Standard Model Higgs Searches at D

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Standard Model Higgs Searches at DØ

• Suyong Choi
• SKKU, Korea
• for DØ Collaboration

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Particles of the Standard Model

• Higgs in the Standard Model
• Mass of elementary particles coupling to
  massive particles stronger
• Electro-weak symmetry breaking

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Limits on MH

• Current limits
• Direct searches at LEP MH gt 114 GeV _at_ 95 CL
• Fits to electroweak dataMH lt 160 GeV _at_ 95 CL
• MH lt190 GeV if direct search result included
• ? Light Higgs favored
• Tevatron
• Direct Searches rule out or find evidence
• Precision mt and MW measurements

MH8736-27 GeV
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SM Higgs Production at the Tevatron

• Though Higgs production copious, not all channels
  are accessible
• gg?H
• Useful for MHgt140 GeV
• H?WW?llnn
• Background WW
• qq?W/ZH
• MHlt140 GeV
• WH?lnbb
• ZH?llbb, nnbb
• Background Wbb, Zbb, top

pb

• Sensitivity studies have shown that all channels
  must be studied, CDF D0 combination is essential

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Tevatron Collider Status

• Excellent performance
• Steady increase in instantaneous luminosity
• gt85 data collecting efficiency

4.3 fb-1
3.7 fb-1
Results presented today are based on 1.0 2.3
fb-1 of data
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W Associated Production WH???bb

• Sensitive for MHlt140 GeV
• Large ? x Br

b jets
Two b-jets form a resonance
b jets
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WH Search

• Single and double b-tagged jet samples are
  analyzed separately and optimized

193 observed 204 ? 31 expected 2.3 WH
expected
Expected signal x 10
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Neural Network Selection

• 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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WH 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

Expected limits
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WH Search Results
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Z Associated Production ZH???-bb

• Clean
• Small cross section x Br
• MHlt140 GeV

b jets
b jets
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Neural 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

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Higgs Limits from ZH???-bb Analysis
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ZH???bb

• 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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Missing ET jets

• Data 2.1 fb-1
• MET gt 50 GeV
• Jets
• 2 or 3 jets, pTgt20 GeV
• 2 leading jets should not be back-to-back
• Wjets and multijets dominant
• Multijet background due to mismeasured jet ET

signal x 500
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Multivariate Analysis

• Boosted decision tree result using 26 variables
  after b-tagging

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ZH???bb Search Result

• Systematic uncertainties

• Limits
• best limit in W/ZH

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H?WW????-?

• Important for mHgt140 GeV
• Final state 2 leptons MET
• Cannot reconstruct MH
• Data 2.3 fb-1
• 1.1 (IIa data) 1.2(IIb data)
• ee, em, ??
• Selection
• 2 oppositely charged leptons
• Large MET
• Di-lepton mass
• min( MT(e), MT(m) )
• ????
• H T
• ? reduce Z, Wjets, tt-bar
• Analysis optimized for each MH

Signal
1.1fb-1
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NN Analysis

• Preselection

• After final selection

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H?WW

• Systematic uncertainty
• Combine distributions from different channels
• Statistical uncertainty Correlated systematics

• Factor 2.4 away from SM For MH160 GeV

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Other SM Higgs Searches

• WH?WWW
• 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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Combined DØ SM Higgs Results

• Correlations of systematic errors taken into
  account

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Combined CDF and DØ Results
Factor 1.1 away!
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Prospects for MHlt140 GeV

• 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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Prospects for MHgt140 GeV

• 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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Expected Higgs Sensitivity in 2009/2010

• Assuming 2 experiments

2010 2009
2009
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Summary

• 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
• CDFD0 results combined
• Many improvements expected to raise sensitivity
  in a broad range of MH - most exciting years to
  come!

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Standard Model Higgs Searches

• WH
• ZH
• H?WW

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The DØ Detector

• Tracking
• Precision silicon vertex detector
• Scintillating fiber tracker
• 2T B-field
• Calorimetry
• Liquid Argon-Uranium
• ?lt4
• Excellent linearity and resolution
• Muon detector
• Low punch throughs
• 1.8T toroidal B-field
• Trying to exploiting full capabilities

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Search for WH???bb

• Data 1.7 fb-1 with e and ?
• Event preselection
• lepton pTgt15 GeV
• Missing ET ETgt20 GeV
• 2 Jets pTgt20 GeV
• hlt2.5
• Veto on
• Additional high pT track
• 4th jet
• Background is well understood

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Tevatron _at_ Fermilab

• _at_ ?s1.96 TeV
• Circumference 6 km
• The only place to directly look for Higgs and
  supersymmetric particles until LHC

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b-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

Primary vertex
Secondary vertices
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Neural 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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Search for H?WW ????-?

• Important channel for mHgt140 GeV
• Final state2 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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Combined SM Higgs Results

• Use as inputs the discriminant outputs from each
  analysis

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Kinematic Distributions of b-jets
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b-tagging in ZH???-bb

• Use both single and double-tagged events
• S/B are different
• Single tag
• ?b45, ?j0.5

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b-tagging in ZH???-bb

• Double tag
• ?b72, ?j6 per jet

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Neural Network Analysis

• 9 kinematic variables used
• 10k signal events and 100k background events
• NN architecture optimized to yield best
  significance

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MET b-jets
40340 events
439 events

• Asymmetric b-tagging requirements on the two
  hadronic jets
• maximizes sensitivity

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Standard Model Higgs Decays

• Higgs prefers to decay to massive particle
  kinematically allowed
• bb for MH lt 140 GeV
• WW for MHgt 140 GeV

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ZH???-bb Selection

• Data 1.1 fb-1
• Dileptons
• pTgt15 (10) GeV for e (?)
• In well-instrumented region of the detector and
  isolated
• 70 GeV lt M?? lt 110 GeV
• Jets
• pTgt15 GeV and ?lt2.5
• b-tagging

• After dileptonjets selection before b-tag