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Measuring the t-tbar Cross-Section in the Dilepton Channel at CDF*

Measuring the t-tbar Cross-Section in the Dilepton Channel at CDF*. J. Incandela for C. Mills Jan. 17, 2008 DOE Site Visit UC Santa Barbara * PhD Thesis. PRD in preparation. Top Dilepton Selection. 1) One well-identified high-E T lepton electron or muon Triggers the event in the data.

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Measuring the t-tbar Cross-Section in the Dilepton Channel at CDF*

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  1. Measuring the t-tbar Cross-Section in the Dilepton Channel at CDF* J. Incandela for C. Mills Jan. 17, 2008 DOE Site Visit UC Santa Barbara * PhD Thesis. PRD in preparation

  2. Top Dilepton Selection • 1) One well-identified high-ET lepton • electron or muon • Triggers the event in the data 3) Missing transverse energy (“MET”) • 2) One “track lepton” = high-pT isolated track • doesn’t distinguish e// • Increased acceptance 4) Two or more jets Note that we do not require a b tag (this was done later by T. Speltzer (Toronto) CDF top cross section measurement

  3. q q q Main Backgrounds Makes 2 Leptons Has MET  l+ q q W W/Z g/ Z q l l- q / q’ W/Z q g • Drell-Yan • Large cross-section • Needs mis-measured ET (false MET), extra jets • W+jets • Large cross-section • Real MET + 1 lepton • Needs extra jets, one misidentified as 2nd lepton • Diboson • Small cross-section • Real MET • Needs extra jets • NB: tt lepton+jets with one jet faking a lepton is also an important background CDF top cross section measurement

  4. Skiing to the Cross-Section B Background: Estimated number of events from sources other than top Nobs Number of candidate events in the data A Acceptance = Number selected / Number generated  L dt Integrated luminosity CDF top cross section measurement

  5. MET Selection • MET based on calorimetry • Drastically reduces backgrounds which do not contain neutrinos • Corrections performed for: • Isolated muon pT • ET significantly less than isolated track pT • Jet energy corrections Threshold 25 GeV CDF top cross section measurement

  6. Selecting Against Background • 1) Z Veto: Require MET > 40 GeV if dilepton invariant mass 76 < M < 106 • ~96% of top events pass • ~50% of Drell-Yan ee/ pass • 2) Opposite-signlepton • ~100% efficient for top (and most backgrounds) • ~ 60% efficient for W+jets with a fake lepton CDF top cross section measurement

  7. Selecting Against Background 3) Delta phi cuts* • Zee/: MET points at mis-measured object • Require that no object points directly at MET ~86% efficient for top ~30%for Zee/ 25o 5o 5o tight lepton track lepton jets* *exempt event if MET > 50 GeV CDF top cross section measurement

  8. Acceptance by the Numbers • Acceptance x Branching Ratio = 0.87±0.06% • Get 0.35% using full lepton selection on both leptons • Systematics • Lepton Identification (1.6%) • Identifying leptons in the messy top event environment. • Parton Distribution Functions (0.5%) • Jet Energy Scale (1.3%) • Affects ≥2 jets requirement • Initial/Final State Radiation (1.7%) • Affects jet counting • Monte Carlo Generator (1.5%) • Total: 3.1 % CDF top cross section measurement

  9. q Z & Diboson Backgrounds • Can’t isolate in data so use MC simulation: • cross-sections well known • Measure acceptance MC, as for signal • Jet multiplicity taken “pure” Z + jets data sample • Invert cross-section formula: N = A Ldt • Systematic uncertainties: • Lepton ID, jet energies and production   +  l q e, g/ Z W q q W q Z e, l l - l  W Z ’ Z g  l+ /K q l’ g l-  g q’ g g q’ g g g CDF top cross section measurement

  10. Z/g*ee/mm : The Basic Problems • Z/g*ee/mm events pass selection if: • One (or more) objects in the event is mis-measured enough to generate large MET • And there are additional jets • But we do not trust the MC to do either 1. or 2. correctly. How do we correct it? • Additional jets from a “pure” Z + jets data sample • For MET, what do we do? CDF top cross section measurement

  11. Z/g*ee/mm : Established Method Data with missing ET but close to Z mass (any number of jets) • Select dilepton sample in the data: • Count events with invariant mass inside the Z resonance with large MET • Subtract (small) contributions from other sources • Select dilepton sample in MC simulation: • Find fractions fi with i jets (correct these to match data) and ratios Ri = Number of events with i jets outside Z peak with MET > 25 over number with i jets inside Z peak • Use ratios Ri from 2. to multiply times data event counts from 1. to obtain estimated DY in data with i jets outside Z peak • Total Systematic 25% • Sample statistics (20%) • MC modeling of MET (13.5%) CDF top cross section measurement

  12. q Background from Fake Leptons • W+jets with a fake lepton • Want to know: probability of jets associated with a W to be identified as the second lepton – i.e. an isolated track • This is a true isolated track, but of hadronic, not leptonic, origin • Basic question:  W q l q / q’ q g N charged tracks in a jet fragmentation/ hadronization parton (q or g) ? - or - CDF top cross section measurement

  13. Signs of Trouble • Fake rates usually derived from jet triggers: • CDF: “jetXX” = trigger with at least one jet with ET > XX GeV • Fake rates derived from different trigger samples don’t match! CDF top cross section measurement

  14. q and g jets • MC: Quark jets more likely to fake a lepton: • Multijets dominated by gluon jets • Leading jet in W/Z/g+jets tends to be quark jet CDF top cross section measurement

  15. q vs. g jets: Evidence from Data CDF top cross section measurement

  16. A Smarter Fake Rate from +jets • . What makes W+jets and +jets different? • . W is massive • Require  with E= 80 GeV g / W q q / q’ g CDF top cross section measurement

  17. Test in Data • 80 GeV +jets sample predicts Z+jets fakes 1 jet events: Predict 100 ± 13 observe 101 2 jet events: Predict 28 ± 2 observe 26 ≥3 jet events: Predict 12 ± 1 observe 13 ≥2 jet events: Predict 40 ± 2 observe 39 CDF top cross section measurement

  18. Opposite-Sign Fakes • Opposite-charge requirement, but fake rate has no sign info • Fake leptons a fluke of fragmentation • Might think 50/50 chance for opposite sign • But, recall the leading diagram • Quark has the opposite charge! • W+jets opposite-sign fraction • Measured in fakes-enhanced subset of zero jet data • Dependence on number of jets from simulation 1 jet: 67% 2 jet: 63% ≥3 jet: 59% q(+2/3) W+ q’(-1/3) g CDF top cross section measurement

  19. Fakes from Top 1) One legitimate  for MET 2) Typically four jets 3) Have ud, or cs quarks from W decay  high fake rate • Top “Lepton + jets” channel: • Fake rate approx. same as for W+jets • Find 23% of fake lepton events are from top • Estimate 67% are opposite-sign • Total systematic on all fakes ~20% • (dominated by largest discrepancy seen in MC tests – 1 jet bin case) CDF top cross section measurement

  20. Signal, Background, and Data • Compare predicted and observed event counts in 1 fb-1 (uncertainties include statistical and systematic components): • Cross-section at Mt = 175 with opposite-sign events with ≥ 2 jets: • Compare 6.7-0.9+0.7 (theoretical prediction for Mt = 175 GeV) CDF top cross section measurement

  21. Summary • Great acceptance e, ,  • Few detector requirements • Measured cross-section in 1.01 fb-1 of CDF data • = 8.3 ± 1.3 (stat.) ± 0.7 (sys.) ± 0.5 (lumi.) pb • Cross-checks all ok CDF top cross section measurement

  22. Additional Material CDF top cross section measurement

  23. Cross-Section vs. Top Mass • Remeasure acceptance • Higher mass  more energetic decay products • Uncertainties are statistical ± systematic • Implied mass between 180-185 GeV/c2 CDF top cross section measurement

  24. A Bit of History • Run II • Goal: high-acceptance analyses • 19 candidates in 200 pb-1, measured 7.0-2.4+2.9 pb (6.7-0.9+0.7 pb predicted for s = 1.96 TeV) • Run I • 9 dilepton candidates in 109 pb-1 • Measured 8.2-3.4+4.4 pb (5.2-0.7+0.4 pb predicted for s = 1.8 TeV) • High, but with huge uncertainty, and odd kinematic features • Speculation: new physics? CDF top cross section measurement

  25. Acceptance for  leptons 85% of decay modes have 1 charged track No distinction between lepton flavors (e, , or ) Don’t require fiducial regions of calorimetry, muon detectors Rejection of jets using isolation SpT of tracks in cone small relative to candidate pT “Track Leptons” electron tau (hadronic) muon jet tracking EM cal hadronic cal muon CDF top cross section measurement

  26. Z/g*ee/mm : Established Method Monte Carlo: Use to get fraction of DY outside Z peak CDF top cross section measurement

  27. Z/*  ee/ Systematic • Statistical uncertainties ~20% • Modeling of extra jets • Data/MC scale factor uncertainty 5.5% • How well does the Monte Carlo model events with false MET? • Look at Z events in data, MC • Require lepton or MET pointing at jet • What fraction of events are above MET cut (25 GeV) in data? (1.7%) • Where would you have to place cut in MC to have the same fraction above the cutoff? (24 GeV) • Remeasure Monte Carlo ratios Ri • Rederive Drell-Yan estimate ≥2 jet bin: • number drops by 13.5% • Total 25% all events WW, ttbar etc MET points at a jet 25 CDF top cross section measurement

  28. Fakes: Established Method From jet trigger data Fake rate definition: Number of isolated tracks Total number of jets Function of jet ET, h CDF top cross section measurement

  29. Fakes: Established Method From jet trigger data How many W (real lepton) + fake lepton +  2 jet events? CDF top cross section measurement

  30. Questioning the Assumptions • Use Monte Carlo to check the basic assumption: • Do jets in pure-QCD events have the same fake rates as jets in W+jets events? • Answer: no. CDF top cross section measurement

  31. Why different fractions? • Parton Distribution Functions (PDFs) • What’s carrying the momentum in the proton? • 1960 GeV available to make particles at the Tevatron • Multijet, g+jets, W+jets at relatively low x (~100 GeV) • Incoming gluons preferred gluons x = momentum fraction fp(x) = probability to find parton p with x up down strange x CDF top cross section measurement

  32. anti-red hadrons hadrons Why different fake rates? • FR (quark jets) >> FR (gluon jets) • Quarks and gluons have to make colorless objects • Cartoon of fragmentation: quark gluon CDF top cross section measurement

  33. Why different fractions? • Gluons dominate at relatively low x in parton distribution functions (PDFs) Leading 22 diagrams (highly schematic) g / W q W/Z or prompt photon production: q / q’ g g g Multijet production: g g CDF top cross section measurement

  34. Test in Simulated Data • Identical procedure to data: 80 GeV +jets predicts W+jets fakes 1 jet events: Predict 5470 ± 150 observe 4480 2 jet events: Predict 1332 ± 200 observe 1050 ≥3 jet events: Predict 304 ± 67 observe 226 ≥2 jet events: Predict 1640 ± 210 observe 1270 CDF top cross section measurement

  35. Fake Leptons from Top • What is the fake rate? • Approx. same as for W+jets • Estimate 67% are opposite-sign? • What fraction of our “W+jets” are from top? • Predict from simulation • Start with top xsec 6.7pb (175 GeV), measure cross-section, iterate until changes are small • Find 23% of fake lepton events are from top • predict 518 ± 45 • observe 424 CDF top cross section measurement

  36. W + fake + jets Systematics • Statistical: “error bars” on fake rate (6%) • Systematic on absolute fake rate (18%) Use the largest discrepancy in MC tests i.e. for 1 jet bin • Opposite-sign (5%) • Statistical uncertainty CDF top cross section measurement

  37. Check: “tight-tight” • Require track lepton also matched to tight lepton • Candidate sample is subset of candidates in default analysis • Background estimates use same methods CDF top cross section measurement

  38. Check: 15/15, 25/25 • Move track lepton pT and jet ET threshold both to 15 or 25 GeV • Scale factors and background estimation techniques unchanged CDF top cross section measurement

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