ttbar Cross-Section Studies leading to ttbar Resonance D. Jana*, M. Saleem*, F. Rizatdinova**, P. Skubic* *University of Oklahoma, **Oklahoma State University 1

Introduction = e , in our case Branching ratio for 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: 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; W+jet for now). = e , in our case Branching ratio for this channel is ~14% 2

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 before likelihood, which is already good enough to measure the X-section. 3