September 27, 2005FAKT 2005, ViennaSlide 1 B-Tagging and ttH, H → bb Analysis on Fully Simulated...
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Transcript of September 27, 2005FAKT 2005, ViennaSlide 1 B-Tagging and ttH, H → bb Analysis on Fully Simulated...
September 27, 2005 FAKT 2005, Vienna Slide 1
B-Tagging and ttH, H → bb Analysis
on Fully Simulated Events
in the
ATLAS Experiment
A.H. WildauerUniversität Innsbruck
CERN ATLAS Computing Group
September 27, 2005 FAKT 2005, Vienna Slide 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, 2005 FAKT 2005, Vienna Slide 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, Hbb channel on AOD level
September 27, 2005 FAKT 2005, Vienna Slide 4
• promising discovery channel for a light Standard Model Higgs Boson
ttHjjb lb 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 (εb4!!)
• 1 W has to decay leptonicaly (trigger!)• 1 neutrino: missing energy!
W
W
September 27, 2005 FAKT 2005, Vienna Slide 5
Signal and Background
• fully simulated events with “initial” detector layout (2 pixel layers)
Signal:
• ttH(120) → lb 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
September 27, 2005 FAKT 2005, Vienna Slide 6
• 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
B-Tagging
BB a0 < 0
a0 > 0Secondary Vertex
Primary Vertex
Jet-Axis
Lepton
September 27, 2005 FAKT 2005, Vienna Slide 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!
)σ(a
a)S(a
0
00
z Signed Impact Significance rφ Signed Impact Significance
September 27, 2005 FAKT 2005, Vienna Slide 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:
tracks Bkg(S)
Sig(S)lnW
September 27, 2005 FAKT 2005, Vienna Slide 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: u1uR
Ru\εb 70% 60% 50%
1D 5 13 34
2D 24 62 180
CB 31 81 220
• numbers are without 2nd vertex tagger
• !performance depends heavily on truth matching and jet cleaning!
• more important: performance in an actual analysis (e.g. )BS
September 27, 2005 FAKT 2005, Vienna Slide 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 Selectionb
b
bb
ℓ
jj
H W
W
t
t
→ 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
September 27, 2005 FAKT 2005, Vienna Slide 11
Cut Flow Signal
ttH(120) AOD TDR J. Cammin(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
• 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, 2005 FAKT 2005, Vienna Slide 12
Cut Flow ttjj Background
ttjj AOD TDR J. Cammin(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
• cut flow in the background with largest cross section: ttjj
September 27, 2005 FAKT 2005, Vienna Slide 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: 174.7 ± 7.7 GeV
t→lb: TDR: 174 ± 8.8 GeV, J.Cammin: 174.6 ± 8.6 GeV
• tail in the Higgs mass spectrum due to mismatched b quarks
GeV GeV GeV
September 27, 2005 FAKT 2005, Vienna Slide 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 lb ljj with BR ~ 29%
ttH(120) AOD TDR J. Cammin(improved analysis)
All Events 3166 3166 3166
1l 6j 1609 1462
4 bjets 128 117 120
2 tops reco 61 70 117
Higgs reco 22 25 47
ttjj AOD TDR J. Cammin(improved analysis)
All Events 4.1M 4.1M 4.1M
1l 6j 730k 631k
4 bjets 1435 410 410
2 tops reco 533 192 377
Higgs reco 29 5 53
simulated events after cuts
Signal: ~300 events left
ttjj background: ~10 events left
→ no detailed analysis possible
September 27, 2005 FAKT 2005, Vienna Slide 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 Wl reconstruction under study
• lack of simulated events
a lot more are needed (factor 10)
• might need to use fast simulation for more background …