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					Higgs Searches at CMS

        Maiko Takahashi
     Imperial College London

   QMUL Seminar 28 March 2008
                                      Outline
    Outline:
    •      Overview of CMS experiment
    •      CMS Reach for SM Higgs Boson Discovery
    •      CMS Reach for MSSM Higgs Boson Discovery

    Remarks:
    •      Most of the analysis results presented today are taken from Physics Technical
           Design Report (TDR) volume II (2006)
              low luminosity scenario (L = 2 x 1032 cm-2s-1)
               (full detector simulation with underlying event and pile-up)
              evaluation of discovery reach at 10’s of fb-1
    •      Major software framework change after completion of Physics TDR
    •      On-going ‘2007’ analyses focuses on early data up to 1 fb-1, some preliminary
           results and plans are included today

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                          LHC and CMS
                                                        Large Hadron Collider
                                                        • p-p collision @ 14 TeV
                                                        • nominal L = 1034 cm-2s-1
                                                        • 40MHz collision rate
                                                           (25ns bunch spacing)




 Compact Muon Solenoid
 • 1 of the 2 general purpose
   detectors at LHC
 • ~100m underground
 • opposite of the LHC ring
   from Meyrin site (& ATLAS)

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           CMS Detector I




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                          CMS Detector II
  Central Tracker:    Si pixel (100 μm x 150 μm) + strip (100-200 μm pitch),
                       total of 14 layers (max)
  ECAL:               PbWO4 crystals (~260, ~1,  x  = 0.0174 x 0.0174)
                       + Si preshower
  HCAL:               Brass/Steel absorber + plastic scintillator
                       (~11 A,  x  = 0.087 x 0.087)
  Muon System:        Drift Tube/Cathode Strip/Resistive Plate
  Solenoid:           4T field

  Track Resolution:   σpT/pT = a x pT + b (a = 1.5% - 6%, b = 0.5%) pT in GeV/c
  Vertex Resolution: σZ ~ 40 m
  ECAL Resolution:    σ/E = a * 1/sqrt(E) + b * 1/E + c
                       (a = 3.63%, b = 0.124GeV, c = 0.26% at 2004 ECAL testbeam)
  ECAL + HCAL Resolution: σ/E = a * 1/sqrt(E) + c
                       (a = 120%, c = 6.9% at 2006 ECAL+HCAL combined testbeam)

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           CMS Reconstruction I




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                    CMS Reconstruction II
 Muon:
    • central track matched with segments from muon chambers
    • momentum reconstructed from the fully extended track
    • tracker and/or calorimeter isolation
 Electron (Photon):
      • ECAL cluster with(out) associated central track
      • Gaussian Sum Filter provides better track fitting
      • identification based on shower shape, EM fraction, cluster-track matching etc
      • Tracker and/or calorimeter isolation
 Jet:
        • interative cone algorithm for majority of early analyses
        • midpoint cone and kT algorithms also available
        • Jet+Track provides improvement for jet energy resolution
        • particle-flow techniques are under development
        • jet energy scale from  + jets (or Z + jets)
         many analyses still use MC jet correction
        (scale factors obtained from true hadron-level jets using a dedicated sample)
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                 CMS Reconstruction III
 Hadronic tau:
     • identification using tracks (1 or 3 prongs within a fixed cone)
     • tracker isolation + electron rejection based on HCAL deposition
     • reconstruction conventionally done using calorimeters
     • particle-flow algorithm now available
      great improvement shown by the working group
 b-jet:
      • identification using track impact parameter,
      secondary vertex, leptons within b-jets etc.
      • momentum reconstruction in calorimeters
 Missing ET:
      • sum of calorimeter energies
      • correction using energy scales on jets in the event
      • some analyses use sum of high pT objects               Tau reconstruction
       benefit from improvements in individual objects


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                  Standard Model Higgs
    Primary goal of CMS (and ATLAS) is to detect Higgs boson
    • direct searches at LEP sets exclusion limit on its mass: MH > 114.4 GeV/c2
    • upper limit set by precision measurement of W and top masses
     relatively low mass is favoured

    Main decay channels
    • low mass scenario
            H  
            H  
    • high mass scenario
            H  ZZ             (ZZ  4e/4μ)
            H  WW             (WW  2μ2)
     Many of them were the original benchmark channels
       for the design of the CMS detector


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            SM Higgs Production at LHC




    Gluon fusion dominates by an order of magnitude
    Vector-boson fusion becomes comparable at high mass
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           SM Higgs Branching Ratio




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                             Inclusive H  
                   cut-based
              (before optimisation)




           CMS Note 2006/112                                  CMS Note 2006/112
   Greatly benefit from the excellent ECAL resolution
   Event Selection
       • 2 isolated high pT photons, primary vertex association & kinematical cuts
       • Neural Net optimisation for different shower shape and detector regions
   Backgrounds: high pT prompt photons,  + jet, multi-jet
     estimation by fitting the side-bands in the di-photon mass distribution
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              qqH, H   : Kinematics
 Vector Boson Fusion (VBF)                                 EW                 QCD
     • signature of forward- backward qurak jets
     • central rapidity gap = hadronic activities
     heavily suppressed in EW process
  additional signature to distinguish signal             D. Zeppenfeld, TeV4LHC 2004

 H  
     • one leptonic decay  for trigger reason
     • hadronic decay of  distinct from QCD jets
 Backgrounds: EW +jets, Z + jets, tt, W+jets
 Basic Event Selection                                        rapidity of 3rd parton
      • 1 high pT isolated lepton                            w.r.t. 2 leading partons
      • 1 high pT isolated had of opposite charge
      • vertex association
      • 2 leading jets (excl. had) in opposite
      rapidity region tagged as VBF jets
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                    qqH, H   : Analysis
 VBF Kinematical Variables:
   (jj), (jj), M(jj), MT(l,ETmiss)
 Central Rapidity Gap Selection
     • Central Jet Veto (with vertex association)
     • Track Counting Veto (Les Houches 2007)                                        MH = 135GeV/c2
                                                                                     σ/M = 9.1%
 Full Mass Reconstruction
 using collinear approximation of ETmiss
    project onto visible  decay
                                                                 CMS Note 2006/088
 On-going effort towards early analysis
  further development of techniques and plan for validation using early data
     • new methods for Central Rapidity Gap Selection
     • different methods and performance for di- mass reconstruction
     • modeling of di- mass shape for Z  using Z μμ data
     • estimation of Z  ee background and electron rejection in  ID

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                            H  ZZ  4l

                                                                   lumonosity for
                                                                     5σ discovery
                                                                   M = 200 GeV/c2


                                                ZZ  ee
                    ZZ  


       CMS Note 2006/122                  CMS Note 2006/136
   “Golden Channel”- clean signature with small background
   Event Selection (mass dependent thresholds)
       • 2 pairs of high pT isolated leptons (same flavour, opposite charge)
       • Vertex association + impact parameter cut
       • di-lepton (Z) mass cut
   Backgrounds: ZZ, tt, Zbb  estimation from side-band data
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                        H  WW  ll




                                      CMS Note 2006/112
    Difficult channel because of ETmiss  counting experiment
    Event Selection
        • 2 oppositely signed high pT isolated leptons from same vertex
        • jet veto, large ETmiss, di-lepton mass, relatively small (ll)
    Backgrounds: di-boson (WW, WZ, ZZ), tt, Wt
      background normalisation to data using additional and/or inverted cuts
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               H  WW  ll Updates




           http://cms-physics.web.cern.ch/cms-physics/public/HIG-07-001-pas-v3.pdf

    New results from on-going effort for early analysis
    Changes/Improvements
        • additional Drell-Yan Z/* contribution evaluated
        • multivariate analysis (Boosted Decision Tree) to increase sensitivity

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                   CMS Reach for SM Higgs




    CMS Physics TDR II 2006                        CMS Physics TDR II 2006
    5σ discovery regions for the SM Higgs boson
    •      H   most sensitive at low mass
    •      H  ZZ / WW very promising for early discovery if MH  130 GeV/c2
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                                MSSM Higgs
    Minimal Supersymmetric extension of Standard Model (MSSM)
    • 5 Higgs particles: h, H, A, H±
    • 2 independent parameters at leading order:
                normally chosen as MA and tan

    Main decay channels covered by CMS analyses
    • Neutral Higgs
              H/A  , μμ
              H/A  bb
              A  Zh
              h  , h  
    •      Charged Higgs
              H±  
              H±  tb

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           MSSM Higgs Production at LHC




    gg/qq  bbH dominates for high tan
            additional signature of b-quark jets to distinguish the signal
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              MSSM Higgs Branching Ratio




    •      Decays to bb and  dominates over all mass range for high tan
    •      Similar behaviour for h and H
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                            bbH/A, H/A
    Look for an excess in reconstructed mass (collinear approx. of ETmiss)
    Event Selection
        • 2 oppositely charged high pT hadronic tau
        • at least 1 additional jet passing b-tagging
        • jet and lepton vetoes depending on the channels
    Backgrounds: tt, Z/*, W+jets, Wt, QCD multi-jet
    Expected Number of Events: O(101-102) signal events at 30 fb-1
                                                                     CMS Note 2006/126


           H/A    j+j                H/A    μ+j                H/A    e+j




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                        bbH/A, H/Aμμ
 Provide best measurement of Higgs width
 (excellent muon reconstruction)
 Discovery reach at
 • low MA
 • intensive coupling (Mh ~ MA ~ MH)
 • Decoupling (MA >> Mh, MA ~ MH)
 Event Selection
     • 2 isolated high pT muons                         CMS Physics TDR II 2006
     • at least 1 jet passing b-tagging
     • missing ET cut against tt
 Backgrounds: tt, Z/*, Zbb
 Constraints on tan
     • mass width A sensitive to tan
     (measured width is A + (MH - MA))
     • 10-20% uncertainty on tan 
          measurement at 30fb-1
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                                  A  Zh
 Branching Ratio high at low tan
 Possible to exploit MA and Mh at the same time
 (gg  A, Zh  llbb case considered)
                                                           Z+jets
 Event Selection                                              tt
     • 2 oppositely signed high pT isolated leptons         Zbb
                                                           signal
     • 2 high ET jets passing b-tagging
     • missing ET and Z mass cut against tt
 Backgrounds: tt, Zbb, ZZ, WZ, Z+jets, W+jets




                                       CMS Note 2006/063
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                  MSSM Charged Higgs
 Single charged Higgs production
   with association to heavy quarks




 Decay dominated by  and tb




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                    pp  tH±, H±  
 Fully hadronic final state:   had, t  bjj
     neutrino from H± decay only
     very small background contributions
 Helicity Correlations
      • polarization opposite for signal H±  
      and background W±  
      • more energetic charged pions from  decay
 Event Selection
     • hadronic tau identification with high pion
     momentum fraction (pTtrk/pT)
     • b-tagging and top mass window cut
     • large missing ET and lepton veto
 Backgrounds: tt, W+jets, Wt, QCD multi-jet


                                                       CMS Note 2006/100
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              CMS Reach for MSSM Higgs I




        CMS Physics TDR II 2006                    CMS Physics TDR II 2006
    5σ discovery regions for the neutral MSSM Higgs boson
    •      H/A important over most of the MA range concerned
    •      H/Aμμ helps at low MA, and A  Zh covers the low tan region
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             CMS Reach for MSSM Higgs II




     CMS Physics TDR II 2006                        CMS Physics TDR II 2006
    5σ discovery regions for the charged and light neutral MSSM Higgs boson
    •      hadronic final state of t-quark from H± production stronger at high MA
    •      studies sensitive down to very low tan at low MA
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                     Summary and Outlook
    CMS searches for the SM and MSSM Higgs Boson presented
    (most of the studies are part of the Physics TDR effort in 2006)

    A major software upgrade since then:
    •      more coherent, standardised analysis environment
    •      many improvements on object identification and reconstruction methods

    Advanced analyses and preparation towards early data are on-going
    •      evaluation of the Standard Model background
    •      new methods and techniques (particle flow, multivariate)
    •      mis-calibrated/aligned MC samples to represent early data

           new results for a selected set of analyses
           expected to be approved and become public later this Spring



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           Back Up




28/03/08   Higgs Searches at CMS   30

				
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