Search for the SM Higgs Boson in HZZ and HWW at the LHC

1
Search for the SM Higgs Boson in H-gtZZ and
H-gtWW at the LHC

• Sinjini Sengupta
• University of Minnesota
• CMS Collaboration
• The 16th International conference on
  Supersymmetry and the Unification of Fundamental
  Interactions.

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Outline

• SM Higgs limits, production and decay modes
• The CMS detector
• About the H-gtWW analysis
• About the H-gtZZ analysis
• SM Higgs Discovery Potential

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SM Limits on the Higgs Mass

• Theoretical limits finite and positive Higgs
  coupling
• Experimental limits
• Direct (from LEP) mH gt 114.4 GeV _at_ 95 CL
• Indirect (from EW data) mH lt 144 GeV

Ref Phys.Lett.B 565(2003)
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SM Higgs Production
Gluon-gluon fusion (dominant mode)
Vector Boson Fusion (forward jets in final state)
Associated production (additional leptons or
jets in the final state)
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SM Higgs Decay

• For mH lt 140 GeV/c2
• bb dominant decay mode but is hard to see because
  of large QCD background.
• ?? main discovery channel due to ECAL resolution
• For 160 lt mH lt 180 GeV/c2
• H-gtWW dominates
• For 140 lt mH 600 GeV/c2
• H-gtZZ-gt4l is important at higher mass

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CMS experiment
Superconducting Solenoid
Muon Detectors
Electromagnetic Calorimeter
Forward HCAL
Hadronic Calorimeter
Tracker
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The Muon Detector

• Drift Tubes (DT) in the barrel region
• Cathode Strip Chambers (CSC) in the endcap region
• Resistive Plate Chambers (RPC) as dedicated
  trigger detectors in both barrel and endcap.

5 wheels in the Barrel
30 (F) sectors
4 layers of muon chambers, ?2.4
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Higgs?WW?l?l? (where le,µ)
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Signal

• Main discovery channel for the Higgs Boson in the
  mass range 2MW lt mH lt 2MZ
• 3 Signal topologies ee-, µµ-, eµ
• Geared for ?L1032cm-2s-1, 100 pb-1 of data
• Pythia is re-weighted to follow NLO PT
  distribution.

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Background

• Primary backgrounds
• Continuum WW uncertainty 17 at 5 fb-1
• tt-gt2µ uncertainty 16 at 5 fb-1
• ?/Z-gt2l
• Other backgrounds ZW-gt3l, tWb-gt2l, ZZ-gt2l
• Most samples generated with Pythia

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Analysis features

• Analysis uses full detector simulation
• Single and di-lepton (e,µ) triggers at L1 and HLT
• 2 charged leptons (e or µ). Leptons are required
  to be well identified, well reconstructed and
  isolated
• 30 GeV lt PTmax lt 55 GeV
• PTmin gt 25 GeV
• 2 neutrinos in the final state. Require ETmiss gt
  50 GeV

mH 160 GeV/c2
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Analysis features

• Due to presence of 2 neutrinos, there is no mass
  peak.
• Counting experiments
• Accurate background estimates from data are
  needed.
• Good reconstruction tools needed.
• Well identified lepton reduces
• Wjet bkg
• To reduce tt and tWb bkg
• all central jets (? lt 2.5)
• with PT gt 15 GeV are
• rejected. (this gets rid of 90
• tt but keeps 50 signal)
• Backgrounds are the dominant
• source of systematics
• (20 for 1 fb-1)

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Jet veto and Acoplanarity
??ll lt 45 12 lt mll lt 40 GeV
Invariant mass before and after central jet veto
cut.
Azimuthal angular separation between The
di-lepton pair after all other cuts. (effective
in reducing WW, ZZ and WZ backgrounds)
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Looking ahead

• Currently cut vs multi-variate analysis is being
  studied.
• First results are very promising
• Shows an excess in signal events.

5s discovery potential can be achieved very
rapidly. Need to understand detector and
backgrounds well.
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Higgs?ZZ?4l (4l 4e, 4µ or 2e2µ)
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The Golden Channel

• Cleanest discovery channel for mH gt 140 GeV/c2

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Signal and Background

• Signal
• Potential for discovery over a wide mass range
• Possible topologies 4e, 4µ, 2e2µ
• 2e2µ has twice the rate as 4e an 4µ channels
• Backgrounds
• qq -gt ZZ/?
• Irreducible, dominant background
• gg -gt tt -gt WWbb
• Zbb
• Reducible backgrounds
• non-isolated leptons from b decays

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Event Selection 2e2µ
2e2µ before selection

• Require 2 pairs of leptons
• Flavor must be conserved
• Charge must be conserved
• Leptons must be isolated
• Originate at primary vertex
• Reconstruct to dilepton invariant mass mZ
• Analysis requires lepton identification with high
  efficiency and resolution down to low PT ( 5
  GeV/c)

mH 200 GeV
mH 140 GeV
after selection
Selection cuts were optimized as a function of mH
but mass independent cuts are being developed.
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Event Selection 4µ

• High muon efficiency
• Underlying event is calibrated with data.

Largest systematic uncertainties are from
background. Vary with mH
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mZ Resolution

• For mH lt 180 GeV/c2, one Z is on shell, other is
  off shell
• For the on shell Z
• Z -gt 2µ has 1.14 mass resolution
• Z -gt 2e has 1.8 mass resolution (worse
  resolution due to Bremsstrahlung)

CMS note 2006/136 mH 130 GeV/c2
Ratio of the measured to the true invariant mass
for µµ- pairs
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Isolation and Impact Parameter

• Isolation is important for reducing contamination
  from b-decay leptons
• Isolation is in the tracker only
• A cut is applied on the sum of the PT of the
  tracks in a cone of radius ?R around the signal
  lepton
• Signal efficiency 90
• Impact Parameter
• Leptons from b quarks dont
• come from the primary vertex
• Apply cuts on the transverse
• and the 3D distance of fitted
• tracks from the vertex as well
• as on the impact parameter
• significance.

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Significance
mH 200 GeV
mH 140 GeV

• Expected events for signal and background for an
  integrated luminosity corresponding to a
  discovery significance of 5s.
• Systematic uncertainties (5-30 from backgrounds)
  have little impact on the discovery potential.

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Discovery Potential for the Higgs

• Luminosity requirements
• 100 pb-1 will exclude certain regions with a
  95 CL
• 1-10 fb-1 has a discovery potential for mH
  mWW at 5s
• 30 fb-1 has a discovery potential of upto mH
  600 GeV

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Conclusions

• The H -gt WW and H -gtZZ analyses are already
  well developed at CMS
• Studies are being updated and new results are
  expected shortly
• Multivariate approach over the counting strategy
  in the case of H -gt WW shows significant early
  promise
• 5s discovery potential for the Higgs Boson over a
  large mass range at the LHC with 5 fb-1 of data.
• H -gt ZZ has the cleanest signal and has the best
  discovery potential for a heavier Higgs Boson.

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Backup Slides
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Cut Flow Table for H-gt ZZ -gt 2e2µ
mH 140 GeV/c2. Errors are statistical only. All
values (except number of events) are in fb
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Cut Flow Table for H -gt ZZ -gt 2e2µ
mH 200 GeV/c2. Errors are statistical only. All
values (except number of events) are in fb
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Results for H -gt ZZ -gt 2e2µ
Expected number of events from signal and
background processes for an integrated luminosity
corresponding to a 5s significance.
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Results for H-gt WW -gt 2e
Expected number of events for a luminosity of 10
fb-1 Selection cuts optimized for mH 150
GeV/c2.