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Measurement of the tt Cross Section in the Dilepton Channel at Fermilab’s Run II

Measurement of the tt Cross Section in the Dilepton Channel at Fermilab’s Run II. David Goldstein. Outline. Why was top expected? History of searches Why study top? CDF II Detector (b tagging, RASNIKs) Cross-section definiton/goals for summer 2003 dilepton analysis

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Measurement of the tt Cross Section in the Dilepton Channel at Fermilab’s Run II

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  1. Measurement of the tt Cross Section in the Dilepton Channel at Fermilab’s Run II David Goldstein

  2. Outline • Why was top expected? • History of searches • Why study top? • CDF II Detector (b tagging, RASNIKs) • Cross-section definiton/goals for summer 2003 dilepton analysis • Physics objects, acceptance increases • Backgrounds • Data events in Run II • Cross-section results • Future directions for dilepton top analyses David Goldstein, UCLA - CERN EP Seminar 27/10/03

  3. The role of top in the SM  f gfa Zaxial f f  • Top quark was expected in the Standard Model (SM) of electroweak interactions as a partner of b quark in SU(2) doublet of weak isospin for the third generation of quarks. • In each generation, complete doublet is required for cancellation of [triangle/axial/chiral] anomaly. • Without cancellation, SM is not renormalizable or unitary. David Goldstein, UCLA - CERN EP Seminar 27/10/03

  4. History • (1964) Cronin & Fitch discover CP violation…3x3 CKM matrix suggested a top quark… • b quark (5 GeV !) discovered in 1977 at Fermilab • (1979) Searches at PETRA e+e- collider (ps up to ~47 GeV) • (1981) SppS hadron collider turns on (√s = 630 GeV): • (1984): • PETRA Mtop > 23.3 GeV • UA1 evidence for top Mtop» 40 GeV ! (based on ~200 nb-1) • (1987) TRISTAN e+e- collider turns on (√s up to ~60 GeV) • (1988) Tevatron hadron collider turns on (√s 1800 GeV) • (1988): • UA1 subsequent results: Mtop > ~60 GeV (~700 nb-1) Backgrounds very important to get right ! • CDF Mtop > 77 GeV • (1989) SLC & LEP turn on (√s at Z pole) David Goldstein, UCLA - CERN EP Seminar 27/10/03

  5. History • (1990): • TRISTAN Mtop > 30.2 GeV • CDF Mtop > 85 GeV • UA2 Mtop > 69 GeV • (1992): • CDF Mtop > 91 GeV • (1993): • SLC/LEP indirect evidence Mtop even higher (130 GeV +) • (1994): • D0 Mtop > 131 GeV, CDF silent… • CDF & D0 evidence for top in 1994 • (1995): Discovery! • Top finally found (175 GeV); mass near electroweak scale, ~40 times heavier than the b quark! 16 years, 5 colliders, 7 collaborations, 103 physicists ! David Goldstein, UCLA - CERN EP Seminar 27/10/03

  6. top decay modes • All hadronic (BR: 36/81 , 44%) • final state: 6 jets (QCD background very high). • Lepton+jets [e,μ] (BR: 24/81, 30%) • final state: 4 jets, 1 lepton, missing transverse energy. • Dilepton [e,μ] (BR: 4/81 , 5%) • final state: 2 jets, 2 leptons, missing transverse energy “Golden mode”. angle ‘s produce missing transverse energy in the detector David Goldstein, UCLA - CERN EP Seminar 27/10/03

  7. Motivation for top studies •  vs. missing energy plot from Run I dilepton analysis intriguing… • Short lifetime and high mass of top (relative to b quark) will allow for precise tests of top CKM values, W polarization studies. David Goldstein, UCLA - CERN EP Seminar 27/10/03

  8. Motivation for top studies CDF/D0 2 fb-1goal! CDF/D0 2 fb-1goal! David Goldstein, UCLA - CERN EP Seminar 27/10/03

  9. Tevatron collider in Run II • The Tevatron is a proton-antiproton collider with 980 GeV/beam • 36 p and p bunches 396 ns between bunch crossing • Increased from 6x6 bunches with 3.5ms in Run I • Increased instantaneous luminosity: • Run II goal 30 x 1031 cm–2 s-1 • Current: 3 - 4.5 x 1031 cm–2 s-1 • (Run I peak inst. lum. ~2 cm-2 s-1) David Goldstein, UCLA - CERN EP Seminar 27/10/03

  10. Tevatron collider in Run II March 2001 March 2002 March 2003 David Goldstein, UCLA - CERN EP Seminar 27/10/03

  11. The upgraded CDF detector • New bigger silicon, new drift chamber. • Upgraded calorimeter, muon coverage. • Upgraded DAQ/trigger, esp. displaced track trigger. David Goldstein, UCLA - CERN EP Seminar 27/10/03

  12. CDF silicon detectors & b tagging Silicon Vertex Tag • Signature of a b decay is a displaced vertex: • Long lifetime of b hadrons (c ~ 450 m)+ boost • B hadrons travel Lxy~3mm before decay with large charged track multiplicity • B-tagging at hadron machines established: • crucial for top discovery in Run I • essential for Run II physics program Achieving optimum resolution requires relative alignment of silicon detectors stable to ~10 microns. Monitor with RASNIK system... David Goldstein, UCLA - CERN EP Seminar 27/10/03

  13. RASNIK locations in situ David Goldstein, UCLA - CERN EP Seminar 27/10/03

  14. RASNIK principle • Illustration of the RASNIK principle: • Portion of the ‘coded mask’ from a system in situ after digitization: • A system with cover removed: 20.0 m David Goldstein, UCLA - CERN EP Seminar 27/10/03

  15. RASNIK data • RASNIK data from a recent month ( ‘SVX’ system). • CDF data taking periods indicated in blue. • RMS of motions during data taking within tolerance for b tagging resolution. • Systems return to previous position after large disturbances. 0 6 12 18 24 hours David Goldstein, UCLA - CERN EP Seminar 27/10/03

  16. Measuring σttbar in dilepton channel • Top already observed, goal for summer 2003 analysis: • Increase acceptance as much as possible. • Add higher angle lepton detection. David Goldstein, UCLA - CERN EP Seminar 27/10/03

  17. Physics objects in the analysis • Two leptons: • ‘Primary’ • Must be isolated (sum of energy in cone around lepton must be less than 10% of lepton energy). • Must have associated track. • ‘Secondary’ • Some allowed to be non-isolated. • Some allowed to not have an associated track. • Never both of the above. • Missing energy (’s) • At least two jets: • b tag not required. David Goldstein, UCLA - CERN EP Seminar 27/10/03

  18. Physics objects in the detector • An electron at CDF… • A jet at CDF… • A muon at CDF… An electron in plug calorimeter David Goldstein, UCLA - CERN EP Seminar 27/10/03

  19. Analysis overview • Data for the dilepton cross-section analysis collected between March 2002 - May 2003 (126 pb-1). • Three primary data sets: • Central electrons (1 main central electron trigger), ~700K events. • Central muons (3 main central muon triggers), ~600K events. • Plug electrons (1 main trigger [plug EM + missing transverse energy]), ~2.6M events. • Analysis increased acceptance over Run I measurement by »30%: • Included CMIO (Central Minimum Ionizing Object) muons • Recovered events previously vetoed by Z mass window cut • Included electrons using Run II upgrade endplug detectors David Goldstein, UCLA - CERN EP Seminar 27/10/03

  20. Acceptance increases (#1,2) • CMIO (Central Minimum Ionizing Object) muons... • Muon detector coverage at CDF not hermetic. Can recover significant acceptance by including CMIO’s which are not fiducial to muon chambers. • Effectively isolated tracks + some (central) calorimeter cuts. • Recovering events under the Z mass window… David Goldstein, UCLA - CERN EP Seminar 27/10/03

  21. Acceptance increases (#2 cont.) • ~25% of e/e or μ/μ dilepton top events have an invariant mass within the Z window, (76 < M < 106) GeV. • Lose ~12% of the overall acceptance by enforcing Z window cut. The problem... Lepton invariant mass distribution from top Monte Carlo. David Goldstein, UCLA - CERN EP Seminar 27/10/03

  22. Acceptance increases (#2 cont.) • Under the Z mass window: • Using cuts on jet significance and on the phi angle between the missing transverse energy in the event and the nearest jet: • Recover 90% of e/e and μ/μ top events. • Reject 80% of Z events. The solution... Z Monte Carlo tt Monte Carlo David Goldstein, UCLA - CERN EP Seminar 27/10/03

  23. Acceptance increases (#3) • Plug electrons… • Developing calorimeter definition of electrons in new endplug detectors buys large gain in acceptance for analysis (~30% over winter 2003 tdl analysis). • Adding tracks to plug electrons: • Tracks necessary for stripping plug-based data set. • Cannot use wire chamber (COT) • Stand-alone silicon tracking difficult. • Solution ! develop calorimeter-seeded silicon tracks. David Goldstein, UCLA - CERN EP Seminar 27/10/03

  24. Acceptance increases (#3 cont.) • Calorimeter based plug electron ID variables developed by hand-scans of events from test beam data, MC samples, Run II data. • Cut values tuned using central/plug Z decays (data) and background samples from jet data. • Calorimeter-only plug electrons give ‘secondary lepton’ for the analysis. David Goldstein, UCLA - CERN EP Seminar 27/10/03

  25. Acceptance increases (#3 cont.) • Calorimeter-seeded tracking: • Two points + a curvature define a unique helix. • Beam spot + precise EM shower coordinates from plug shower maximum detector provide 2 points. • Plug EM calorimeter gives shower energy … curvature. • Define two hypothetical track trajectories corresponding to positive and negative charges. • Look for hits in the silicon detectors which support one or both of these hypotheses. • Choose best fit track if basic quality cuts satisfied. • Calorimeter plug electron cuts + calorimeter-seeded track gives ‘primary lepton’ for the analysis. David Goldstein, UCLA - CERN EP Seminar 27/10/03

  26. Acceptance increases (#3 cont.) • Check that plug definitions are working by measuring W and Z boson cross-sections AS CROSS CHECK in the endplugs: • ¢B(W!e) = (2.43§0.25) nb SM prediction at 1.96 TeV: ¢B(W!e) = (2.72§0.13) nb • ¢B(Z!e+e-) = (259§25) pb SM prediction at 1.96 TeV: ¢B(Z!e+e-) = (252§9) pb Note: Plug/plug Z’s David Goldstein, UCLA - CERN EP Seminar 27/10/03

  27. Acceptance summary • With increases, # reconstructed dilepton decays all MC top events • Of the 0.74%: • By decay channel: • e/e: 24% • e/μ: 53% • μ/μ: 23% • By detector region: • Central/central: 78% • Central/plug: 21% • Plug/plug: 1% • Sensitivity to new physics an important goal! £  factors = 0.74% (~ 5% max.) David Goldstein, UCLA - CERN EP Seminar 27/10/03

  28. Events in Run II data (summary) HT = Σ (jet ET’s + lepton ET’s + missing transverse energy) Require ¸ 2 jets with at least 15 GeV transverse energy Topological cuts on missing tranverse energy Jet significance (/Z) David Goldstein, UCLA - CERN EP Seminar 27/10/03

  29. Systematic errors Source Uncertainty (%) Lepton ID Scale Factors + Trigger Efficiencies 2.0 Jet Corrections 5.6 ISR/FSR 1.6 PDF’s 7.7 MC Generators 3.9 Total 10.6 Statistical error is 30% David Goldstein, UCLA - CERN EP Seminar 27/10/03

  30. Run II cross-section summary NNLL Theoretical prediction @NLO ≈ 6.7 pb David Goldstein, UCLA - CERN EP Seminar 27/10/03

  31. Events in Run II data David Goldstein, UCLA - CERN EP Seminar 27/10/03

  32. Kinematic distributions from Run II Dilepton analysis David Goldstein, UCLA - CERN EP Seminar 27/10/03

  33. Kinematic distributions from Run II Dilepton analysis HT = Σ (jet ET’s + lepton ET’s + missing transverse energy) David Goldstein, UCLA - CERN EP Seminar 27/10/03

  34. Conclusion • Question suggested by Run I  vs. MET plot answered. • Top cross-section measured in the dilepton channel is consistent with Standard Model predictions at NLO. • Dilepton kinematic distributions are not suggestive of new physics so far. • Focus of dilepton channel analyses switching to a priori minimization of uncertainties, detailed analyses of kinematics. David Goldstein, UCLA - CERN EP Seminar 27/10/03

  35. Backup slides: David Goldstein, UCLA - CERN EP Seminar 27/10/03

  36. Backup slides: David Goldstein, UCLA - CERN EP Seminar 27/10/03

  37. Backup slides: • Believe NI leptons not faked by b’s because of ET cut on electron • Not many b’s with electrons > 20 GeV • Et spectrum of electrons from b’s in Wbb plotted at right • This plot is just to give a qualitative sense for this contribution • If this is true, shouldn’t we start to see b’s if we lower the cut? David Goldstein, UCLA - CERN EP Seminar 27/10/03

  38. Backup slides: muons electrons David Goldstein, UCLA - CERN EP Seminar 27/10/03

  39. Acceptance increases (#3 cont.) David Goldstein, UCLA - CERN EP Seminar 27/10/03

  40. Systematic errors (backgrounds) Background Source Uncertainty (%) Z → tt 2-jet efficiency 10 Jet energy scale 32 WW/WZ MC Generator 40 Jet energy scale 17 DY (ee, mm) Method 50 Jet energy scale 32 Fakes Method 21-50 David Goldstein, UCLA - CERN EP Seminar 27/10/03

  41. Backup slides: • Rescale lepton ID efficiencies to match those observed in Z data; Scale Factors applied: • CMUP: 0.94 +/- 0.01 • CMX: 1.00 +/- 0.01 • CMU: 0.97 +/- 0.02 • CMP: 0.96 +/- 0.02 • TCE: 0.98 +/- 0.01 • NI TCE: 0.78 +/- 0.07 • PEM: 0.96 +/- 0.05 • NI MUONS: 0.85 +/- 0.09 • Apply track efficiencies • Decreases overall acceptance by 6.6% David Goldstein, UCLA - CERN EP Seminar 27/10/03

  42. Backup slides: David Goldstein, UCLA - CERN EP Seminar 27/10/03

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