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CDF Measurements of Top Quark Cross Section and Mass

CDF Measurements of Top Quark Cross Section and Mass. Jason Nielsen (LBNL) for the CDF Collaboration. Why Study Top Quark Physics?. Fits into third generation - CKM. Large mass interesting in itself. Just lighter than tungsten nucleus. CDF/D0 2 fb -1 goal. Mass near EWSB scale.

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CDF Measurements of Top Quark Cross Section and Mass

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  1. CDF Measurements of Top QuarkCross Section and Mass Jason Nielsen (LBNL) for the CDF Collaboration

  2. Why Study Top Quark Physics? Fits into third generation - CKM Large mass interesting in itself • Just lighter than tungsten nucleus CDF/D0 2 fb-1goal Mass near EWSB scale • Central role in these theories • Corrections to MW mt2, ln(MH) Heavy enough to decay to exotics • On-shell charged Higgs, SUSY Lake Louise Winter Institute, Feb. 2004

  3. Top Quark Production and Decay s = 5.8 - 7.4 pb (Cacciari et al.) Pair production: Central, spherical events Large transverse energy Cross section increases ≈30% with Tevatron √s increase to 1.96 TeV (Single top production is factor of 2 smaller) Top event signatures (from W bosons) Expect 2 b jets from top pair production Lake Louise Winter Institute, Feb. 2004

  4. Tevatron Run 2 at Fermilab Tevatron successes in early 2004: • Record luminosity 5.9E31 • 3pb-1 integrated in a single store • First store w/ antiprotons from Recycler New CDF results use 200pb-1 collected through Oct. 2003 (cf. 110 pb-1 from Run 1) Lake Louise Winter Institute, Feb. 2004

  5. CDF II Detector at the Tevatron • Continuing work to incorporate upgraded detectors in data analysis • Accurate detector simulation vital to precision physics measurements Lake Louise Winter Institute, Feb. 2004

  6. Top Data Samples Define samples counting leptons and jets Establish component contributions to samples Optimize event selections for top physics and new physics In both cases, the sample composition is important Develop tools to increase purity Cross section results validate top-enriched samples • can also point toward new physics Lake Louise Winter Institute, Feb. 2004

  7. Top Dilepton Cross Section Tight e/m selection complemented by e/m + tracks selection Lepton + track sample has looser ID requirements for second lepton Sensitive also to t lepton final states 2 lepton + 2 jets sample is small but very clean for top signal Lake Louise Winter Institute, Feb. 2004

  8. Tagging Tools: Vertexing and Soft Muons B hadrons in top signal events are long-lived and massive may decay semileptonically Identify low-pt muon from decay Vertex of displaced tracks 55% 0.5% Top Event Tag Efficiency False Tag Rate (QCD jets) 15% 3.6% Lake Louise Winter Institute, Feb. 2004

  9. Cross Section Results using Tagging Counting experiments with vertex tag and soft muon tag in 3,4-jet bins • Estimate backgrounds in the lepton + jets sample from first principles: • Using data as much as possible (non-W QCD, fake tags) • Some MC calculations for diboson and W + heavy flavor backgrounds Lake Louise Winter Institute, Feb. 2004

  10. Kinematic Fits to Lepton + Jets Sample Provides another way of estimating the background contribution Sample before tagging Vertex-tagged sample Lake Louise Winter Institute, Feb. 2004

  11. CDF Top Cross Section Summary 6.9+1.6(stat.) ± 0.9 (syst.) -1.8 4.1+4.0(stat.) ± 2.2 (syst.) -2.8 4.5+1.4(stat.) ± 0.8 (syst.) -1.3 6.9+2.7(stat.) ± 1.3 (syst.) -2.4 108 pb-1 126 pb-1 108 pb-1 4.7 ± 1.6(stat.) ± 1.8 (syst.) 195 pb-1 202 pb-1 7.6 ± 3.4(stat.) ± 1.5 (syst.) 126 pb-1 Lake Louise Winter Institute, Feb. 2004

  12. Comparison with Theory Lake Louise Winter Institute, Feb. 2004

  13. Search for Single Top Quark Production q’ l q q l t t W n W n b b q’ b g b b Best hope for constraining Vtb at the Tevatron 1.98±0.24 pb 0.88±0.11 pb Final state is W + exactly 2 jets • Tag one b jet Search strategy employs • Likelihood fit to HT (combined search) • Likelihood fit to b Q*h (t-channel only) • Q of lepton, h of light quark jet Lake Louise Winter Institute, Feb. 2004

  14. Single Top Search Results Sample composition Cross section limits at 95% confidence level: • Combined search: s < 13.7 pb (a priori sensitivity 14.1 pb) • T-channel search: s < 8.5 pb (a priori sensitivity 11.3 pb) Lake Louise Winter Institute, Feb. 2004

  15. Top Mass Measurement Challenges Many combinations of leptons and jets: which one is correct? • Choose assignment kinematically most consistent with top • Use all combinations, but weight them Link observables to parton-level energies • Detector - Generation -Simulation • Largest uncertainties come from this difficult relation Final likelihood fit methods • Derive mass templates from top MC and fit data to most likely template • Add event kinematic information, possibly including matrix element Lake Louise Winter Institute, Feb. 2004

  16. Top Mass Results: Dilepton Reconstruction 6 dilepton candidates in 125.8 pb-1 Kinematic fit is underconstrained For each lepton-b pair assignment • Calculate best raw mass, from most probable combination Fit to signal and background templates Likelihood fit result: mtop = 175 ± 17(stat.) ± 8(syst.) GeV/c2 Lake Louise Winter Institute, Feb. 2004

  17. Top Mass Results: Vertex-Tagged Events CDF Run 2 Preliminary (108pb-1) Likelihood fit result: mtop = 177.5 +12.7(stat.) ±7.1(syst.) GeV/c2 -9.4 22 vertex-tagged events from lepton+4 jet sample Reconstruct mt via kinematic fit Choose single combination with best 2 from over-constrained fit Compare to templates from • Top signal MC • Backgrounds (fixed relative) Lake Louise Winter Institute, Feb. 2004

  18. Summary • Quality top physics results arriving now • Early CDF passes on Run 2 data complete • No unexpected physics seen so far… • Uncertainties will decrease; no surprises here • Tevatron success - more data - precise results! • The near future is rich with opportunity. • Improvements in current measurements • Plenty of new measurements and searches • W helicity, top branching ratios, top resonances, all-hadronic channels Lake Louise Winter Institute, Feb. 2004

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