Top Quark Physics at D0

1. Top Quark Physics at D0 Yi Jiang University of Science & Technology of China • Introduction • Top quark production cross section • Top quark mass measurement • Single top physics • Spin correlation • Summary

2. Tevatron Collider in Run II • The Tevatron is a proton-antiproton • Collider with 980 GeV/beam =1.96TeV in RunII (1.8TeV in RunI) • 36 P and Pbar bunchesa396 ns • between bunch crossing Increased from 6X6 bunches with 3.5ms in Run I • Increased instantaneous luminosity • Run II goal • Current: ~

3. Run II D0 Data Taking Status 85~90%

4. D0 Detector (Run II)

5. PVrt/IP~15mm Silicon Microstrip Detector (SMT) Vertex resolution: ~10mm(design) Primary Vertex vs. Impact parameter

6. Center Fiber Tracker (CFT) SMT combines vertex and tracking capabilities and provides good primary and secondary vertex resolutions.

7. y q j x Z The Calorimeter Resolution: s/E ~ 15%/√E(GeV) “fine” EM 50%/√E(GeV) “coarse” jet sMET ~ a+b*ST +c*ST2 (run1) ST scalar sum of ET a~1.89GeV, b~6.7E-3, c~9.9E-6/GeV

8. J/Psi: Local/Global Muon Detector

9. D0 Detector Performance

10. Motivation for the Top Quark Studies (I) • Top quark has been discovered by CDF and D0 in 1995; • Top quark mass ~175GeV and strong Yukawa coupling ~1; • - Study of the topquark provides an excellent probe • of the electroweak symmetry breaking mechanism; • - New physics may be discovered in either its • production or decays; • - Top quark spin can be directly observed. • Tevatron is the only palce to study top quark properties • before LHC operation.

11. Motivation for the Top Quark Studies (II) Top Mass, W Mass Measurement

12. Top Physics Understanding • Program • Top production & decay • Tools • Cross section • Mass • Single top • Spin correlation • W helicity

13. Top Quark Production at Tevatron Top-antitop quark Pair Production (mainly) Single top quark production (not yet observed)

14. Top Quark Decay In the standard model, the top quark is short lived and decay almost exclusively to W and b quark

15. Methodology& tools Full characterization of the chosen final state signature in term of SM background processes (control region) [ Optimize signal for best measurement precision How to separate signal from background: a Top events have very distinctive signatures 8 Decay products (leptons, neutrinos, jets) have large PT 8 Event topology: central and spherical 8 Heavy flavor content: always 2 b jets in the final state Tools (need multipurpose detectors) 8 Lepton ID: detector coverage and robust tracking 8 Calorimetry: hermetic and well calibrated 8 B identification: algorithms pure and efficient 8 Simulation: essential to reach precision goals

16. Production cross section RunI~100 events

17. Top cross section: dilepton channels

18. CDF & D0: dilepton channels ------------------------------------------------------

19. Top cross section: lepton+jets “Golden” mode for top studies: ~ 30% yield and relatively clean

20. Lepton+jetschannel: topological analysis • Preselect a sample enriched in W events • Evaluate QCD multijet background • from data for each jet multiplicity • bin using “matrix” method • e+jets:due to fake jets (po and g) • m+jets: due to heavy flavor decays • Estimate real W+4 jets contribution • with scaling law • Additional topological cuts: • ≥ 4 jets • HT>180 GeV (e) • Aplanarity>0.06 • HT(jets,pT(W))>220GeV (μ) “Matrix” method Nloose = NW + NQCD Ntight = sig  NW + qcd  NQCD

21. D0: b tagging Soft lepton tag b tagging efficiency

22. Lepton+jets: topological cuts and SLT

23. Cross section from topological analyses

24. D0: lepton+jets channels with b-tagging

25. CDF: lepton+jets channels with b-tagging

26. D0: e+jets channels with matrix element method • use the signal and background process matrix elements to calculate the • observation probability function; • for each pre-selected event(e+X), calculate the probability of being the • signal and background; • fit the data with the discriminator plot to extract the probability of • signal and background; • use likelihood function to extract the signal event fraction of the total • pre-selected events. simulation result: Discriminator: background signal events signal probability background probability D(x)

27. Run II cross section summary

28. Cross section √s dependence

29. First Run II look at all jets channel • Challenging signature: Very low S/B ! • 9 cross section & mass measured in Run I (CDF, D0) • Tools needs: • kinematical quantities, neural networks, b-tagging … D0 Run I all hardonic channel

30. Top mass measurement

31. Lepton + Jets mass method • Additional complications from • background events • detector effect (mismeasurement + resolution) • initial and final state radiations

32. Lepton + Jets mass method

33. Mass from lepton + jets (Run I)

34. Mass from alljets (Run I)

35. Dilepton mass method The final state momentum and angular information is sensitive to the top quark mass.

36. Dilepton mass method D0: Run I CDF: Run I

37. First look at top mass in Run II (CDF)

38. Single top physics Run I results:

39. Search for single top in Run II

40. Spin correlation

41. Spin correlation

42. Spin correlation D0 Run I Result:

43. W boson helicity if b quark mass=0, W polarizations can be analyzed from the angular or PT distributions of the charged leptons.

44. W boson helicity

45. Summary • The Tevatron is the top quark factory until LHC: • First Run II results cover a variety of channels and topics • CDF and D0 are exploiting their upgraded detector features • Several top properties studied using Run I data (limited statistic) • There is a big potential to improve crucial aspects of physics • analyses (tracking in jets, physics object identification, • b-tagging optimization and many others). A very rich top physics program is underway: let’s see what the top quark can do for us!