Maiko_Takahashi by yaofenjin


									Higgs Searches at CMS

        Maiko Takahashi
     Imperial College London

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

    •      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

28/03/08                             Higgs Searches at CMS                                 2
                          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)

28/03/08                        Higgs Searches at CMS                            3
           CMS Detector I

28/03/08      Higgs Searches at CMS   4
                          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)

28/03/08                           Higgs Searches at CMS                            5
           CMS Reconstruction I

28/03/08         Higgs Searches at CMS   6
                    CMS Reconstruction II
    • 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
        • 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)
28/03/08                            Higgs Searches at CMS                               7
                 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
      • 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

28/03/08                          Higgs Searches at CMS                             8
                  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

28/03/08                          Higgs Searches at CMS                            9
            SM Higgs Production at LHC

    Gluon fusion dominates by an order of magnitude
    Vector-boson fusion becomes comparable at high mass
28/03/08                        Higgs Searches at CMS     10
           SM Higgs Branching Ratio

28/03/08           Higgs Searches at CMS   11
                             Inclusive H  
              (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
28/03/08                              Higgs Searches at CMS                          12
              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
28/03/08                          Higgs Searches at CMS                                 13
                    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

28/03/08                                 Higgs Searches at CMS                                    14
                            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
28/03/08                         Higgs Searches at CMS                              15
                        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
28/03/08                          Higgs Searches at CMS                         16
               H  WW  ll Updates


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

28/03/08                            Higgs Searches at CMS                            17
                   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
28/03/08                            Higgs Searches at CMS                      18
                                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

28/03/08                           Higgs Searches at CMS        19
           MSSM Higgs Production at LHC

    gg/qq  bbH dominates for high tan
            additional signature of b-quark jets to distinguish the signal
28/03/08                              Higgs Searches at CMS                   20
              MSSM Higgs Branching Ratio

    •      Decays to bb and  dominates over all mass range for high tan
    •      Similar behaviour for h and H
28/03/08                             Higgs Searches at CMS                   21
                            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

28/03/08                           Higgs Searches at CMS                                22
                        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
28/03/08                        Higgs Searches at CMS                             23
                                  A  Zh
 Branching Ratio high at low tan
 Possible to exploit MA and Mh at the same time
 (gg  A, Zh  llbb case considered)
 Event Selection                                              tt
     • 2 oppositely signed high pT isolated leptons         Zbb
     • 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
28/03/08                          Higgs Searches at CMS             24
                  MSSM Charged Higgs
 Single charged Higgs production
   with association to heavy quarks

 Decay dominated by  and tb

28/03/08                         Higgs Searches at CMS   25
                    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
28/03/08                           Higgs Searches at CMS                   26
              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
28/03/08                           Higgs Searches at CMS                     27
             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
28/03/08                             Higgs Searches at CMS                          28
                     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

28/03/08                             Higgs Searches at CMS                         29
           Back Up

28/03/08   Higgs Searches at CMS   30

To top