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Measurement of the Single Top Production Cross Section at CDF

This presentation discusses the measurement of the single top production cross section at CDF experiment, including the top quark properties and the theoretical cross section. It also explores the technical motivation and analysis strategies used for the measurement.

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Measurement of the Single Top Production Cross Section at CDF

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  1. Measurement of the Single Top Production Cross Section at CDF On Behalf of the CDF Collaboration Koji Nakamura Univ. of Tsukuba PHENO 2008 @ Wisconsin Madison 29 Apr. 2008

  2. Top Quark at Tevatron • Top quarks are predominantly produced in pairs by the strong interaction • Discovered in 1995 at Tevatron • Studying properties like cross section, mass, charge, Whelicity ~ 6.7 pb ~ 2.9 pb • Electroweak single top production allows • The theoretical cross section is 0.4xsttbar • Evidence was seen by D0 and CDF experiment PHENO 2008 (Dec.2006) (Aug. 2007)

  3. Why Single Top Quark? Production rate is proportional to |Vtb|2 st = (1.98 ± 0.25) |Vtb|2pb ss = (0.88 ± 0.11) |Vtb|2pb Top Polarization study Single top quarks are 100% polarized in SM Can test this with angular distribution of top decay Probe Non Standard Model phenomena Can search for heavy W’ boson or H± Technical Motivation Test of the methodology for Higgs search (the same final state as the WHlνbb signal) PHENO 2008

  4. 2 or 3 high Pt jets (Pt>20 GeV) Single top at CDF Event topology Singletop production with decay into lepton + 2 jets final state One high Pt lepton (Pt>20 GeV) Large Missing Energy (Et>25 GeV) Singletop Signal is hidden under the huge bkg  Multivariate analyses are needed Top pair Dominant process of 4 jets bin counting method is possible PHENO 2008

  5. Analysis strategy Signal Model Background Model CDF Data Set Blind analysis Event Selection Split in sub set of different purity W+2jets W+3jets 1 tag ≥2 tag 1 tag ≥2 tag Multivariate Analysis Combined Analysis • Both s- and t-channel as the signal • Fixed st / ss Ratio to SM • Sensitive to Discovery and |Vtb| Boosted Decision Tree Matrix Element Discriminant Separate Search • Ether s- or t-channel is the signal • Not fixed st / ss Ratio • Sensitive to New Physics Likelihood Function Neural Network PHENO 2008

  6. Boosted Decision Tree Matrix Element Matrix element: Different for each process. extension of a cut based analysis ≥ 20 variables used for training Optimize series of binary cuts Parton distribution functions Transfer functions: jet energy measurement Calculate for each leaf purity as BDT output … Neural Network Discriminant Likelihood Function i=variable index k=sample index PHENO 2008 Use 10-15 variables

  7. Systematic Uncertainties Dominant Systematic uncertainty is the normalization of data based bkg estimation. All rate and shape variation systematic uncertainty are included cross section and significance fit PHENO 2008

  8. Result I Cross section measurement Neural Network Likelihood Function Matrix Element Boosted Decision Tree All results are consistent with SM prediction PHENO 2008

  9. Result II Significance Neural Network Likelihood Function Matrix Element Boosted Decision Tree PHENO 2008

  10. Result III Combination I Super-Discrimintant Combine ME, NN, LF analyses into one by using discriminants as inputs to a neural net. “Neuro- Evolution of Augmenting Topologies” PHENO 2008

  11. Result IV Combination II (cross check) Asymmetric Iterative BLUE • Generate correlated PEs and measure correlations. • Parameterize uncertainties as function of single top s • Iterate to avoid biases. “Best Linear Unbiased Estimator” Original BLUE : L. Lyons NIM A270 110 (1988) How correlated single analyses are : LF-ME 58.9% ME-NN 60.8% LF-NN 74.1% 29 April 2008 PHENO 2008 PHENO 2008 11

  12. Result V CKM matrix element |Vtb| The cross section measurement is also converted directly into a measurement of |Vtb| : Assuming no anomalous coupling Superdiscriminant: |Vtb|=0.88 ± 0.14 (exp.) ± 0.07 (theory) Result is consistent with |Vtb|~1 (|Vtd|,|Vts| << |Vtb|) PHENO 2008

  13. Result VI S- channel optimized search This is a first result of the optimization search • Used double tag to separate s- and t-ch •  use “Loose” second tag to increase statistics • Likelihood base signal background separator • t-ch was treated as a background. Result : Cross section Best Fit 95% CL. Upper limit Exp. Limit : 2.36 pb (bkg. Only) Obs. Limit : 2.77 pb Result is consistent with SM prediction PHENO 2008

  14. Conclusion Precise measurement of cross section and |Vtb| : |Vtb|=0.88 ± 0.14 (exp.) ± 0.07 (theory) Significance extrapolation assuming √∫Ldt dependence. Extrapolation of Expected Error on|Vtb|. PHENO 2008

  15. Thank you! PHENO 2008

  16. Expected candidate events PHENO 2008

  17. Acceptance Gain for Muon PHENO 2008

  18. A Neural Network b-tagging tool PHENO 2008

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