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Top Production Cross Section from CDF

Top Production Cross Section from CDF. Anyes Taffard. University of Illinois. On behalf of CDF Collaboration. 32nd International Conference on High Energy Physics. Why Study Top Quark Physics ?. Experimentally, still know very little about the top quark

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Top Production Cross Section from CDF

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  1. Top Production Cross Section from CDF Anyes Taffard University of Illinois On behalf of CDF Collaboration 32nd International Conference on High Energy Physics

  2. Why Study Top Quark Physics ? • Experimentally, still know very little about the top quark • Run I: ~110 pb-1, ~100 top candidates • Overall consistency with SM prediction, statistic limited • Only know fermion with mass near electroweak scale • Theoretical models proposed to solve the problems of the SM often have top playing a leading role: • SUSY: Large top mass causes EWSB • In many dynamical symmetry breaking models, top interactions are modified (e.g technicolor) • Probe for physics beyond the SM • Non SM production (Xtt, Xll+jets+ET) • Non-SM decay: Heavy enough to decay to exotic particles (tXb) • On-sheel charged Higgs, SUSY • Signal of today, background of tomorrow

  3. Tevatron & CDF • Proton-antiproton collisions • s=1.96 TeV • Main injector • 150 GeV proton storage ring • New DAQ • New Track Trigger • New Silicon ||<2 • Improved b-tagging • New Drift Chamber • New Plug Calorimeter • Increase acceptance • Upgrade Muon Detectors

  4. Tevatron & CDF: Luminosity 2003 2002 2004 Initial Luminosity (E30) CDF acquired Luminosity (pb-1) Store Number days Record Peak Luminosity: July 16, 2004: 10.3 x 1031 cm-2sec-1 With mixed pbar (recycler & accumulator) Acquired luminosity in 2004 already surpassed 2003 total CDF ~450 pb–1 total on tape In this talk:160 pb-1 <Ldt <200 pb-1

  5. Pair Production 85% qq, 15 % gg (fractions reversed @ LHC) Central, spherical events Large transverse energy Cross section increases 30% with Tevatron s increase to 1.96 TeV Single top production is a factor of 2 smaller BR(tWb) 100 % Both W’s decay via Wl (l = e or ; 5%) final state l l bb : dilepton One W decays via Wl (l = e or ; 30%) final state l qq bb : lepton+jets Both W decays via Wqq (44%) final state: qq qq bb: all hadronic Top Quark Production & Decay Bonciani et al., Nucl. Phys. B529, 424 (1998) Kidonakis and Vogt, Phys. Rev. D68, 114014

  6. Measuring tt Cross Section Improve S:B with b-tagging Background estimate: most challenging part Efficiency & Acceptance: Measured from MC (Pythia) & Data • Starting point for all top physics • Events triggered on one high momentum lepton or multi-jets • Optimized event selection for top physics and new physics • Define top sub-samples by counting lepton and jets  2 jets in dilepton channel  3 jets in l+jets channel  6 jets in all hadronic channel

  7. Dilepton Channel Small sample, but highest S/B Backgrounds: Z/g*l+l- WW, WZ, ZZ W+jets (fake leptons) Background can be further reduced with an HTcut. Ht: Scalar summed ET of jets, leptons, and missing ET

  8. Counting Experiments Higher purity, lower statistical significance Lower Purity, higher acceptance (~20% from t) Standard Run I method(ee,mm,em) Looser selection: e/m + track 13 candidates: 1 ee, 3 mm, 9 em 1st Run II paper Combined Result: hep-ex/0404036 • With higher statistics in Run II observe good agreement with SM

  9. Inclusive Dilepton Analysis No cuts other than two identified leptons If same flavor, Z e+e-/m+m- dominates: require significant Etmiss Advantages: increase acceptance Goal: search for new physics Fit data to tt,WW, Z t+t- contribution in 2D (Etmiss, Njet) plane CDF RunII Preliminary Top Cross Section WW Cross Section J.M.Campbell and R.K.Ellis, Phys.Rev.D60 113006 (1999)

  10. Larger sample due to BR, but less pure Improve S:B by requiring b-tagging 2nd vertex semileptonic decay L+jets Channel  3 Jets Candidate Event Display m

  11. Using Vertex Tagging • B decay signature: displaced vertex • Long life time c ~ 450 m • Travels Lxy~3mm before decay • Top event tagging efficiency: 52% • False tag rate per QCD jets: 0.5% @least 1 b-tag & HT>200 GeV

  12. Using Double b-Tag Double b-tag event essential for top mass measurement: reduces combinatorics Improve b-tagger: Increase per jet tagginge: 10.8 % 12.0% False tag rate: x 3.4 Significance up by 18%: tt expectation increases from 8.7  13.0 3x more background Loose SecVtx Standard SecVtx

  13. Using Soft Lepton Tag Tagging B may decay semileptonically Leptons id challenges: softer spectrum than leptons from W/Z non-isolated(cannot use calorimetry information) Id low pT muon Background dominated by fake tag Punch though, decay in flight Top event tagging efficiency: 15% False tag rate (QCD jets): 3%

  14. Using Kinematics Fits • Determined signal fraction using kinematics shape Fit leading jet ET, require @ least 1 b-tag Fit NN output (7 kin. var.)

  15. All Hadronic Channel Final State: 4 jets from W, 2 b-jets (data sample: multi-jets trigger) Large statistics, but huge QCD background: S:B1:2500 Increased S:B by requiring: ET>320GeV Topological cuts (aplanarity, centrality) Event selection efficiency 6.2% S:B  1:24 @ least 1 b-tag jet: S:B  1:4 Measure Xs with  6 jets &  1 b-tag jet

  16. Single Top s-channel production (W*) t-channel production (Wg fusion) s= 1.98 pb s= 0.88 pb B.W. Harris et al. • Probe EW coupling, direct determination of Vtb • Sensitive to new physics • t-channel: anomalous couplings, FCNC • s-channel: new charged gauge boson • Strategy: • Isolate W+exactly 2 jets & 1 b-tag jet • Non-top 89%: W+jets (62%), false tags (25%), QCD (10%) • Top: 11% • Likelihood Fit to Ht(combined):to discover • Likelihood Fit to Q*h(t-channel): to see new physics • Q of lepton, h of light quark jet

  17. Single Top cont. Entries 42 CDF Run II Limits at 95% C.L.: Combined: 17.8 pb (β95=6.2/4.8) t-channel: 10.1 pb (β95=5.1/5.6) s-channel: 13.6 pb (β95=15.4/13.7) (Format: β95=observed/expected)

  18. Summary • x2 Run I dataset • Observed consistent with SM prediction @ mtop=175 GeV/c2 • 1st paper out, more coming soon. • Near future: already x2 more data on tape

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