Transcript ttbar Cross-Section Studies leading to ttbar Resonance

ttbar Cross-Section Studies
leading to ttbar Resonance
D. Jana*, M. Saleem*, F. Rizatdinova**, P. Skubic*
*University of Oklahoma, **Oklahoma State University
1
Introduction
 This work is aimed to study the ttbar cross-section for ttbar events decaying to lepton+jet(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:
tt  WWbb  ( jj )(l )bb
l= e , in our case
 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.
Branching ratio for
this channel is ~14%
 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; W+jet for now).
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Preliminary study
 We are using the following available Samples:
Signal : ttbar “lepton+jets", sample: 5200 Total events ~ 137500 ;
Background: W + jets (Samples: 8240, 8241, 8242, 8243)
Ntotal
nreco X-sec(pb)
nexpect Luminosity(pb-1) (=n/N)
W+2partons : 18200
22
214.01
25.86
100
0.12 %
W+3partons : 11250
116
123.92
127.77
1.03 %
W+4partons : 6000
424
53.66
379.19
7.10 %
W+5partons : 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 > 25 GeV & |eta|<2.2 ; Missing Et > 20 GeV
3 jets with Pt > 40 GeV & |eta| <2.2 ; 4th jet with Pt > 20 GeV and |eta|<2.2
Require minumum 4 jets
The Final BG is the sum of the number of expected events using:
 x (=nreco/Ntotal) x £ = Nexpect(~ 825 ) for W+n 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 1:1
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before likelihood, which is already good enough to measure the X-section.