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					                     Standard Model Higgs
                        Searches at DØ

                           Suyong Choi
                               SKKU, Korea
                           for DØ Collaboration




Suyong Choi (SKKU)                SUSY 2008       1
                            Particles of the Standard Model




                                             Higgs in the Standard Model
       Suyong Choi (SKKU)     PASCOS 2006, Columbus, Ohio elementary particles
                                             1. Mass of
                                                – coupling to massive particles stronger
                                             2. Electro-weak symmetry breaking
Suyong Choi (SKKU)                       SUSY 2008                                     2
                          Limits on MH
   • Current limits
          – Direct searches at LEP
              MH > 114 GeV @ 95% CL
          – Fits to electroweak data
            MH < 160 GeV @ 95% CL
          – MH <190 GeV if direct search
            result included
           Light Higgs favored                        MH=87+36-27 GeV


   • Tevatron
          – Direct Searches : rule out
            or find evidence
          – Precision mt and MW
            measurements

Suyong Choi (SKKU)                         SUSY 2008                     3
           SM Higgs Production at the
                   Tevatron
   • Though Higgs production
     copious, not all channels are
     accessible

        gg→H



                                       pb
          – Useful for MH>140 GeV
          – H→WW→llnn
          – Background: WW


        qqW/Z+H
          –   MH<140 GeV
          –   WH→lnbb                         •     Sensitivity studies have shown
                                                    that all channels must be studied,
          –   ZH→llbb, nnbb
                                                    CDF + D0 combination is essential
          –   Background: W+bb, Z+bb, top
Suyong Choi (SKKU)                      SUSY 2008                                        4
             Tevatron Collider Status
   • Excellent
     performance                                         4.3 fb-1
          – Steady
            increase in
                                                                    3.7 fb-1
            instantaneous
            luminosity

          – >85% data
            collecting
            efficiency
                                        Results presented
                                        today are based on
                                        1.0 ~ 2.3 fb-1 of data
Suyong Choi (SKKU)          SUSY 2008                                    5
                W Associated Production
                      WH→nbb
   • Sensitive for MH<140 GeV
          – Large  x Br



         q'                                          W



                           W*
                                                                  b jets
         q                                           H

                       Two b-jets form a resonance
Suyong Choi (SKKU)                       SUSY 2008       b jets            6
                     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
Suyong Choi (SKKU)                  SUSY 2008                       7
         Neural Network Selection
   • Variables for ANN
          – pT of two jets
          – Opening angle of jets
          – Dijet system pT and
            mass
          – pT (lepton +n)
   • Observation in
     agreement with
     background only
     hypothesis


Suyong Choi (SKKU)                  SUSY 2008
                     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
                        Sources                   Error
                                                   Expected limits
                        Luminosity                 6%
                        Trigger                  3~10%
                        Jet energy scale         2~6%
                        B-tagging                4~6%
                        Background x-            16~20%
                        section
Suyong Choi (SKKU)                   SUSY 2008                       9
                     WH Search Results




Suyong Choi (SKKU)          SUSY 2008
Suyong Choi (SKKU)   SUSY 2008   11
                 Z Associated Production
                       ZH→+-bb
   • Clean
   • Small cross section x Br
          – MH<140 GeV
                     q                 Z



                           Z*
                                                    b jets
                     q                 H

                                           b jets
Suyong Choi (SKKU)         SUSY 2008                         12
            Neural Network Analysis
                           • No significant excess
                           • Set limits on  x Br
                                   – NN output distributions
                                   – Systematic errors and
                                     correlations considered
                           • Systematics
                                  Sources            Error
                                  Jet energy scale   1~7%
                                  B-tagging           7%

                                   – Background error : 28%
                                   – Signal eff. error : 8%


Suyong Choi (SKKU)    SUSY 2008
                      Higgs Limits from
                     ZH→ +-bb Analysis




Suyong Choi (SKKU)           SUSY 2008
                            ZH→nnbb




   • Advantage of large branching fraction of Z→nn
          – MET + 2 jets – cannot reconstruct Z explicitly
          – Large multijet background
          – Recovers leptonic decays of WH and ZH, where leptons were
            not reconstructed explicitly

Suyong Choi (SKKU)                  SUSY 2008                           15
                     Missing ET + jets
   • Data: 2.1 fb-1
   • MET > 50 GeV
   • Jets                                         signal x 500
          – 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

Suyong Choi (SKKU)                    SUSY 2008
                     Multivariate Analysis
   • Boosted decision tree result using 26 variables
     after b-tagging




Suyong Choi (SKKU)            SUSY 2008
              ZH→nnbb Search Result
   • Systematic                      • Limits
     uncertainties                         – best limit in W/Z+H
    Sources            Error
    Luminosity         6.1%
    Trigger            5.5%
    Jet energy scale   1~7%
    B-tagging           6%
    Background x-      6~16%
    section
    Heavy flavor       50%
    fractions


Suyong Choi (SKKU)             SUSY 2008
                         H→WW→+n-n
   • 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, em, mm

   • Selection
          –   2 oppositely charged leptons        Signal
          –   Large MET
          –   Di-lepton mass
          –   min( MT(e), MT(m) )
          –   
                                                           1.1fb-1
          –   HT
             reduce Z, W+jets, tt-bar

   • Analysis optimized for each MH

Suyong Choi (SKKU)                           SUSY 2008         19
                     NN Analysis
   • Preselection              • After final selection




Suyong Choi (SKKU)       SUSY 2008
                                  H→WW
   • Systematic uncertainty

      Sources                   Error
      Trigger                    5%
      Jet energy scale         5~10%
      Muon momentum              11%
      resolution (mm)
      Muon ID efficiency      -5% +8%
      WW x-section               4%

                                              •     Factor 2.4 away from SM For
   • Combine distributions from                     MH=160 GeV
     different channels
          – Statistical uncertainty +
            Correlated systematics
Suyong Choi (SKKU)                      SUSY 2008                                 21
          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
Suyong Choi (SKKU)                 SUSY 2008   22
              Combined DØ SM Higgs
                    Results
   • Correlations of systematic errors taken into account




Suyong Choi (SKKU)                 SUSY 2008
                Combined CDF and DØ
                      Results




                                    Factor 1.1 away!




Suyong Choi (SKKU)      SUSY 2008                      24
       Prospects for MH<140 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

Suyong Choi (SKKU)                  SUSY 2008                  25
       Prospects for MH>140 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




Suyong Choi (SKKU)                  SUSY 2008             26
        Expected Higgs Sensitivity
              in 2009/2010
   • Assuming 2 experiments




                                        2010
                                        2009


                     2009




Suyong Choi (SKKU)          SUSY 2008          27
                         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
          – CDF+D0 results combined

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

Suyong Choi (SKKU)              SUSY 2008
                 Standard Model Higgs
                       Searches
                 WH
                 ZH
                 H→WW


Suyong Choi (SKKU)        SUSY 2008     29
                     The 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
Suyong Choi (SKKU)                     SUSY 2008   30
                     Search for WH→nbb
   • Data: 1.7 fb-1 with e and m

   • 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
Suyong Choi (SKKU)                     SUSY 2008   31
                     Tevatron @ Fermilab
   • pp @ s=1.96 TeV

   • Circumference 6 km

   • The only place to directly
     look for Higgs and
     supersymmetric particles
     until LHC




Suyong Choi (SKKU)                SUSY 2008   32
                        b-jet Tagging
   • D0 developed                                     Detector view
     sophisticated Neural
     Network based algorithm
          – Lifetime of a b hadron is                        Primary
                                                             vertex
            quite long (a few mm)
          – Superb efficiency
                                                                       Secondary
          – Samples and performance                                    vertices
            derived from data
          – Fakes are due to finite
            resolutions of the tracking               – Decay length
            detector
                                                      – Impact parameter
                                                      – Measurement errors

Suyong Choi (SKKU)                        SUSY 2008                                33
            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

Suyong Choi (SKKU)           SUSY 2008                          34
             Search for H→WW →+n-n
   • 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

Suyong Choi (SKKU)                       SUSY 2008                                    35
     Combined SM Higgs Results
   • Use as inputs the discriminant outputs from each
     analysis




Suyong Choi (SKKU)        SUSY 2008
        Kinematic Distributions of b-
                    jets




Suyong Choi (SKKU)   SUSY 2008
              b-tagging in ZH→+-bb
   • Use both single and
     double-tagged events
          – S/B are different
   • Single tag
          – b=45%, j=0.5%
                     Electron   Muon
                     Channel    Channel
     Observed           73         87
     Expected         57  21   101  38
     Higgs             0.23       0.31
     (mH =115GeV)

Suyong Choi (SKKU)                       SUSY 2008
              b-tagging in ZH→+-bb
   • Double tag
          – b=72%, j=6% per jet
                        Electron    Muon
                        Channel     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                 0.23       0.30
     (mH =115GeV)



Suyong Choi (SKKU)                           SUSY 2008
            Neural Network Analysis




   • 9 kinematic variables used
   • 10k signal events and 100k background events
   • NN architecture optimized to yield best
     significance
Suyong Choi (SKKU)       SUSY 2008
                      MET + b-jets


                        40340 events               439 events




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


Suyong Choi (SKKU)                     SUSY 2008
       Standard Model Higgs Decays
                                 • Higgs prefers to
                                   decay to massive
                                   particle kinematically
                                   allowed

                                 • bb for MH < 140 GeV

                                 • WW for MH> 140 GeV




Suyong Choi (SKKU)   SUSY 2008                              42
                     ZH→+-bb Selection
   • Data: 1.1 fb-1                       • After dilepton+jets
   • Dileptons                              selection before b-tag
          – pT>15 (10) GeV for e (m)
          – In well-instrumented
            region of the detector
            and isolated
          – 70 GeV < M < 110 GeV


   • Jets
          – pT>15 GeV and ||<2.5
          – b-tagging

Suyong Choi (SKKU)                  SUSY 2008

				
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