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

Top Quark at Tevatron Electroweak single top production allows Top quarks are predominantly produced in pairs by the strong interaction Discovered in 1995 at Tevatron Studying properties like cross section, mass, charge, W helicity ~ 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 29 April 2008 PHENO 2008 (Dec.2006) (Aug. 2007)

Why Single Top Quark? Production rate is proportional to |Vtb|2 st = (1.98 ± 0.25) |Vtb|2 pb ss = (0.88 ± 0.11) |Vtb|2 pb 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) 29 April 2008 PHENO 2008

Single top at CDF Event topology 2 or 3 high Pt jets (Pt>20 GeV) 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 29 April 2008 PHENO 2008

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 29 April 2008 PHENO 2008

Matrix Element Neural Network Likelihood Function Boosted Decision Tree 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 29 April 2008 PHENO 2008 Use 10-15 variables

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 29 April 2008 PHENO 2008

Cross section measurement Result I Neural Network Likelihood Function Matrix Element Boosted Decision Tree All results are consistent with SM prediction 29 April 2008 PHENO 2008

Significance Result II Neural Network Likelihood Function Matrix Element Boosted Decision Tree 29 April 2008 PHENO 2008

Combination I Result III Super-Discrimintant Combine ME, NN, LF analyses into one by using discriminants as inputs to a neural net. “Neuro- Evolution of Augmenting Topologies” 29 April 2008 PHENO 2008

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

CKM matrix element |Vtb| Result V 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|) 29 April 2008 PHENO 2008

S- channel optimized search Result VI 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 29 April 2008 PHENO 2008

Conclusion √∫Ldt dependence. 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|. 29 April 2008 PHENO 2008

Thank you! 29 April 2008 PHENO 2008

Expected candidate events 29 April 2008 PHENO 2008

Acceptance Gain for Muon 29 April 2008 PHENO 2008

A Neural Network b-tagging tool 29 April 2008 PHENO 2008