standard model higgs searches at d Ø
DESCRIPTION
Standard Model Higgs Searches at D Ø. Suyong Choi SKKU, Korea for D Ø Collaboration. Particles of the Standard Model. Higgs in the Standard Model Mass of elementary particles – coupling to massive particles stronger Electro-weak symmetry breaking. Limits on M H. Current limits - PowerPoint PPT PresentationTRANSCRIPT
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Suyong Choi (SKKU) SUSY 2008 1
Standard Model Higgs Standard Model Higgs Searches at DØSearches at DØ
Suyong ChoiSKKU, Korea
for DØ Collaboration
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Suyong Choi (SKKU) SUSY 2008
Suyong Choi (SKKU) PASCOS 2006, Columbus, Ohio
2
Higgs in the Standard Model1. Mass of elementary particles
– coupling to massive particles stronger2. Electro-weak symmetry breaking
Higgs in the Standard Model1. Mass of elementary particles
– coupling to massive particles stronger2. Electro-weak symmetry breaking
Particles of the Standard Model
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Suyong Choi (SKKU) SUSY 2008 3
Limits on MLimits on MHH
• Current limits– Direct searches at LEP
MH > 114 GeV @ 95% CL
– Fits to electroweak dataMH < 160 GeV @ 95% CL
– MH <190 GeV if direct search result included
Light Higgs favored
• Tevatron– Direct Searches : rule out
or find evidence– Precision mt and MW
measurements
MH=87+36-27 GeV
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Suyong Choi (SKKU) SUSY 2008 4
SM Higgs Production at the SM Higgs Production at the TevatronTevatron
• Though Higgs production copious, not all channels are accessible
gg→H– Useful for MH>140 GeV
– H→WW→ll– Background: WW
qqW/Z+H– MH<140 GeV
– WH→lbb
– ZH→llbb, bb
– Background: W+bb, Z+bb, top
• Sensitivity studies have shown that all channels must be studied, CDF + D0 combination is essential
pb
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Suyong Choi (SKKU) SUSY 2008
Tevatron Collider StatusTevatron Collider Status
• Excellent performance– Steady
increase in instantaneous luminosity
– >85% data collecting efficiency
5
Results presentedtoday are based on1.0 ~ 2.3 fb-1 of data
4.3 fb-14.3 fb-1
3.7 fb-13.7 fb-1
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Suyong Choi (SKKU) SUSY 2008
W Associated Production W Associated Production WHWH→→bbbb
• Sensitive for MH<140 GeV– Large x Br
6
q
'q
*W
W
H b jets
b jets
Two b-jets form a resonance
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Suyong Choi (SKKU) SUSY 2008 7
WH SearchWH Search
• Single and double b-tagged jet samples are analyzed separately and optimized
193 observed204 31 expected2.3 WH expected
Expected signal x 10
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Suyong Choi (SKKU) SUSY 2008
Neural Network Neural Network SelectionSelection
• Variables for ANN– pT of two jets
– Opening angle of jets
– Dijet system pT and mass
– pT (lepton +)
• Observation in agreement with background only hypothesis
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Suyong Choi (SKKU) SUSY 2008
WH Search ResultsWH Search Results
• No excess observed Set limits on cross section x Br
• Limits obtained by fitting the NN output– ST and DT treated as independent channels
• Systematics
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Expected limitsSources Error
Luminosity 6%
Trigger 3~10%
Jet energy scale 2~6%
B-tagging 4~6%
Background x-section
16~20%
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Suyong Choi (SKKU) SUSY 2008
WH Search ResultsWH Search Results
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Suyong Choi (SKKU) SUSY 2008 11
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Suyong Choi (SKKU) SUSY 2008 12
Z Associated Production Z Associated Production ZH→ZH→++--bbbb
• Clean• Small cross section x Br
– MH<140 GeV
q
q
*Z
Z
Hb jets
b jets
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Suyong Choi (SKKU) SUSY 2008
Neural Network AnalysisNeural Network Analysis• No significant excess• Set limits on x Br
– NN output distributions– Systematic errors and
correlations considered
• Systematics
– Background error : 28%– Signal eff. error : 8%
Sources Error
Jet energy scale 1~7%
B-tagging 7%
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Suyong Choi (SKKU) SUSY 2008
Higgs Limits from Higgs Limits from ZH→ZH→++--bb Analysisbb Analysis
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Suyong Choi (SKKU) SUSY 2008 15
ZHZH→→bbbb
• Advantage of large branching fraction of Z→ – MET + 2 jets – cannot reconstruct Z explicitly– Large multijet background– Recovers leptonic decays of WH and ZH, where leptons were
not reconstructed explicitly
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Suyong Choi (SKKU) SUSY 2008
Missing EMissing ETT + jets + jets
• Data: 2.1 fb-1
• MET > 50 GeV
• Jets– 2 or 3 jets, pT>20 GeV
– 2 leading jets should not be back-to-back
• W+jets and multijets dominant– Multijet background due
to mismeasured jet ET
signal x 500
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Suyong Choi (SKKU) SUSY 2008
Multivariate AnalysisMultivariate Analysis
• Boosted decision tree result using 26 variables after b-tagging
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Suyong Choi (SKKU) SUSY 2008
ZHZH→→bb Search Resultbb Search Result
• Systematic uncertainties
• Limits– best limit in W/Z+H
Sources Error
Luminosity 6.1%
Trigger 5.5%
Jet energy scale 1~7%
B-tagging 6%
Background x-section
6~16%
Heavy flavor fractions
50%
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Suyong Choi (SKKU) SUSY 2008 19
H→WW→H→WW→++--• Important for mH>140 GeV
– Final state: 2 leptons + MET– Cannot reconstruct MH
• Data: 2.3 fb-1
– 1.1 (IIa data) + 1.2(IIb data)– ee, e,
• Selection– 2 oppositely charged leptons – Large MET– Di-lepton mass– min( MT(e), MT() ) – H T
reduce Z, W+jets, tt-bar
• Analysis optimized for each MH
Signal
1.1fb-1
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Suyong Choi (SKKU) SUSY 2008
NN AnalysisNN Analysis
• Preselection • After final selection
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Suyong Choi (SKKU) SUSY 2008 21
H→WWH→WW
• Factor 2.4 away from SM For MH=160 GeV
• Systematic uncertainty
• Combine distributions from different channels– Statistical uncertainty +
Correlated systematics
Sources Error
Trigger 5%
Jet energy scale 5~10%
Muon momentum resolution ()
11%
Muon ID efficiency -5% +8%
WW x-section 4%
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Suyong Choi (SKKU) SUSY 2008
Other SM Higgs SearchesOther SM Higgs Searches
• WHWWW– 3 lepton final state– Recovers sensitivity
MH ~ 140 GeV
• H – Not a discovery
channel at the Tevatron
– Analysis with less model dependence
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Suyong Choi (SKKU) SUSY 2008
Combined DØ SM Higgs Combined DØ SM Higgs ResultsResults
• Correlations of systematic errors taken into account
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Suyong Choi (SKKU) SUSY 2008
Combined CDF and DØ Combined CDF and DØ ResultsResults
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Factor 1.1 away!
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Suyong Choi (SKKU) SUSY 2008
Prospects for MProspects for MHH<140 <140 GeVGeV
• We achieved 1.7 factor improvement in sensitivity since 2005– not including gains due to lumi
• We expect additional x2 gain in sensitivity– Optimized b-tagging with inner silicon Layer 0– semileptonic b-tags– dijet mass resolution– lepton efficiencies– refined multivariate analyses
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Suyong Choi (SKKU) SUSY 2008
Prospects for MProspects for MHH>140 >140 GeVGeV
• We achieved 1.7 factor improvement in sensitivity since 2005 (not including gains due to lumi)
• We expect additional x1.4 gain in sensitivity– lepton efficiencies– multivariate analyses
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Suyong Choi (SKKU) SUSY 2008
Expected Higgs Expected Higgs Sensitivity in 2009/2010Sensitivity in 2009/2010
• Assuming 2 experiments
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2009
20102009
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Suyong Choi (SKKU) SUSY 2008
SummarySummary
• We searched for Standard Model Higgs boson in all the sensitive channels using the DØ data
• Results in agreement with expected backgrounds
• In 2008, we may be able to exclude new MH range beyond that of LEP– CDF+D0 results combined
• Many improvements expected to raise sensitivity in a broad range of MH - most exciting years to come!
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Suyong Choi (SKKU) SUSY 2008 29
Standard Model Higgs Standard Model Higgs SearchesSearches
WH
ZH
H→WW
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Suyong Choi (SKKU) SUSY 2008 30
The DØ DetectorThe DØ Detector• Tracking
– Precision silicon vertex detector– Scintillating fiber tracker– 2T B-field
• Calorimetry– Liquid Argon-Uranium– ||<4– Excellent linearity and
resolution
• Muon detector– Low punch throughs– 1.8T toroidal B-field
• Trying to exploiting full capabilities
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Suyong Choi (SKKU) SUSY 2008 31
Search for WHSearch for WH→→bbbb
• Data: 1.7 fb-1 with e and
• Event preselection– lepton: pT>15 GeV
– Missing ET: ET>20 GeV
– 2 Jets: pT>20 GeV
||<2.5
• Veto on– Additional high pT track
– 4th jet
• Background is well understood
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Suyong Choi (SKKU) SUSY 2008 32
Tevatron @ FermilabTevatron @ Fermilab
• @ s=1.96 TeV
• Circumference 6 km
• The only place to directly look for Higgs and supersymmetric particles until LHC
pp
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Suyong Choi (SKKU) SUSY 2008 33
b-jet Taggingb-jet Tagging
• D0 developed sophisticated Neural Network based algorithm– Lifetime of a b hadron is
quite long (a few mm)– Superb efficiency– Samples and performance
derived from data– Fakes are due to finite
resolutions of the tracking detector
Detector view
– Decay length– Impact parameter– Measurement errors
Primaryvertex
Secondaryvertices
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Suyong Choi (SKKU) SUSY 2008
Neural Network AnalysisNeural Network Analysis
• After final selection • Neural network used to maximize sensitivity– pT of leptons– m
– MET– angles between MET
and leptons– minimum transverse
mass
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Suyong Choi (SKKU) SUSY 2008 35
Search for H→WW →Search for H→WW →++--
• Important channel for mH>140 GeV
• Final state:2 leptons + MET– Explicit mass cannot be
reconstructed– Signal / background
separation by exploiting event topology differences
WW decays from a spin 0 particle leptons prefer to decay in the same direction
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Suyong Choi (SKKU) SUSY 2008
Combined SM Higgs Combined SM Higgs ResultsResults
• Use as inputs the discriminant outputs from each analysis
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Suyong Choi (SKKU) SUSY 2008
Kinematic Distributions of Kinematic Distributions of b-jets b-jets
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Suyong Choi (SKKU) SUSY 2008
b-tagging in ZH→b-tagging in ZH→++--bbbb
• Use both single and double-tagged events– S/B are different
• Single tag b=45%, j=0.5%
Electron Channel
Muon Channel
Observed 73 87
Expected 57 21 101 38
Higgs (mH =115GeV)
0.23 0.31
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Suyong Choi (SKKU) SUSY 2008
b-tagging in ZH→b-tagging in ZH→++--bbbb
• Double tag b=72%, j=6% per jet
Electron Channel
Muon Channel
Observed 24 53
Expected 28 8 46 13
Z + jets (udsgc)
10 19
Z + bb 12.0 18.5
Tt-bar 3.1 5.3
Higgs (mH =115GeV)
0.23 0.30
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Suyong Choi (SKKU) SUSY 2008
Neural Network AnalysisNeural Network Analysis
• 9 kinematic variables used• 10k signal events and 100k background events• NN architecture optimized to yield best
significance
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Suyong Choi (SKKU) SUSY 2008
MET + b-jetsMET + b-jets
• Asymmetric b-tagging requirements on the two hadronic jets– maximizes sensitivity
40340 events 439 events
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Suyong Choi (SKKU) SUSY 2008 42
Standard Model Higgs Standard Model Higgs DecaysDecays
• Higgs prefers to decay to massive particle kinematically allowed
• bb for MH < 140 GeV
• WW for MH> 140 GeV
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Suyong Choi (SKKU) SUSY 2008
ZH→ZH→++--bb Selectionbb Selection
• Data: 1.1 fb-1
• Dileptons– pT>15 (10) GeV for e ()
– In well-instrumented region of the detector and isolated
– 70 GeV < M < 110 GeV
• Jets– pT>15 GeV and ||<2.5
– b-tagging
• After dilepton+jets selection before b-tag