B-Tagging and ttH, H bb Analysis on Fully Simulated Events in the - PowerPoint PPT Presentation

B-Tagging and ttH, H → bb Analysis on Fully Simulated Events in the ATLAS Experiment A.H. Wildauer Universität Innsbruck CERN ATLAS Computing Group September 27, 2005 FAKT 2005, Vienna Slide 1

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 September 27, 2005 FAKT 2005, Vienna Slide 2

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 September 27, 2005 FAKT 2005, Vienna Slide 3

ttH → jjb l ν b bb • promising discovery channel for a light Standard Model Higgs Boson complex final state W • 6 jets where 4 are b-jets ( ε b 4 !!) • 1 W has to decay leptonicaly (trigger!) W • 1 neutrino: missing energy! → efficient b-tagging very important for signal reconstruction → channel has to be fully reconstructed to reduce combinatorics September 27, 2005 FAKT 2005, Vienna Slide 4

Signal and Background • fully simulated events with “initial” detector layout (2 pixel layers) Signal: ttH(120) → l ν b jjb bb ( 0.52 pb , H(120) → bb 70%) • • m H 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 September 27, 2005 FAKT 2005, Vienna Slide 5

B-Tagging • b-tagging: identify jets which come from a b-quark • How? By using the properties of B-hadrons: • longer lifetime Lepton • reconstructable 2 nd vertex Jet-Axis • semileptonic decay modes a 0 > 0 Secondary Vertex a 0 < 0 B B “Dependencies”: Primary Vertex - Tracking - Vertex reconstruction - Jet finding September 27, 2005 FAKT 2005, Vienna Slide 6

Impact Parameter Tagging • most common way to tag b-jets: the signed IP significance distribution a 0 = S(a ) 0 σ (a ) 0 • better than IP alone: give higher weight to well measured tracks! r φ Signed Impact Significance z Signed Impact Significance • knowledge of primary vertex important to calculate IP! September 27, 2005 FAKT 2005, Vienna Slide 7

Likelihood/Weight • Significance distributions are used as input pdfs to calculate a jet weight   Sig(S) ∑ =   W ln   Bkg(S)   tracks • or a normalized b-tag likelihood. • typical likelihood/weight plot for combined tagging in z and rphi: September 27, 2005 FAKT 2005, Vienna Slide 8

B-Tagging Performance • B-Tagging performance is given in 2 connected quantities: 1 = R • light jet rejection R u at a given b-jet selection efficiency ε b : ε u u R u \ ε b 70% 60% 50% 1D 5 13 34 2D 24 62 180 CB 31 81 220 • numbers are without 2 nd vertex tagger • !performance depends heavily on truth matching and jet cleaning! S • more important: performance in an actual analysis (e.g. ) B September 27, 2005 FAKT 2005, Vienna Slide 9

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 H W ν Event Selection ℓ t b b b → 1 µ (e) with p t > 20(25) GeV, | η |<2.5 → 6 jets with p t > 20 GeV, | η |<5. → 4 jets tagged as b-jets (cut defined at ε b = 60%) j → 2 reconstructed tops with | ∆ m top |<20 GeV j W b → this leaves 2 b-jets for the reco of the Higgs t September 27, 2005 FAKT 2005, Vienna Slide 10

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) AOD TDR J. Cammin ttH(120) (improved analysis) All Events 100 % 100 % 100 % 1l 6j 50.8 46.2 4 bjets 4.15 (8) 3.8 (8) 3.8 2 tops reco 2.0 ( 48 ) 2.3 ( 60 ) 3.7 Higgs reco 0.7 (35) 0.8 (35) 1.5 • numbers in () are relative to previous cut • problem with top reconstruction? (might be at W → l ν reco) September 27, 2005 FAKT 2005, Vienna Slide 11

Cut Flow ttjj Background • cut flow in the background with largest cross section: ttjj AOD TDR J. Cammin ttjj (improved analysis) All Events 100 % 100 % 100 % 1l 6j 17.7 15.4 4 bjets 0.035 (0.2) 0.01 (0.1) 0.01 2 tops reco 0.013 (37) 0.0047 (47) 0.01 (92.3) Higgs reco 0.0007 (5.4) 0.0001 (2.1) 0.0013 (13) • selection efficiency and background rejection comparable • ttbb (QCD) background also comparable with earlier analyses September 27, 2005 FAKT 2005, Vienna Slide 12

Reconstructed Masses in Signal m =172.2 GeV m =173.3 GeV σ = 10.1 GeV σ = 9.3 GeV GeV GeV GeV t → jjb: TDR: 174 ± 11.7 GeV, J.Cammin: 174.7 ± 7.7 GeV t → l ν b: TDR: 174 ± 8.8 GeV, J.Cammin: 174.6 ± 8.6 GeV • tail in the Higgs mass spectrum due to mismatched b quarks September 27, 2005 FAKT 2005, Vienna Slide 13

Number of expected Events at 30 fb -1 30 fb -1 is the anticipated ttH(120) AOD TDR J. Cammin • (improved analysis) integrated luminosity after 3 years of low lumi run All Events 3166 3166 3166 1l 6j 1609 1462 4 bjets 128 117 120 • tt is always forced to decay to l ν b ljj with BR ~ 29% 2 tops reco 61 70 117 Higgs reco 22 25 47 AOD TDR J. Cammin ttjj simulated events after cuts (improved analysis) All Events 4.1M 4.1M 4.1M Signal: ~300 events left 1l 6j 730k 631k ttjj background: ~10 events left 4 bjets 1435 410 410 2 tops reco 533 192 377 → no detailed analysis possible Higgs reco 29 5 53 September 27, 2005 FAKT 2005, Vienna Slide 14

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 ν reconstruction under study • lack of simulated events → a lot more are needed (factor 10) • might need to use fast simulation for more background … September 27, 2005 FAKT 2005, Vienna Slide 15