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Top physics results

Explore the latest findings in top physics including large cross-section, quark decays, Higgs mass uncertainties, and more. Presented at the UK HEP forum in Cosener's on September 7, 2011.

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Top physics results

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  1. Top physics results Greg Heath University of Bristol

  2. Motivation • Large cross-section • About 20× Tevatron at 7 TeV • Opportunity to study “bare” quark • Decays before hadronisation • Handles on New Physics • Higgs mass uncertainties • New phenomena coupling to top • Top as background to discovery channels • Exercise full detector and reconstruction in complex multi-parton events • Leptons, jets, MET, b-tagging, tau-id UK HEP forum, Cosener's, 7th September 2011

  3. Theory predictions Top production and decay • Top pair production from initial gg pair dominates • Top pair decay modes • All hadronic (6 jets) – large BR, large QCD background • Lepton+jets (4 jets) – smaller BR (per lepton), smaller background • Dileptonic (2 jets) – smallest BR, cleanest • Also consider single top production UK HEP forum, Cosener's, 7th September 2011

  4. Top pair cross-section UK HEP forum, Cosener's, 7th September 2011

  5. 2011 dilepton analyses • Robust cut and count analyses • B-tagging improves efficiency • Systematic errors dominate ATLAS CMS UK HEP forum, Cosener's, 7th September 2011

  6. Lepton+jets analyses with b-tags • b-tagging probability as input to likelihood fits to signal+background • Combined fit to jet multiplicity bins CMS lepton+jets with b-tags ATLAS single lepton with b-tags m+jets e+jets m+jets e+jets UK HEP forum, Cosener's, 7th September 2011

  7. Lepton+jets untagged (ATLAS) 2011 data (0.7 fb-1) Likelihood fit to event kinematics UK HEP forum, Cosener's, 7th September 2011

  8. All-hadronic measurement (CMS) • Kinematic fit to events with b-tagging • Fit mass distribution to estimate t-tbar contribution UK HEP forum, Cosener's, 7th September 2011

  9. CMS – cut-based selection including b-tagging ATLAS – m+jets background minimised using BDT Dilepton measurements with ts UK HEP forum, Cosener's, 7th September 2011

  10. ATLAS CMS Cross-section summary Individual combinations (l+jets and dilepton) UK HEP forum, Cosener's, 7th September 2011

  11. Other cross-section measurements • Single top • t-channel • s-channel and tW • Resonance searches • Charge asymmetry UK HEP forum, Cosener's, 7th September 2011

  12. Single top production • Three EW diagrams contribute to production of single top quarks at hadron colliders • t-channel production has the largest predicted cross-section • Main backgrounds t-tbar and W+jets UK HEP forum, Cosener's, 7th September 2011

  13. t-channel cross-section • t-channel (dominant) • Central b-tagged jet • Forward light jet • Leptonic decay of W • Isolated lepton • Large MT(ln) • Variety of approaches • Backgrounds are large 2 jets 3 jets Boosted decision tree (BDT) Cut-based UK HEP forum, Cosener's, 7th September 2011

  14. t-channel cross-section ATLAS 2011 cut-based: CMS 2010 combined: (dominated by BDT) UK HEP forum, Cosener's, 7th September 2011

  15. 1.2s 95% CL Wt and s-channel single top searches ATLAS UK HEP forum, Cosener's, 7th September 2011

  16. ATLAS dilepton Searches for t-tbar resonance ATLAS l+jets Reject extra jets from radiation CMS l+jets Kinematic fit and b-tagging UK HEP forum, Cosener's, 7th September 2011

  17. Charge asymmetry • Small effect predicted in SM • Top quark h distribution narrower than antiquark • But dominant gg production is symmetric • Measured quantity: • Theory predicts small positive asymmetry • Lepton+jets selection • Fit to identify top and anti-top • Unfold measured asymmetry UK HEP forum, Cosener's, 7th September 2011

  18. Top properties UK HEP forum, Cosener's, 7th September 2011

  19. mt in lepton+jets (ATLAS 2011) • Template method • Pick the “best” jet combination hypothesis per event • Kinematic fit with mW constraint • Simultaneous fit to mt and Jet Scale Factor UK HEP forum, Cosener's, 7th September 2011

  20. mt in lepton+jets (CMS 2010) • Ideogram method • Kinematic fit and per-event likelihood vs mt • Uses all jet combination hypotheses • JES as systematic correction • Additional CMS measurement in dilepton events: UK HEP forum, Cosener's, 7th September 2011

  21. Measurement of pole mass • Direct measurements yield MC mass parameter mt • Theoretical cross-section calculations depend on the pole mass • Mass uncertainty 4× smaller than cross-section uncertainty • Pole mass looks smaller than MC mass ATLAS UK HEP forum, Cosener's, 7th September 2011

  22. t-tbar mass difference CMS • Ideogram method modified • Lepton+jets • Use lepton to distinguish t and tbar • Fit mass of hadronic top only • 2011 data (1.09 fb−1) UK HEP forum, Cosener's, 7th September 2011

  23. Parton level Data vs MC Spin correlation ATLAS • Top quark decays before hadronisation • Spin correlations at production propagated to decay products • Charged lepton from W decay carries full spin information • Measure distribution of Df between leptons in dilepton events • Fit to a mixture of SM and uncorrelated templates • Results consistent with SM expectations • SM fraction FSM=1.06±0.21 UK HEP forum, Cosener's, 7th September 2011

  24. W helicity measurement ATLAS • Test of V−A structure of top quark decay • Angular distribution of lepton and b quark in W rest frame • Fit to LH, RH and longitudinally polarised W • Results consistent with SM expectations • Longitudinal fraction F0=0.75±0.08 • Improved limits on anomalous couplings UK HEP forum, Cosener's, 7th September 2011

  25. Summary and outlook • LHC is a top factory • Initial measurements in l+jets and dilepton channels now systematics dominated • Cross-section precision (7%) approaching that of theory prediction • Single top production confirmed • Top quark properties consistent with SM (so far) • Top is an active field in searches for New Physics • Many studies just beginning UK HEP forum, Cosener's, 7th September 2011

  26. List of sources UK HEP forum, Cosener's, 7th September 2011

  27. Top pair cross section measurements • L+jets: • ATLAS 2010 analysis: 3, 4, >4-jet subsamples (ATLAS-CONF-2011-035) • CMS 2010 analysis similar (arXiv:1108.3773) • B-tagging gives best results • Untagged analysis on 2011 data (ATLAS-CONF-2011-121) • Dilepton: • ATLAS 2011 0.70 fb-1 (ATLAS-CONF-2011-100) • Simple cut-based, no b-tagging • CMS 2011 1.14 fb-1 (CMS PAS TOP-11-005) • Combinations • ATLAS combine the above two results (ATLAS-CONF-2011-108) • CMS combine the two 2010 measurements (arXiv:1108.3773) • All-hadronic and tau dilepton • CMS all-hadronic measurement on 2011 data (CMS PAS TOP-11-007) • ATLAS all-hadronic search on 2010 data (ATLAS-CONF-2011-066) • ATLAS mu+tau measurement (ATLAS-CONF-2011-119) • CMS mu+tau measurement (CMS PAS TOP-11-006) UK HEP forum, Cosener's, 7th September 2011

  28. Production measurements; single top • Spin correlation (ATLAS-CONF-2011-117) • Charge asymmetry • (ATLAS-CONF-2011-106) • (CMS PAS TOP-11-014) • Single top • t-channel measurement (ATLAS-CONF-2011-101) • t-channel measurement (arXiV:1106.3052) • s-channel search (ATLAS-CONF-2011-118) • Search for Wt (ATLAS-CONF-2011-104) UK HEP forum, Cosener's, 7th September 2011

  29. Top mass and properties, exotica • Top mass • Template method (ATLAS-CONF-2011-120) • Mass from cross-section (ATLAS-CONF-2011-054) • Ideogram method (CMS PAS TOP-10-009) • Mass in dileptons (included in arXiV:1105.5661) • Mass difference (CMS PAS TOP-11-019) • W polarisation (ATLAS-CONF-2011-122) • Resonance search • Dilepton channel (ATLAS-CONF-2011-123) • Lepton+jets channel (ATLAS-CONF-2011-087) • CMS search (CMS PAS TOP-10-007) • Exotic searches • Search for FCNC (ATLAS-CONF-2011-061) • Search for anomalous MET (ATLAS-CONF-2011-036) UK HEP forum, Cosener's, 7th September 2011

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