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Measuring the top production cross section using dilepton events

Measuring the top production cross section using dilepton events. Peter Wittich. Why study the top quark? (I). CDF/DØ 2 fb -1 goal. New particle, barely characterized CDF & D Ø discovery in 1995 Top is extremely heavy (m top ≈178 GeV) : Special relation to missing Higgs boson

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Measuring the top production cross section using dilepton events

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  1. Measuring the top production cross section using dilepton events Peter Wittich Fermilab - Peter Wittich

  2. Why study the top quark? (I) CDF/DØ 2 fb-1 goal • New particle, barely characterized • CDF & DØ discovery in 1995 • Top is extremely heavy (mtop≈178 GeV): • Special relation to missing Higgs boson • Strategy: look for discrepancies with Standard Model “Yukawa scale” Fermilab - Peter Wittich

  3. Why study the top quark? (II) • Open Questions: • Is top production described by QCD? Resonant production? • Is BR(t→Wb)≈100%? Non-SM decays? • Is what we call top really top or top plus X, X possibly exotic? • Today’s signal is tomorrow’s background • Understanding of sample of high momentum leptons + missing energy is crucial for exotic searches, e.g., SUSY CDF: Phys. Rev. D63, 091101 (2001) Fermilab - Peter Wittich

  4. _ Fermilab’s Tevatron pp Collider • Tevatron is world’s highest energy collider: √s = 1.96 TeV • Run 2 data-taking started in 2001 • CDF and DØ upgraded • Ultimate goal: >40x Run 1 • Currently: >350/pb • This analysis: 200/pb • Tev is at energy frontier until LHC turn-on (~2007) • Tev is the only lab making top until then! Fermilab - Peter Wittich

  5. Top production & decay @ Tevatron 85% qqbar (NB: qq, gg fractions reversed at LHC) 15% • In p-pbar collisions, top quarks are produced mainly in pairs • At √s=1.96 TeV, qqbar production is dominant process (~85%) • Due to large mtop, no toponium states • Standard-model top decays mainly via t  W b • W daughters label decay mode gg Fermilab - Peter Wittich

  6. Why dilepton events? • Dilepton channel is • Clean – small SM backgrounds • Different experimental requirements from higher-stats channel (l+jets) • Don’t have to identify b quarks • Different backgrounds • Part of a exp. program • SM top: dil, l+jet, all-hadronic all agree w/ one-another • Run I dilepton kinematics caused some excitement • Reminder: CDF (109/pb) • 7 em events out of 9 total • Small event sample intriguing – check with larger sample (×2 larger) Fermilab - Peter Wittich

  7. New physics hints in Run 1 dileptons? Last time we looked, we saw something intriguing here…. Standard Model top or top + something else? Total event energy Kinematics in Dilepton Events Fermilab - Peter Wittich

  8. Supersymmetry – one possibility • Based on fundamental symmetries • Many string theories are supersymmetric • Solves some technical problems of Standard Model • How: double particle spectrum! • Worked before: postulate positron for quantum mechanics • Introduce “super-partners” with different spin • Makes theory self-consistent • Also provides dark matter candidate • But: where are they? • M(positron)=M(electron) • But not so for ~e • SUSY is broken! • Should be visible in near future… Dark Matter Candidate Fermilab - Peter Wittich

  9. Event topology • Two high-momentum, opposite-sign leptons • (e,µ, some t) from W decay • 2 b jets • Heavy flavor ID not used for this analysis • Large missing energy (ETmiss) • Two escaping neutrinos Fermilab - Peter Wittich

  10. CDF detector General, multi-purpose detector • Focus on charged-particle tracking Features: • Silicon tracker • large radius tracking wire chamber (COT) • Magnet (1.4T) • Calorimeter • (|η| < 3.6) • muon chambers • (|η| < 1) Fermilab - Peter Wittich

  11. Measurement strategy • For this measurement we need to • Collect lots of data (Ldt) • Select signal events (Nobs) • Understand our signal acceptance (A) • Understand corrections to this acceptance (ε) • Estimate our backgrounds (Nbgnd) • Consider control region (njet < 2) to test background estimates • We pursue two independent and complementary analysis approaches Fermilab - Peter Wittich

  12. Collecting the data Trigger • Events are triggered on one high-momentum lepton (e or m) • We use data collected up to 9/2003, corresponding to 197 pb-1 • (cf ~108 pb-1 Run I) • Additionally • √s 1.8 TeV→ 1.96 TeV • Electron acceptance • |η| < 1 → |η| < 2  expect more than twice Run I yield Currently updating to 9/2004 Fermilab - Peter Wittich

  13. Aside: Triggering at hadron colliders Like drinking out of a fire hydrant… • Interactions occur at ≥ 1 MHz (every 396 ns) • Have to sort out ≈ 100 Hz to save • This is the job of the trigger system • Combination of custom electronics and computer farms • Even top physics profits from a capable trigger… • CDF’s inclusive trigger a big benefit This analysis Fermilab - Peter Wittich

  14. Event Selection: Two independent analyses • Each analysis is seeded by a single, isolated high-momentum lepton (e or m) • Electron is track plus matching calorimeter cluster consistent with electron test beam • Muon is track plus matching stub in muon chambers • Split occurs when we look for the second lepton “LTRK” analysis • Second lepton is just track isolated in drift chamber (“tl”) • Increase acceptance at expense of purity • Get ~hadronic t & holes in lepton ID coverage • Lower purity, higher statistical significance “DIL” analysis • Two well-identified leptons • Second lepton uses traditional lepton ID in calorimeter, muon chambers • Possibly looser requirements • Higher purity, lower statistical significance Two independent, complementary approaches Fermilab - Peter Wittich

  15. Event selection details (I) • Identify leading lepton • Central muon (|η|<1) or central or forward electron (|η|<2) • ET,pT>20 GeV, isolated in the calorimeter • Re-cluster jets, recalculate missing energy (ETmiss ) with respect to lepton • Luminous region is extended along direction of beam • Defines event vertex in along luminous axis (Z axis) • Look for second lepton • DIL, LTRK as described previously Fermilab - Peter Wittich

  16. Event selection details (II) • Require ETmiss>25 GeV • Reject events with ETmiss co-linear with jets or lepton • LTRK: Reject events with ETmiss parallel or anti-parallel to isolated track • If lepton pair mass is in Z boson mass region, apply additional rejection • Require ≥2 jets • Require leptons to have opposite charge • DIL: • Increase purity by requiring HT≡ (scalar sum of event energy) > 200 GeV Fermilab - Peter Wittich

  17. Signal: Top acceptance LTRK DIL • Determine from PYTHIA Monte Carlo (mT=175 GeV) • Apply trigger efficiencies, lepton ID Monte Carlo correction factors, luminosity weights for different detector categories • DIL: (0.62 ± 0.09)% • LTRK: (0.88 ± 0.14)% [ This acceptance includes the BR(Wlν)=10.8% ] Fermilab - Peter Wittich

  18. Estimate efficiencies from data • Estimate differences between simulation and data for Lepton ID using Z boson data • Select Z boson candidates with one tight lepton leg and one probe leg • Measure efficiencies of probe leg, compare to Monte Carlo efficiencies • LTRK: Estimate “track lepton” (tl) selection efficiencies: • Use W boson sample selected w/o tracking requirement • Compare track efficiency with efficiency for W track in top Monte Carlo simulation Fermilab - Peter Wittich

  19. Backgrounds Categorize backgrounds as instrumental or physics • Instrumental backgrounds • False ETmiss or leptons • Drell-Yan Z →ee, µµ • Mismeasurement gives false ETmiss • W+jets • Jet is mis-id’d as lepton • Physics backgrounds: • Real leptons, ETmiss • Diboson (WW, WZ, ZZ) • Drell Yan Z → DIL: equal LTRK dominant Fermilab - Peter Wittich

  20. Instrumental: Drell-Yan background (I) q e- Z/g* _ e+ q • For optimal ETmissresolution: • Correct jets for uniform calorimeter behavior • Correct ETmiss with these jets • LTRK: for “tl”, correct ETmiss if tl is min ionizing • μ’s • Undermeasured e’s • Large cross section but no intrinsic ETmiss • False ETmiss • Detector coverage isn’t 4π • Reconstruction isn’t perfect • Tails of ETmiss resolution critical • Simulation doesn’t accurately model this Fermilab - Peter Wittich

  21. Drell Yan background (II) Data-based scale ETmiss>25 • DIL and LTRK use special cuts to suppress DY background • Require min δΦ(j or l, ETMiss) • Additionally in “Z window” (76<mll<106 GeV) • DIL: “Jet significance” • LTRK: Boost ETMiss requirement: >40 GeV • Estimate residual contamination • Overall normalization: Z-like data set: Two leptons, high ETMiss, in Z window • Remove signal contribution • Use simulation to estimate background outside of Z window • Low data counts dominant uncertainty “Jet significance” Fermilab - Peter Wittich

  22. Instrumental: fake lepton background Fakes from W+1p Illustrative fake rate example for LTRK One or both leptons are not real • LTRK • Single pions from jets • DIL: • Jet that passes electron requirements • π punch-through to muon chambers • Measure probability from jet sample • Sample with triggered on one jet with more than 50 GeV of energy (a.k.a. ‘Jet50’) • Apply to our data sample Fake rate as fcn(ET) Fake rate as fcn(h) Fermilab - Peter Wittich

  23. Instrumental: fake lepton background • Cross-check technique on different samples • Check scalar prediction and shapes Jet20 # observed (predicted) fakes Jet70 ET of observed (predicted) fake tracks in green (black) Scalar prediction of rates Fermilab - Peter Wittich

  24. Physics backgrounds • Diboson: • WW, WZ, ZZ • Drell Yan Z/g*→tt • In both cases: • Real missing energy • Jets from decays or initial/final state radiation • Estimates derived from Monte Carlo calculations • ALPGEN, PYTHIA, HERWIG • Cross sections normalized to theoretical calculations • Correct for underestimation of extra jets in MC • Determine jet bin reweighting factors for Z →tt from Z →ee, mm data • Reweight WW similarly Fermilab - Peter Wittich

  25. Signal acceptance systematics (I) Fermilab - Peter Wittich

  26. Signal acceptance systematics (II) Fermilab - Peter Wittich

  27. Systematic Uncertainty on Background Estimate • Total syst uncertainty due to background is ±0.6 pb for DIL, ±1.0 pb for LTRK (NB: these are applied to background, not final x-section) Fermilab - Peter Wittich

  28. Signal and background vs data DIL LTRK Good agreement in background region s(tt) signal region Fermilab - Peter Wittich

  29. Cross section results • Both measurements consistent with SM calc • Error is statistics-dominated • Combining results makes for better measurement At NLO @ √s=1.96 TeV for mtop = 175 GeV: hep-ph/0303085 (Mangano et al) DIL: LTRK: (Assume BR(W→lν)=10.8%) Fermilab - Peter Wittich

  30. Double-tagged top dilepton candidate  jet  jet ET • An event with 2 jets and 2 muons • Both jets show displaced secondary vertices from the interaction point: b jet candidates Fermilab - Peter Wittich

  31. Cross-checks: W, Z cross sections • Measure W, Z cross sections • Use analysis tools, selections and samples • Validates acceptance, efficiencies, luminosity estimate • Good agreement with other measurements in all cases LTRK, DIL Gauge bosons as “standard candle” E/p in W boson decays Fermilab - Peter Wittich

  32. Cross-checks: b content • Top decays should have two b quarks per event • We don’t use this info here • Consider b quark content as check • Look for jets consistent with long-lived particles detected in Silicon Vertex tracker  Number of events with detected b-like quark jets consistent w/expectation • DIL: 7, LTRK: 10 • 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 Top Event Tag Efficiency 55% False Tag Rate (QCD jets) 0.5% Fermilab - Peter Wittich

  33. Cross-checks: Tighten 2nd lepton ID Signal background • LTRK sample: apply lepton ID on track lepton • Very few fake leptons, no hadronic t’s • Good agreement with DIL, LTRK Fermilab - Peter Wittich

  34. Cross-check: vary Jet, 2nd lepton thresholds Central value LTRK • Vary jet threshold, 2nd lepton momentum threshold • Changes background composition • Fake dominated → physics dominated → See consistent results (NB Uncerts very correlated) • Choose value with best a-priori significance as central value Fermilab - Peter Wittich

  35. Combining the cross sections • Combining two measurement reduces the largest uncertainty (statistics) • Strategy: divide signal, background expectation and data into three disjoint regions • Use extra information about events (high purity, low purity) and higher stats to get better measurement 3.0 1 2.6 Acceptance ratio: DIL:LTRK:Common 1:2.6:3.0 Fermilab - Peter Wittich

  36. Combination technique α=product over DIL only, LTRK only, overlap • For three regions, maximize combined Poisson likelihood • Be conservative w/ systematics between regions • Treat as 100% correlated, distribute to give largest total systematic 12% reduction in statistical error Fermilab - Peter Wittich

  37. Kinematical distributions With larger statistics, we can start going beyond counting experiments to do shape tests on our selected sample. Use larger statistics of LTRK to examine sample kinematics KS = 75% KS = 66% Data follow expected distribution of top + background Fermilab - Peter Wittich

  38. Flavor distribution Use sample with two identified lepton (DIL) to look at flavor distribution  Flavor distribution is consistent with expectation. Fermilab - Peter Wittich

  39. What’s next for top? • CDF has many other measurements that use the dilepton channel • Dedicated hadronic tau measurement • Detailed kinematic studies • W helicity (dil + l + jets) • Top mass in dilepton channel • Combined cross section dilepton and l + jets channels • Other cross-sections en route to publication • Mass results • Dilepton & lepton + jets • See CDF public top results page for full details http://www-cdf.fnal.gov/physics/new/top/top.html Fermilab - Peter Wittich

  40. Conclusions consistent with SM prediction of • We have measured the tt production cross section in the dilepton decay channel using 197 pb-1 of Run II data • Our result is (mt = 175 GeV/c2, BR(W→lν)=10.8%): • Kinematics, flavor distribution of data also consistent with Standard Model expectation • Paper published • Phys. Rev. Lett. 142001 93 (2004) • First published Run 2 high momentum physics paper from Tevatron! Fermilab - Peter Wittich

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