier analyses

  13. m =173.3 GeV  = 9.3 GeV Reconstructed Masses in Signal m =172.2 GeV  = 10.1 GeV GeV GeV GeV t→jjb: TDR: 174 ± 11.7 GeV, J.Cammin: 174.7 ± 7.7 GeV t→lb: TDR: 174 ± 8.8 GeV, J.Cammin: 174.6 ± 8.6 GeV • tail in the Higgs mass spectrum due to mismatched b quarks

  14. Number of expected Events at 30 fb-1 • 30 fb-1 is the anticipated integrated luminosity after 3 years of low lumi run • tt is always forced to decay to lb ljj with BR ~ 29% simulated events after cuts Signal: ~300 events left ttjj background: ~10 events left → no detailed analysis possible

  15. Conclusion and Outlook • first look at ttH channel with fully simulated events and initial detector layout • “realistic” b-tagging performance looks OK on ttH channel (no SV tagger in use for this analysis so far) • cut flow on sig and bkg in agreement with earlier studies • small discrepancies in the Wl reconstruction under study • lack of simulated events  a lot more are needed (factor 10) • might need to use fast simulation for more background …

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