Freya Blekman

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Freya Blekman NIKHEF / Universiteit van Amsterdam
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Outline

• Introduction
• Triggers
• Analyses
• tt ? all hadrons
• tt ? electron jets
• Conclusion, Prospects

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Physics motivation

• X-sec
• QCD HQ production
• Mass
• Mtop provides restriction on Mhiggs
• Top physics
• spin correlations
• Vtb

DØ Run 1 Mtop
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Tevatron Top Physics

• ?(s)1.96 TeV
• Top pair production (this talk)
• Single top
• t ? W b BR gt 99
• tt analyses classified by W decay channels
• s(pp?tt) 7 pb

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Event topology

• Background reduction most important factor
• Lepton jets Background mainly QCD and W jets
• Hadronic channel Background mainly QCD
• All tt channels
• 2 b-jets!
• ? B-tagging of vital importance

jet
ejets4 jets e ?e
b-jet
jet
b-jet
e
jet
?
jet
b-jet
Hadronic 6 jets
jet
b-jet
jet
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Triggers

• Hadronic channel
• focus on jet multiplicity, high total transverse
  energy ( HT)
• Electron jets channel
• focus on electron identification jet
  multiplicity

tt ? ejets tt hadronic
SignalEfficiency Background Trigger Rate Signal Efficiency Background Trigger Rate Simulated BG Trigger Rate
L1 99 88 Hz 100 36 Hz 47.8 Hz (only pT gt 20 QCD MC)
L2 95 52 Hz 99.0 8 Hz 4.1 Hz
L3 75 6 Hz 97.8 1.4 Hz 1.4 Hz
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tt ? e jets
(L. Phaf, I. Iashvili et al.)

• Finding electrons
• EM ID efficiency
• High pT electrons
• Count Z ? ee- with and without ID cuts
• ID cuts isolation, EM fraction, pT gt 20 GeV,
  shower shape.

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tt ? e jets candidate

• passes Run I cuts
• Isolated electron
• 4 reconstructed jets above
• 15 GeV
• HT 191 GeV
• Aplanarity 0.11

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Offline event selection

• event-shape variables

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tt ? e jets candidate

• passes Run I cuts
• Isolated electron
• 4 reconstructed jets above
• 15 GeV
• HT 191 GeV
• Aplanarity 0.11
• Mtop (?2 fit) 165 GeV

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tt ? e jets candidate
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tt ? hadrons
(F.Blekman,M.Vreeswijk,G.Massaro)

• HT gt 225 GeV
• Aplanarity gt 0.05

• N jets gt 5
• Centrality gt 0.6

After selection signal/bg 1/30 (MC signal eff
44 )
Data Lum 2 pb-1
QCD MC Pythia, pT gt 80
tt MC Pythia,hadronic tt Mtop 174 GeV
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tt ? hadrons candidate event

• 6 jets
• Passes selection cuts
• Secondary vertices tagged
• Mtop (?2 fit) 168 GeV

?
f
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Results Hadronic Channel

• Efficiencies and Purities
• Results for preliminary study on 2 pb-1
• Further study still to be done, but
• promising for larger dataset.

Cuts tt ? hadrons efficiency S/B ratio
Selection (this study) 44 1/30
2 vertex tags non-leading jet cut 25 1/2
ANN output
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Prospects
Expect to have 50 pb-1 reconstructed data by
February 2003

• Lepton jet , di-lepton
• For now Use Analysis similar to Run 1
• (based on event topology)

• All hadronic
• Make use of improved b-tagging

tt decay channel Signal events DØ Run 1 Expected signal by spring 2003
Leptonlepton 6 1 - 5
Leptonjet 20 5 - 27
Hadronic 13 7 - 40
110 pb-1 _at_ 1.8 TeV 50 pb-1 _at_ 1.96 TeV, s 6 pb (expected eff 5 - 30 )
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Conclusion

• We see top events being produced at Tevatron
• Improved b-tagging starting to make a difference
• Potential measurement for the winter conferences
• More interesting high pT physics to come
• Stay Tuned

!
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Event Selection

• event-shape variables ( hadronic channel)

Sphericity
Aplanarity
Sphericity
ttbar QCD data
Thrust
Oblateness