Outline:

1
Search for Pair Production of Stop and Sbottom
Quarks
S.M. Wang, Rong-Shyang Lu Academia Sinica CDF
June 2006

• Outline
• Introduction Stop/Sbottom production
• Data Sample / Trigger
• JetProb tagging
• SM Background
• Optimization Studies
• Results
• Summary

2
Stop/Sbottom Production

• For 3rd gen squarks, large mixing in the L- and
  R-handed weak eigenstates could lead to one of
  the stop and sbottom quark to be light ? large
  production cross section at Tevatron

• Producted in pair (RP conserved) at Tevatron
• Assume that both stop and sbottom are light, and
  only decays in these channels

• Final state consists of large MET and HF jets
• Main SM backgrounds W/Zjets, Di-boson, top,
  QCD multi-jets
• Use Jet Probability algorithm to tag c/b-jets

3
Data Sample Used for Stop/Sbottom Search

• Data Sample
• MET352jets
• Trigger requirements
• Two jets w/ Etgt10 GeV
• METgt35 GeV
• L295 pb-1

4
SM Background Estimation

• Light Flavor contributions (mis-tags)
• Estimate from data by applying the Jet
  Probability mis-tag matrix
• Mis-tag matrix
• Measurement of the mis-tag rate (on data sample
  dominated by light flavor jets) as function of
  jet parameters (Et, h, of tracks, )
• Heavy Flavor contributions
• Estimate from MC samples (W/Zjets(b,c),
  di-boson, top, QCD(bb,cc))
• Apply tagging efficiency scale factor to correct
  MC tagging rate
• QCD Multi-jet normalization
• Select a control region dominated by QCD
  multi-jet production
• Scaled the QCD MC contribution such that

5
QCD Dominance Region

• QCD dominance region
• 2 or 3 jets (Etgt35, 15, 15 GeV)
• METgt50 GeV
• ?1 jet tagged (_at_ Jet Probability lt5)
• loose lepton 0

Pre-selection cuts
6
EWK Control Region

• Choose another control region to test
  understanding of SM background
• Pre-selection cuts, ?1 loose lepton

7
Selection Cuts

• Optimized selection cuts for stop/sbottom in
  three stop/sbottom mass regions

Selection Cuts

• Clean-up cuts
• Remove beam-halo
• Veto events w/ jets entering into central
  calorimeter crack, or into the chimney
• No charged lepton
• 2 or 3 jets
• Et cuts on 1st and 2nd leading jets vary
  depending on M(stop), M(sbottom)
• METgt50 GeV or higher (also depending on M(stop),
  M(sbottom) )
• 1st and 2nd leading jets not back-to-back
• Jets not pointing along direction of MET
• ?1 jet tagged (_at_ JPlt5 for stop search, _at_ JPlt1
  for sbottom search)

Reduce QCD
8
Expected Background in Stop Search
Low Medium High
Wjets 11.52.432.62 9.312.302.12 3.951.540.90
Zjets 9.930.502.02 7.340.431.49 4.140.340.84
Diboson 2.520.110.48 1.990.090.38 0.890.060.17
Top 5.230.190.82 4.900.190.77 3.930.170.62
QCD 32.55.208.11 17.94.004.47 2.551.470.64
Mistags 75.42.2210.7 53.51.987.60 27.21.513.87
Total 13715.8 94.911.1 42.75.28
9
Expected Background in Sbottom Search
Low Medium High
Wjets 5.501.151.16 1.530.550.32 0.510.360.11
Zjets 5.950.361.10 2.660.250.49 1.020.160.19
Top 4.220.170.56 2.880.140.38 1.160.090.16
QCD 18.84.014.70 2.601.500.65 02.05-0
Mistags 20.50.632.30 8.090.470.91 1.980.250.22
Total 55.07.24 17.82.31 4.672.16-0.67
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Signal Acceptance and Systematic Uncertainties
Sbottom ( M(neutralino)70 GeV)
Stop ( M(neutralino)55 GeV)
low high
M(stop) (GeV) 90 130
Acceptance 0.54 2.4
low high
M(sbottom)(GeV) 120 200
Acceptance 3.1 6.2
Source Uncertainties()
Luminosity 6
Trigger 5
Tagging 12 (JPlt5) , 8.6 (JPlt1)
Jet Energy 4 20
ISR/FSR 5
PDF 2
Total 13.4 25.2
11
Un-Blinded Signal Region
Stop Low Medium High
SM (Total) 13715.8 94.911.1 42.75.28
Data 151 108 43
Sbottom Low Medium High
SM (Total) 55.07.24 17.82.31 4.670.67
Data 60 18 3
No significant excess in data
12
Compare Data and MC (Stop, high mass)
13
Compare Data and MC (Sbottom, medium mass)
14
Observed Limits (Stop)
15
Observed Limits (Sbottom)
16
Mass Limits (Stop)
17
Mass Limits (Sbottom)
18
Sbottom Candidate (high mass)
Run 184957 Evt 962706
MET145 GeV

• Both jets passed Jet Probability cut JPlt1

Et55 GeV
JP8.2e-6
Et100 GeV
Et55 GeV
Et100 GeV
JP6.6e-3
19
Stop/Sbottom Candidate
Run 185377 Evt 2077415
20

• Summary
• Searched for stop and sbottom pair production in
  the METjets signature
• Searched regions do not show hints of SUSY
• Preliminary results set exclusion regions in the
  stop/sbottom mass vs neutralino mass plane