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

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

3 Decay Channels and Cross Sections σ tt th = 833 pb σ tt th = 833 pb Dileptonic channel: tt → WbWb → lν l bl ’ v l ’ b Dileptonic channel: tt → WbWb → lν l bl ’ v l ’ b Lepton can be e, μ, τ Lepton can be e, μ, τ Γ = 10.3 % Γ = 10.3 % Semileptonic channel: tt → WbWb → lν l bqq ’ b Semileptonic channel: tt → WbWb → lν l bqq ’ b Lepton can be e, μ, τ Lepton can be e, μ, τ Γ = 43.5 % Γ = 43.5 % Assuming lepton universality: Assuming lepton universality: Γ(tt →e νbqq’b) = (1/3) 43.5 % = 14.5% Γ(tt →e νbqq’b) = (1/3) 43.5 % = 14.5% σ e th = * 833 pb = 121 pb σ e th = * 833 pb = 121 pb Similarly for semileptonic channel with muon. Similarly for semileptonic channel with muon. Hadronic channel: Hadronic channel: tt → WbWb → qq ’ bq ’’ q ’’’ b tt → WbWb → qq ’ bq ’’ q ’’’ b Γ = 46.2 % Γ = 46.2 %

4 TTbar Sample Sample used to provide top signal was csc Sample used to provide top signal was csc Event generators were Herwig. Event generators were Herwig. σ = 461 pb, N = 5*10 5 events, L = 1.08*10 3 pb -1 σ = 461 pb, N = 5*10 5 events, L = 1.08*10 3 pb -1 Sample included dileptonic and semileptonic events where the lepton is e, μ, τ. Sample included dileptonic and semileptonic events where the lepton is e, μ, τ. Reconstruction was performed using Full Simulation, Reconstruction was performed using Full Simulation, Jet reconstruction was performed using Cone4. Jet reconstruction was performed using Cone4. Muon reconstruction was performed using muid. Muon reconstruction was performed using muid. Common Ntuples: Common Ntuples:/castor/cern.ch/user/g/ghodbane/ntuples/ /csc Analysis ran over 637,711 top events. Analysis ran over 637,711 top events. Analysis considered only semileptonic events where lepton is e or μ. 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. TTbar background consisted of all dileptonic events and any semileptonic events involving a tau.

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

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

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

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

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

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

11 TTbar Signal and Background Leptonic sample is csc , σ = 461 pb, N = 5*10 5,L = 1.08*10 3 pb -1 Leptonic sample is csc , σ = 461 pb, N = 5*10 5, L = 1.08*10 3 pb -1 Hadronic sample is csc , σ = 460 pb, N = 2*10 5, L = 5.42*10 2 pb -1 Hadronic sample is csc , σ = 460 pb, N = 2*10 5, L = 5.42*10 2 pb -1 Hadronic sample is weighted by L leptonic / L hadronic ~ 2. Hadronic sample is weighted by L leptonic / L hadronic ~ 2. Semi.With eSemi.With μSemi.With τDi.With e, e Di.With e, μ Di.With e, τ Di.With μ, μ Di.With μ, τ Di.With τ, τ Had.5200Had.sample5204 TrueInitialEvents169,218169,763169,51014,31428,17328,08614,09228,35914, ,538 True Final Events 30,19623,9914,5691,8001,6032, , Obs.FinalEvents37,41229,093 2,450 e 1,978 μ 1024 e 776 μ 776 μ 872 e 731 μ 1,412 e 1,110 μ 132 e 116 μ 773 e 591 μ 147 e 117 μ 1 e 4 μ 24 e 24 e 84 μ 84 μ ε 17.9 % 14.1 % 2.3 % 12.6 % 5.7 % 9.0 % 1.8 % 4.8 % 1.9 % 35.7 % 35.7 % % % Π 81.1 % 82.2 %

12 Backgrounds Dileptonic ttbar events:Dileptonic ttbar events: Should be reduced by requiring exactly one lepton.Should be reduced by requiring exactly one lepton. Futher reduced by requiring 4 high-p T jets.Futher reduced by requiring 4 high-p T jets. ttbar events where at least one W decays to tau lepton:ttbar events where at least one W decays to tau lepton: Identify and discard events with tau leptons?Identify and discard events with tau leptons? Hadronic ttbar events:Hadronic ttbar events: Should be reduced by requiring one high-p T lepton.Should be reduced by requiring one high-p T lepton. QCD dijets:QCD dijets: Should be reduced by requiring one high-p T lepton.Should be reduced by requiring one high-p T lepton. W → eν, W → μν, W → τν:W → eν, W → μν, W → τν: Should be reduced by requiring 4 high-p T jets.Should be reduced by requiring 4 high-p T jets. W+n jets where W →e ν, W → μν, W → τνW+n jets where W →e ν, W → μν, W → τν Samples are reconstructed using the Full Simulation, Cone4.Samples are reconstructed using the Full Simulation, Cone4. Reference:

13 QCD Dijet Events, Electron Channel Sample p T [GeV] p T [GeV] σ [pb] σ [pb] Lumi [pb -1 ] N initial N initial N observed csc < p T < < p T < * * ,1500 csc < p T < < p T < * * ,2000 csc < p T < < p T < * ,0000 csc < p T < < p T < * ,3000 csc < p T < *10 2 csc < p T < * *10 3 csc < p T 2240 < p T *10 4 Events were generated with Herwig. TTbar events were excluded from N observed. No non-ttbar QCD dijets survived the event selection.

14 W+n jets, W →e ν Background, Electron Channel processsample σ [pb] N initial true true N selected N observed Efficiency ε [%] ε [%]Purity Π [%] Π [%]Lumi [pb -1 ] Ttbarsignalcsc ,218 30,356 30,356 37,412 37, *10 2 W →e ν+X csc ,225 20,500 20,500 N/A N/A W+3 partons, W →e ν csc ,000 10,000 N/A N/A 2,978 2, *10 3 W+4 partons W →e ν csc ,050 19,050 N/A N/A 4,631 4, *10 4 W+5 partons W →e ν csc ,000 7,000 N/A N/A 1,861 1, *10 4 Desired events are semileptonic ttbar events with electron. W →e ν events produced with Herwig. W+n partons events produced with Alpgen / Herwig.

15 W+n jets, W →μ ν Background, Electron Channel processsample σ [pb] N sample true true N selected N observed Efficiency ε [%] ε [%]Purity Π [%] Π [%]Lumi Ttbarsignalcsc ,218 30,356 30,356 37,412 37, *10 2 W →μ ν+X csc ,318 87,100 87,100 N/A N/A W+3 partons, W →μ ν csc ,000 2,000 N/A N/A *10 3 W+4 partons W →μ ν csc *10 4 W+5 partons W →μ ν csc ,000 3,000 N/A N/A *10 4 Signal events are semileptonic ttbar events with electron. W →μ ν events produced with Herwig. W+n partons events produced with Alpgen / Herwig.

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

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