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1 ttbar Cross-Section Studies D. Jana*, M. Saleem*, F. Rizatdinova**, P. Gutierrez*, P. Skubic* *University of Oklahoma, **Oklahoma State University Slide 2 2 For all quarks, there are qqbar bound states (mesons), LHC is the good place to study the existence of ttbar bound state. Existence of ttbar resonance will simply imply physics beyond SM. Several theoretical models (MSSM) predict existence of ttbar pair resonances. 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. Motivation (1) Slide 3 3 At LHC: Production of more than 8 million ttbar pairs(~ 830 pb -1 ) in one year at initial luminosity (~10 33 cm -2 /s) LHC is also a Top factory. Excellent opportunity to understand top quark at 14 TeV. We need to understand top quark very well before doing any other physics (Higgs or SUSY or any other physics with in the SM or beyond the SM). ttbar production cross-section study is also first priority at 14 TeV (excellent chance for students to graduate ) Plan is to continue with this study and understand the ttbar production which will be helpful understanding of the ttbar background for several analyses and also proceed towards the ttbar resonance study (later). Motivations (2) Slide 4 4 Decay Process Multi-jet channel: 65.5 % of all ttbar events Dilepton channel: 4.9 % of all ttbar events Single lepton plus jet channel: 29.6% of all ttbar events Problem : Large QCD background Background process: Drell-Yan process, Z-boson + jets, 2 W-boson jets and bbar pair production = e, , Slide 5 5 40 Topology of single lepton + jets decay channel Slide 6 Current Goal Our aim is to measure the ttbar x-section using likelihood method. No B-tagging is assumed at this moment. Need to find topological variables that will help to separate ttbar from Background (e.g; W+jet). The first line is the best discrimnator. Approximated in the 2 nd line neglecting the correlation among the variables. Finally, this is transformed to a expression: where we have log of the ratio of the topological variable i used to build the the discriminant. Slide 7 7 MC Data Signal : ttbar e+jets", M top = 170 GeV ; sample: 6201 Total signal events ~ 85000 ; Background: W + jets N total n reco X-sec(pb) Luminosity(pb -1 ) W+2partons : 12000 15 2032 100 W+3partons : 11250 125 771 W+4partons : 5500 436 273 W+5partons : 4950 709 91 Run on these samples separately and calculated the selection efficiency after passing all the cuts (next page). The Final BG is the sum of the number of expected events using: x (=n reco /N total ) x = N expect (~ 910 ) for W+n jets. The expected number for the ttbar signal events: x (=n reco /N total ) x = N expect 896.1 x 7.9 x 100 = ( ~ 707919 ) for ttbar We are using the following available Samples: Slide 8 8 Basic Selection Criteria Only one lepton (e) with Pt > 20 GeV & |eta| 20 GeV 3 jets with Pt > 40 GeV & |eta|20 GeV and |eta| 0.4 E/P : Slide 9 9 1 st Leading Jet 1 st light jet 2 nd light Jet 2 nd Leading Jet Jet Momentum Distribustion 1 st leading 2 nd leading 4 th leading 3 rd leading In the early data, we will not have B-tagging performance well understood and will not be applied. Assumption: the 2 leading jets are b-jets. The next two leading jets (3 rd, 4 th ) are considered as light jets. Jet Momentum (GeV) Slide 10 10 Invariant jj mass (two light jets) Transverse W mass (electron & neutrino) (MeV) MeV Invariant Mass Distribustion for W Slide 11 11 Comparison of variables to discriminate between signal & background From the previous experience (with D0): We are planning to use the following discriminating variables, to differentiate Our signal form the Background. -Aplanarity - Centrality -Sphericity - H T - K T - - Slide 12 12 Event Probability Aplanarity Slide 13 13 Event Probability Centrality = Scalar Sum of the p T of the jets = Scalar Sum of the energy of the jets Centrality is defined: Slide 14 14 Event Probability Sphericity Sphericity(S): Measures summed p 2 trans w.r.t. event axis. S=1 (for Isotropic events, ttbar..) S =0 (for less isotropic, like w+jet, QCD..) Something not right Slide 15 15 Event Probability H T (MeV) H T -> Scalar sum of the pT of the four leading jets Slide 16 16 Event Probability KTKT Slide 17 17 Event Probability Delta Phi (electron, Missing E T ) Slide 18 18 Event Probability Invariant mass of W (combining 2 light jets) Slide 19 19 Event Probability Jet Pt (Pt of a leading Jet) Slide 20 20 Cut flow for W+jets Cuts W+2par (Effeciency) W+3par lepton cut(Etacone>40, eta20GeV) 4.63%3.90% 1st Jet Cut 40 GeV 4.03%3.62% 2nd Jet Cut 40 GeV 2.15%2.51% 3rd Jet Cut 40 GeV 0.60%0.68% 4th Jet Cut 20 GeV 0.001% 0.01% W+4par 37.72% 27.96% 26.65% 20.89% 10.22% 7.92% W+5par 36.65% 25.05% 24.46% 21.33% 15.03% 14.32% Slide 21 21 Cutflow Cuts Event Number Efficiency lepton cut(Etacone>40,eta20GeV) 1620918.9% 1st Jet Cut 40 GeV 1522417.7% 2nd Jet Cut 40 GeV 1289215.0% 3rd Jet Cut 40 GeV 8124 9.5% 4th Jet Cut 20 GeV 6802 7.9% Total Signal events : 85966 Slide 22 22 Future Plan Plan is to plot Likelihood discriminant distributions for ttbar & W+jets once we get enough W+jet events After we find Likelihood distributions, we will find top mass dependence of the ttbar production cross-section. We have not yet applied b-tagging, as 2 of our jets will be tagged as b-jets. During the initial running we might not have B-tagging, well understood. In that case we might have to choose from the following: Do not use any B-tagging initially Or only tag one of the jet as B-jet (will be helpful to remove some of the background). background studies (W + jets) already started. Will be working on the others: Z-boson + jets, Z-boson pairs, W-boson pairs, W- boson & Z-boson production ). Will report to this group meeting for suggestions and comments. Slide 23 23 Backup.. Slide 24 24 Normalized momentum tensor = Momentum vector of a reconstructed object 0 Cartesian Co-ordinates After standard diagonalization, we can find out 3 eigen values of Aplanarity ( which is a measure of the flatness of the events) is defined by In the sum, we have jets and electron (from W decay) which has best discriminating power between signal & background Aplanarity defination Slide 25 25 Sphericity