Study of Z Jets Production and Higgs Search at D0

1
Study of Z Jets Production and Higgs Search at
D0
Marc Buehler

• Tevatron Status
• D0 Detector
• SM Higgs
• Z(ee) Jets Analysis
• Summary

2
Tevatron Collider in Run II

• Colliding protons and antiprotons at ?s1.96TeV
  (Run I 1.8TeV)
• 36 x 36 proton and antiproton bunches with 396ns
  bunch crossing time (RunI 6x6/3.5ms)
• Increased instantaneous luminosity (currently 4-5
  x 1031 cm-2 s-1)
• Expected integrated luminosity is 2fb-1 by 2007

3
The upgraded D0 Detector

• New tracking silicon and fibers in 2T magnetic
  field
• Upgraded muon system
• Upgraded DAQ/trigger (displaced-vtx trigger soon
  !)

4
D0 Data Taking
Delivered 410pb-1
On Tape 320pb-1
Data collection efficiency 90
Start of physics quality data in Spring 2002

• Run I 166pb-1 delivered / 130pb-1 recorded
• Integrated luminosity used for this analysis is
  170pb-1 (data taken up till Oct 03)

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The SM Higgs in a Nutshell

• Last missing piece of the SM
• Explains electroweak symmetry breaking
• Gives mass to the particles of the SM
• Light SM Higgs preferred
• MH 117 GeV, MHlt 251GeV _at_
  95CL
• LEP's direct search MH gt 114.4GeV _at_
  95CL

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Higgs Search Strategy

• Production cross sections are small depending on
  Higgs mass
• H?bb dominant for MHlt 140 GeV
• Searches can be performed in W/Z associated
  production to handle backgrounds
• H?WW() final states can be explored at higher
  masses

• Need several fb-1 of data to detect the Higgs !!!

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Z(ee) Jets Analysis

• Too early to go ''Bump Hunting'' !!!
• The goal for this analysis is to measure the
  Z(ee) Jets production cross section
• Determination of the reliability of QCD
  calculations for heavy boson production, testing
  the theoretical predictions of cross sections
• Study the production properties of high energy
  hadronic jets associated with Z boson production

• Event selection includes
• 2 central (?lt1.1) and isolated electrons with
  pT gt 20 GeV and at least one track matched
  electron
• DiEM invariant mass cut 80 GeV lt Mee lt 100 GeV
• Jet Energy Scale corrected Jets with pT gt 20 GeV
  passing all jet quality cuts

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Background substraction

• Substracting QCD background from Z and DY through
  fitting of Mee distributions
• Gauss Breit-Wigner for Z
• Exponential for QCD and DY
• Fitting to Z/??ee- Pythia MC to determine
  fraction of DY 1.99
• Fitting to Mee data distribution

Signal Z DY 6150 Background QCD66
Mee (GeV)
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Electron Distributions in Z Events

• Points represent data with statistical errors -
  histograms represent Pythia MC (additional energy
  and momentum smearing applied) GEANT detector
  simulation
• Approximately 6,000 Z candidate events in 170pb-1
  of data

D0 Run II Preliminary
D0 Run II Preliminary
D0 Run II Preliminary
D0 Run II Preliminary
D0 Run II Preliminary

• DiEM invariant mass distribution

• Transverse momentum and ? distributions for
  leading and second leading electron

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Jet Properties in Z?ee- Events

• Points represent data with statistical errors -
  histograms represent ALPGEN/Pythia MC GEANT
  detector simulation (use ALPGEN for event
  generation and interface it with Pythia for
  parton showering)

D0 RunII Preliminary

• Dijet invariant mass

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Jet Properties in Z?ee- Events
D0 RunII Preliminary
D0 RunII Preliminary
D0 RunII Preliminary
D0 RunII Preliminary

• ? distributions for leading and second leading jet

• pT distributions for leading and second leading
  jet

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Summary

• Properties of Z(?ee-) Jets events are being
  studied as a first step towards Higgs search in
  the ZH?ee- bb channel
• Kinematic properties of events have been compared
  to MC simulations
• Next step Calculation of Cross Sections vs Jet
  Multiplicity/pT and comparison to theoretical
  predictions
• More data coming up soon
• Higgs hunting at the Tevatron has begun !!!

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Backup Slides ...
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Analysis Event Selection

• Electrons
• EMFraction gt 0.9
• Isolation lt 0.15
• HMatrix7 lt 12.0
• pt gt 20.0GeV
• ? det lt 1.1
• At least one trackmatched object (based on ?2 of
  the trackmatch based on E/p and the matching
  distance)
• 80GeV lt Mee lt 100GeV

• Jets
• 0.05 lt EMFraction lt 0.95
• CHFraction lt 0.4
• HotFraction lt 10.0
• N90 gt 1
• L1 confirmation
• Removing jets overlapping with EM objects with ?R
  of 0.4
• JES corrected pt gt 20GeV

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SM Higgs Decay Modes
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Z Distributions
D0 Run II Preliminary
D0 Run II Preliminary

• Transverse momentum and rapidity distributions
  for the reconstructed Z Boson