ttbar Cross-Section Studies

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

2. Motivation (1) • 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.

3. Motivations (2) • At LHC: • Production of more than 8 million ttbar pairs(~ 830 pb-1) in one year at initial luminosity (~1033 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).

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 ,,

5. Topology of singlelepton + jets decay channel 40

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 2nd 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.

7. MC Data • We are using the following available Samples: Signal : ttbar “e+jets", Mtop = 170 GeV ; sample: 6201 Total signal events ~ 85000 ; Background: W + jets Ntotal nreco 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 (=nreco/Ntotal) x £ = Nexpect(~ 910 ) for W+n jets. The expected number for the ttbar signal events:  x (=nreco/Ntotal) x £ = Nexpect 896.1 x 7.9 x 100 = ( ~ 707919 ) for ttbar

8. Basic Selection Criteria • Only one lepton (e) with Pt > 20 GeV & |eta|<2 • Missing Et > 20 GeV • 3 jets with Pt > 40 GeV & |eta| <2 (All standard cuts used) • 4th jet with Pt > 20 GeV and |eta|<2 Electron isolation cut (deltaR, distance from the jet axis) > 0.4 E/P :

9. Jet Momentum Distribustion 1st leading 2nd leading 1st Leading Jet 3rd leading 4th leading 2nd Leading Jet Jet Momentum (GeV) • 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 (3rd, 4th) are considered as “light” jets. 2nd light Jet 1st light jet Jet Momentum (GeV)

10. Invariant Mass Distribustion for W Transverse W mass MeV Invariant jj mass (MeV) (electron & neutrino) (two light jets)

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 • HT • - KT • -  • -

12. Event Probability Aplanarity

13. Event Probability Centrality is defined: Centrality = Scalar Sum of the pT of the jets = Scalar Sum of the energy of the jets

14. Event Probability Something not right Sphericity Sphericity(S): Measures summed p2trans w.r.t. event axis. S=1 (for Isotropic events, ttbar..) S =0 (for less isotropic, like w+jet, QCD..)

15. Event Probability HT(MeV) HT -> Scalar sum of the pT of the four leading jets

16. Event Probability KT

17. Event Probability Delta Phi (electron, Missing ET)

18. Event Probability Invariant mass of W (combining 2 light jets)

19. Event Probability Jet Pt (Pt of a leading Jet)

20. Cut flow for W+jets

21. Cutflow Total Signal events : 85966 21

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.

23. Backup…..

24. Aplanarity defination 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 24

25. Sphericity 25