ttbar Cross-Section Studies leading to ttbar Resonance

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ttbar Cross-Section Studiesleading to ttbar
Resonance
D. Jana, M. Saleem, F.
Rizatdinova, P. Skubic University
of Oklahoma, Oklahoma State University

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Introduction

• This work is aimed to study the ttbar
  cross-section for ttbar events decaying to
  leptonjet(s) channel.
• Further this is aimed to extend this work to
  determine the discovery potential of narrow ttbar
  resonance with the Atlas detector.
• ttbar will be the main background to many Higgs
  and SUSY searches. Examples ttH (SM Higgs boson
  search), Stop pair production, heavy charged
  Higgs boson searches.
• Any new physics related to EWSB should be
  coupled to the Top (t), leading to deviations
  from SM ttbar production rate. (e.g new
  resonaces ).
• For our study we use
• Backgrounds involved (W jets major Bkg.)
  already in progress.
• others Z-boson jets, Z-boson pairs, W-boson
  pairs, W-boson Z-boson production).
• Plan is to measure the ttbar x-section using
  likelihood method.
• No B-tagging is assumed at this moment.
• During our initial study we have looked into the
  variables Centrality, KT, ??(e, MET)
• these will help to separate ttbar from
  Background (e.g Wjet for now).

e , in our case
Branching ratio for this channel is 14
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Preliminary study

• We are using the following available Samples

Signal ttbar leptonjets", sample 5200
Total events 137500

• Background W jets (Samples 8240, 8241, 8242,
  8243)
• Ntotal nreco
  X-sec(pb) nexpect Luminosity(pb-1)
  ?(n/N)
• W2partons 18200 22 214.01
  25.86 100 0.12
  
• W3partons 11250 116 123.92
  127.77 1.03
  
• W4partons 6000 424 53.66
  379.19 7.10
• W5partons 4950 649 22.31
  292.5 13.11
• Run on these samples separately and calculated
  the selection efficiency after passing all
  the cuts.
• one lepton (e) with Pt gt 25 GeV etalt2.2
  Missing Et gt 20 GeV
• 3 jets with Pt gt 40 GeV eta lt2.2 4th jet
  with Pt gt 20 GeV and etalt2.2
• Require minumum 4 jets
• The Final BG is the sum of the number of expected
  events using
• ? x ?(nreco/Ntotal) x
  Nexpect( 825 ) for Wn jets.
• Expected number for the ttbar signal events(
  2296 ) with ?(n/N)7.59
• Looking at these numbers and the efficiencies our
  S/B ratio is expected to be 11 before
  likelihood, which is already good enough to
  measure the X-section.

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