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SEARCH FOR TOP INTO TAU DILEPTON CHANNELSpp ?
tt ? WbWb
t? leptons
Stéphane Tourneur for the CDF collaboration LPN
HE - Université Paris 6/IN2P3- CNRS APS
04/23/2006

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Search for top into taus main motivations

• Goal 1 First time 3 s observation
• Goal 2 Check BR(t?t?b) against Standard Model
• BR(WB) 100 ? Or Hb ??

• A good training camp for beyond SM searches
• Ex background of SUSY
• t decay dominant at large tan ß

H-?
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Top production decay at Tevatron
Mainly produced in pairs via strong interaction
NLO QCD prediction s(tt) 6.70.7 pb
(mtop175 GeV)
At vs1.96 TeV 85 qq 15 gg
-0.9
Decays exclusively via electroweak interaction
t?Wb
Final state characterized by number and type of
charged leptons from decay of W and W- bosons
5 of all ttbar
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Measurement strategy

• A counting experiment count events passing the
  signal selection
• For a first evidence of the signal optimize S/vB

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Measurement strategy
High Pt central ?online trigger

• A counting experiment count events passing the
  signal selection
• For a first evidence of the signal optimize S/vB

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Measurement strategy
High Pt central ?online trigger

• A counting experiment count events passing the
  signal selection
• For a first evidence of the signal optimize S/vB

Central ? hadrons
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Measurement strategy
High Pt central ?online trigger

• A counting experiment count events passing the
  signal selection
• For a first evidence of the signal optimize S/vB

2 high Pt jets

Central ? hadrons
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Measurement strategy
High Pt central ?online trigger

• A counting experiment count events passing the
  signal selection
• For a first evidence of the signal optimize S/vB

2 high Pt jets

High missing Transverse energy MET gt 20 GeV
Central ? hadrons
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Kinematic cuts

• Discriminative variables against Z and W
• Lepton Pt gt 20 GeV
• 1st Jet Et gt 25 GeV
• 2nd Jet Et gt 15 GeV
• Ht t Etlepton Pt2 Jets EtMEt gt 205 GeV

CDF simulation
gt25 GeV
1st jet Et (GeV)
CDF simulation
Ht Correlated with the high top mass
gt15 GeV
gt205 GeV
Ht (GeV)
2nd jet Et (GeV)
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Killing Z?tt jets background

• 1 t?e or µ,1 t?hadrons
• Unlike the top signal
• The 2 leptons are close
• MEt points between them

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Killing Z?tt jets background

• 1 t?e or µ,1 t?hadrons
• Unlike the top signal
• The 2 leptons are close
• MEt points between them OR opposite to them

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Killing Z?tt jets background

• 1 t?e or µ,1 t?hadrons
• Unlike the top signal
• The 2 leptons are close
• MEt points between them
• OR opposite to them
• The t ?s energies can be reconstructed
• Veto event if it satisfies the former angular
  requirements and M(t,t) lt 115 GeV
• This Z veto cuts 70 of Z?tt and 8 of top signal

CDF simulation
gt115 GeV
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Background estimation

• Z ? tt 2 jets
• Ndata (Z?tt 2 jets) NMonte-Carlo(Z?tt 2
  jets)x
• W3 jets with a jet faking a t
• Its estimation is the biggest challenge in this
  analysis
• Derived from jet and multijet data
• The probability for a jet to fake a tau is
  parametrized as a function of jet Et, Sum Et and
  N jets
• 30 precision

Ndata(Z?µµ2 jets)
NMC(Z?µµ2 jets)
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control regions

• The background prediction is carefully checked
  before looking at the signal
• N jets control regions 1 e OR µ, 1 t, metgt20
  GeV and 0 OR 1 jet
• Excellent agreement reached in 14 checks like
  these ones
• ? Ready to look at data!!

CDF Run II preliminary (350 pb-1)
CDF Run II preliminary (350 pb-1)
0 jet eQ x tQ
0 jet Ht (GeV)
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Summary of signal and backgrounds
CDF Run II preliminary
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Result 5 events observed in 350 pb-1

• p-value 15 probability for the expected
  background (2.70.4) to have fluctuated to the 5
  observed events or more (CDF run II preliminary)
• Roughly equivalent to a 1 sigma significance for
  the top tau dilepton signal
• A method was set up to estimate with high
  confidence difficult backgrounds like Wjets with
  jets faking taus, and Z?tt 2 jets
• 5 events were expected and we observed 5
• Next to come
• Use the full statistics available on CDF tape (gt1
  fb-1 !)
• Still improve the selection of the signal over
  the background
• Without analysis improvement, a first 3 s
  observation from CDF is expected with less than 3
  fb-1 of data well do it earlier!

Conclusion
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Backup slides
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Variation of the p-value with changes in the
systematic error
Result remarkably stable with variations of the
systematic error on B
1-p
0.85
Sys error
0.42
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Some history of the top into tau dilepton channel

• CDF Run I 109 pb-1
• 2.50.4 background and 1.10.4 top signal
  expected in 109 pb-1
• Observed 4
• 3 events also had b tagged jets vs 0.28 B and
  0.63 S expected!
• CDF Run II 194 pb-1
• 1.30.3 background and 1.00.2 top signal
  expected
• Observed 2
• Result rt G(t?t?q)/GSM(t?t?q) lt 5.2 at 95 CL
• CDF Run II 350 pb-1
• 2.70.4 background and 2.20.1stat only top
  signal expected
• Observed 5
• 15 p-value, 1 sigma evidence for top into tau
  dilepton signal

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Tau identification

• To schematize, it looks for narrow isolated jets
• An inner cone, shrinking with the energy,
  contains 1 or 3 good tracks
• An isolation cone around it and up to 30o
  contains no track and no p0s

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The Run II CDF Detector

• A multilayer multipurpose detector
• Inner silicon tracking
• Drift Chamber
• Solenoid
• EM and Hadronic Calorimeters
• Muon Detectors
• New for Run II
• Tracking 8 layer silicon and drift chamber
• Trigger/DAQ
• Better silicon, calorimeter and muon coverage