September 27, 2005FAKT 2005, ViennaSlide 1 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, 2005FAKT 2005, ViennaSlide 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

September 27, 2005FAKT 2005, ViennaSlide 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

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

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

September 27, 2005FAKT 2005, ViennaSlide 7 most common way to tag b-jets: the signed IP significance distribution better than IP alone: give higher weight to well measured tracks! Impact Parameter Tagging knowledge of primary vertex important to calculate IP! z Signed Impact Significance rφ Signed Impact Significance

September 27, 2005FAKT 2005, ViennaSlide 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:

September 27, 2005FAKT 2005, ViennaSlide 9 B-Tagging Performance B-Tagging performance is given in 2 connected quantities: light jet rejection R u at a given b-jet selection efficiency ε b : Ru\εbRu\εb 70%60%50% 1D D CB numbers are without 2 nd vertex tagger !performance depends heavily on truth matching and jet cleaning! more important: performance in an actual analysis (e.g. )

September 27, 2005FAKT 2005, ViennaSlide 10 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 b b b b ℓ j j H W W t t → 1  (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%) → 2 reconstructed tops with |  m top |<20 GeV → this leaves 2 b-jets for the reco of the Higgs

September 27, 2005FAKT 2005, ViennaSlide 11 Cut Flow Signal ttH(120) AODTDRJ. Cammin (improved analysis) All Events100 % 1l 6j bjets4.15 (8)3.8 (8)3.8 2 tops reco2.0 (48)2.3 (60)3.7 Higgs reco0.7 (35)0.8 (35)1.5 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)

September 27, 2005FAKT 2005, ViennaSlide 12 Cut Flow ttjj Background ttjj AODTDRJ. Cammin (improved analysis) All Events100 % 1l 6j bjets0.035 (0.2)0.01 (0.1) tops reco0.013 (37) (47)0.01 (92.3) Higgs reco (5.4) (2.1) (13) selection efficiency and background rejection comparable ttbb (QCD) background also comparable with earlier analyses cut flow in the background with largest cross section: ttjj

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

September 27, 2005FAKT 2005, ViennaSlide 14 Number of expected Events at 30 fb 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% ttH(120) AODTDRJ. Cammin (improved analysis) All Events3166 1l 6j bjets tops reco Higgs reco ttjj AODTDRJ. Cammin (improved analysis) All Events4.1M 1l 6j730k631k 4 bjets tops reco Higgs reco29553 simulated events after cuts Signal: ~300 events left ttjj background: ~10 events left → no detailed analysis possible

September 27, 2005FAKT 2005, ViennaSlide 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 …