Measuring the Top Quark Cross Section in the Semileptonic Channel

1. Measuring the Top Quark Cross Section in the Semileptonic Channel Andrea Bangert

2. Preview • TTbar decay channels and cross sections • The ttbar sample • Event selection • Reconstructing the top quark • Reconstructing the W boson • Ttbar signal, ttbar background • Physics backgrounds • Next steps

3. Decay Channels and Cross Sections • σttth = 833 pb • Dileptonic channel: tt → WbWb → lνlbl’vl’b • Lepton can be e, μ, τ • Γ = 10.3 % • Semileptonic channel: tt → WbWb → lνlbqq’b • Lepton can be e, μ, τ • Γ = 43.5 % • Assuming lepton universality: • Γ(tt→eνbqq’b) = (1/3) 43.5 % = 14.5% • σeth = 0.145 * 833 pb = 121 pb • Similarly for semileptonic channel with muon. • Hadronic channel: • tt→ WbWb → qq’bq’’q’’’b • Γ = 46.2 %

4. TTbar Sample • Sample used to provide top signal was csc11.005200. • Event generators were MC@NLO, Herwig. • σ = 461 pb, N = 5*105 events, L = 1.08*103 pb-1 • Sample included dileptonic and semileptonic events where the lepton is e, μ, τ. • Reconstruction was performed using Full Simulation, 11.0.42. • Jet reconstruction was performed using Cone4. • Muon reconstruction was performed using muid. • Common Ntuples: /castor/cern.ch/user/g/ghodbane/ntuples/11.0.42/csc.005200 • Analysis ran over 637,711 top events. • Analysis considered only semileptonic events where lepton is e or μ. • TTbar background consisted of all dileptonic events and any semileptonic events involving a tau.

5. Selection • Isolated, high-pT lepton: • Electron or muon • pT > 20 GeV, |η| < 2.5 • For electrons “isolated” meant less than 7.5 GeV energy within cone of ∆R = 0.45 • For muons “isolated” meant less than 7.5 GeV energy within cone of ∆R = 0.2 • Electrons were required to have isEM == 0. • Missing ET > 20 GeV • At least four jets: • with |η| < 2.5 and • pT > 40 GeV • No b-tagging was required.

6. Lepton Isolation • Event selection has not yet been performed. • All electrons from each event are plotted. • Plotted are dileptonic and semileptonic ttbar events. • Plot depicts energy contained in cone of R=0.45 about electron, R=0.2 about muon. • Cuts require less than 7.5 GeV within cone for both electron and muon.

7. Reconstructing the Top Quark • For each selected event: • At least four jets have passed selection cuts. • Form all possible three-jet combinations using all jets present. • Select the three-jet combination with the highest pT. • Take this best three-jet combination to represent the top quark. • Events occurred in semileptonic channel (electron or muon). • MC@NLO event weights were implemented.

8. The Top Quark Mass • Fit mass distribution with sum of Gaussian and Chebyshev polynomials. • The ttbar signal is represented by the Gaussian. • Combinatorial background is represented by sum of polynomials. • mt is defined to be the position of peak of mass distribution. • mt = 167.7 GeV • σ= 11.8 GeV • Generated mass was mt = 175 GeV

9. Reconstructing the W boson • The best top quark candidate from each event is composed of three jets. • Form the 3 possible two-jet combinations. • Select that two-jet combination with the maximal pT. • Take this best two-jet combination to represent the reconstructed W boson. • MC@NLO event weights were implemented.

10. The W Mass • Fit mass distribution with sum of Gaussian and Chebyshev polynomials. • The signal is represented by the Gaussian. • Combinatorial background is represented by sum of polynomials. • mW is defined to be the position of peak of mass distribution. • mW = 79.0 GeV • σ = 8.2 GeV

11. TTbar Signal and Background • Leptonic sample is csc.005200, σ = 461 pb, N = 5*105,L = 1.08*103pb-1 • Hadronic sample is csc.005204, σ = 460 pb, N = 2*105, L = 5.42*102pb-1 • Hadronic sample is weighted by Lleptonic / Lhadronic ~ 2.

12. Backgrounds • Dileptonic ttbar events: • Should be reduced by requiring exactly one lepton. • Futher reduced by requiring 4 high-pT jets. • ttbar events where at least one W decays to tau lepton: • Identify and discard events with tau leptons? • Hadronic ttbar events: • Should be reduced by requiring one high-pT lepton. • QCD dijets: • Should be reduced by requiring one high-pT lepton. • W→eν, W→μν, W→τν: • Should be reduced by requiring 4 high-pT jets. • W+n jets where W→eν, W→μν, W→τν • Samples are reconstructed using the Full Simulation, Cone4. • Reference: http://jarguin.home.cern.ch/jarguin/dc3requests_sm.html

13. QCD Dijet Events, Electron Channel • Events were generated with Herwig. • TTbar events were excluded from Nobserved. • No non-ttbar QCD dijets survived the event selection.

14. W+n jets, W→eνBackground, Electron Channel • Desired events are semileptonic ttbar events with electron. • W→eν events produced with Herwig. • W+n partons events produced with Alpgen / Herwig.

15. W+n jets, W→μνBackground, Electron Channel • Signal events are semileptonic ttbar events with electron. • W→μν events produced with Herwig. • W+n partons events produced with Alpgen / Herwig.

16. Backgrounds • Top signal and background in semileptonic channel with electron. • Sample is weighted to a total luminosity L = 1 pb-1. • W+N jets where W→eν is most significant background. • W+N jets where W→μν is small in this (electron) channel. • No non-ttbar QCD events survive the event selection. • W→eν +X background is not included.

17. Next Steps • Repeat analysis for semileptonic channel where the lepton is a muon. • Understand W→eν+X inclusive sample. • Consider W+n partons events where W→τν • for both electron and muon ttbar samples. • Analyze a b-bbar sample to make sure QCD dijets are insignifficant. • Use leptonic side of semileptonic ttbar decay to reconstruct a second top quark mass.