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Measuring the Charge of the Top Quark at CDF

This research presentation discusses the motivation behind precision measurements of top quark properties and the search for exotic top quarks. It explores the methodology and results of determining the top charge using decay products in different channels. Statistical treatment and sensitivity studies are also presented.

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Measuring the Charge of the Top Quark at CDF

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  1. Measuring the Charge of the Top Quark at CDF Kirsten Tollefson DPF + JPS 2006 Meeting Honolulu, Hawaii

  2. Motivation • Precision measurements of top quark properties • Is it SM top? Expect Q = +2/3 • Or exotic t’ quark with Q = -4/3 (hep-ph 9810531) Exotic t’ SM Top K. Tollefson - DPF, Hawaii

  3. 2.0 fb-1 delivered 1.6 fb-1 to tape _ 1.96 TeV p p Tevatron CDF DØ Main Injector Fermilab and CDF K. Tollefson - DPF, Hawaii

  4. Top Decay • Dilepton: • 2 leptons PT > 20 GeV • ET > 25 GeV •  2 jets, ET > 15 GeV • Lepton+Jets: • Lepton PT > 20 GeV • ET > 20 GeV •  3 jets, ET > 15 GeV • B(tWb) ~ 100% • Classify by W decay / / K. Tollefson - DPF, Hawaii

  5. or W+? Determining Top Charge using Top Decay Products In SM tW+b, can it be tW-b? Need to know: • Charge of W (charge of lepton) • Pairing between W and b • Flavor of b jet Event samples: Dilepton Lepton + Jets K. Tollefson - DPF, Hawaii

  6. Purity = Asymmetry = Dilution = Defining Performance • Define N+ = N+2/3= W+bN- = N-4/3 = W-b • Nright = # events correctly assigned to N+ or N- • Optimize method using (Atrue)  (D2N)-1/2 K. Tollefson - DPF, Hawaii

  7. Top MC Wrong decision Right decision keep Mlb2 x 102 Wb Pairing - Dilepton Channel • 2 leptons + 2 highest Et jets • Max. Mlb2 > 22,000 GeV2/c4 =39%, P=97%, D2=0.34 K. Tollefson - DPF, Hawaii

  8. Wb Pairing - L+Jets Channel • Kinematic mass fitter • Constrained fit, 2<9 • 2 jets b-tagged • Secondary vertex tagger =63%, P=86%, D2=0.33 K. Tollefson - DPF, Hawaii

  9. Use good silicon tracks with PT>1.5 GeV Weight factor X = 0.5 Require 2 b-jets have opposite sign (OS) _ b b Qb-jet ^  a = jet axis, pi = track momentum b Flavor TaggingMomentum Weighted Jet Charge Top MC DIL   =42%, P=73%, D2=0.09 L+Js   =51%, P=73%, D2=0.11 K. Tollefson - DPF, Hawaii

  10. From MC Jet Charge Performance • Dijet data with: • Muon jet and away jet back-to-back • Both jets b-tagged • Expect sign correlation for bb • Opp. sign between jets • Need to correct for: • bc • Mixing • Extrapolate in PT and  from dijet to ttbar sample _ K. Tollefson - DPF, Hawaii

  11. Dijet MC Dijet MC Vertex Mass PTrel PTrel P Jet axis _ Getting bb Fraction • Use PTrel for b fraction of muon jet • Use mass of secondary vertex tracks for b fraction of away jet K. Tollefson - DPF, Hawaii

  12. Backgrounds • Is background more likely to mimic +2/3 or -4/3 events? • If no bias, purity in background pb= 0.5 K. Tollefson - DPF, Hawaii

  13. Top MC Ideal case Expectations with 1 fb-1 • Top MC signal purity ps ~ 70% • Dijet MC signal purity ps ~ 65% • Dijet data signal purity ps ~ 60% K. Tollefson - DPF, Hawaii

  14. Statistical Treatment • Exclude the exotic -4/3 (or SM +2/3) model to X% C.L. • Investigating Binomial and Profile Likelihood methods • What is the probability of observing N+ and N- events in data given the fraction, f+, of true +2/3 events SM Top f+=1 Exotic case f+=0 • Sensitivity studies using the Profile Likelihood method • Frequentist approach with multiple nuisance parameters • NsNs, psps, NbNb, pbpb • Scan over fraction of true +2/3 events (f+) fitting for nuisance parameters to get minimum likelihood f+ K. Tollefson - DPF, Hawaii

  15. Signal purity = 6015% Signal purity = 69.315% SM top Exotic SM top Exotic f+ f+ Sensitivity Studies • Not sensitive to # Nb or purity pb of background • Strong dependence on signal purity ps K. Tollefson - DPF, Hawaii

  16. Summary and Plans • CDF measuring charge of top quark in Dilepton and Lepton+Jets channels • Finalizing work on understanding purity in signal and statistical treatment • Plan to have result for winter conferences using >1 fb-1 of data • Already looking into possible improvements for larger data samples • Add b-tags to dileptons reduces background<5% • Add alternative b-tagging algorithms K. Tollefson - DPF, Hawaii

  17. Backup Slides K. Tollefson - DPF, Hawaii

  18. Expectations for 1 fb-1 K. Tollefson - DPF, Hawaii

  19. Profile Likelihood • L has 5 parts - Ls: Poisson(sig+bg), L: Gaussian(,) • Nuisance parameters are NsNs, psps, NbNb, pbpb • Scan in f+ and fit for nuisance parameters at each point K. Tollefson - DPF, Hawaii

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