Top Quark Physics

1. Top Quark Physics Suyong Choi Korea University

2. Top Quark in the Standard Model • Measurement of Production Cross Sections • Properties of the Top Quark • Summary and Outlook Contents

3. Top quark in the Standard model

4. Discrete quantum numbers • Spin • Weak isospin • Charge • Mass • Lifetime or Decay width • Branching fraction • Coupling – QCD, EW Properties of Top Quark

5. Top quark carries “color” and interacts with gluons • Production of top quark pairs is important probe of QCD interactions QCD

6. Fermion part • Interaction with H, W, , Z Electroweak Lagrangian

7. Strong interaction: • Coupling to neutral gauge boson: • Coupling to charged gauge boson: • Coupling to Higgs field (boson): • Coupling to Higgs field: Interactions of Top Quark in SM

8. Decays to physical states • Top decays almost 100% to W+b Top Quark Decays

9. Decays of Top Quark

10. 1.3 GeV width for 172 GeV top quark • s • Top quarks are produced and decay like free quarks with spin at production information intact • Hadron formation time • Hadron formation is governed by light-quark dynamics • In contrast, B mesons decay isotropically Top Decay Width

11. Top quark polarization is reflected in angular distribution of decay products Polarized Top Quark

12. Top Quark and Electroweak Precision data

13. Radiative corrections to W and Z propagator • Quadratic sensitivity to fermion masses Top Quark Corrections to Electroweak Measurements

14. W and Z masses • Z mass was measured very precisely at LEP experiments • could be inferred with knowledge of • Test of EW theory Top Quark and Electroweak Measurements

15. Top quark mass could bepredicted from precisionmeasurements Prediction from LEP

16. CDF DØ CDF DØ Top Quark Run 2 results Top Mass Distributions from 1995 observation paper

17. Success of the SM

18. LEP EW precision results from Top Quark Mass from Electroweak Data

19. In conjunction with W and Z,we can gain information on Higgs mass Connection with Higgs

20. Top quark is special • Most massive • Interaction only within 3rd generation • top-Higgs coupling ~ 1 • Boundary between metastabilityand stability The Top Quark

21. 5 fb-1 @ 7 TeV 20 fb-1 @ 8 TeV LHC and Experiments

22. Properties • Mass • Decay width • Spin • Coupling • Cross section measurements • Production and decays Physics with Top Quarks

23. Higher cross sectionand higher luminosity at LHC • Top quark factory • Rare processes with top quarks • New physics with top quarks • Tevatron and LHC are complementary Cross Sections at Tevatron and LHC

24. Production

25. Strongly produced • Contribution of andchanges as Pair production diagams Pair Production

26. Lepton+jets per lepton flavor Multijet – Highest statistics, but large backgrounds and combinatorics Lepton+jets – Highest statistics and usually yields best measurement Dilepton – Smaller statistics but clean, less combinatoric, solving for 2 neutrino momenta not trivial channels

27. Lepton+Jets • 1 charged lepton • 4 hadronic jets (2 are b-quark jets) • Missing ET • Problem • How to correctly assign jets to top or antitop • How to reconstruct neutrino momentum Reconstructing tt-bar Events

28. 1 unknown: neutrino • , • 3 constraints: • Problem of combinatorics • 2 fold ambiguity – if 2 b-jets tagged • 6 fold ambiguity – if 1 b-jet tagged Top Quark Reconstruction in L+Jets

29. Have to consider • experimental uncertainties on measurements • finite widths of W and top • Numerically minimize event-by-event Top Quark Reconstruction in L+Jets

30. Top Production Cross Section

31. Experimental error comparable to theory error • QCD explains well the inclusive pair production Pair Production Cross Section

32. s and t channel • Electroweak production • Cross section of the sameorder as pair production • Sensitive probe of withoutthe assumption of 3 generationof quarks W associated Single Top Production

33. Single Top Production t-channel

34. Signal Region Control Region significance Observation of Wt Single Top Production

35. From single top quark production cross section, we can measure directly without assuming 3 generation of quarks • Current best direct measurement: Measurement of

36. Properties

37. Tevatron: GeV– 0.5% accuracy Mass of Top Quark

38. CPT violated if • and distinguished by electric charged of lepton Mass Difference of and

39. In SM, top quark width at NLO is • 1.29 GeV/c2 • Lifetime of • Decay width reflected in reconstructed mass distribution • CDF measures Decay Width of Top Quark

40. B-jet charge calculatedfrom tracks associatedwith b-jet Electric Charge of Top Quark

41. Use lepton angular distribution in top rest frame • W from top decays are either left-handed or longitudinal W Polarization from Top

42. On average, spin of top and antitop are unpolarized, but event-by-event, their spins are correlated • Most prominent in initial state: aligned top spin • For gg mostly anti-aligned spins • Results depend on spin quantization axis chosen Relativistic top Produced at Rest Spin Correlation in Production

43. and the spins of top quarks are correlated • Due to , spin state of top at production reflected in decay products • Lepton is the most sensitive probe of top spin polarization • Tevatron and LHC has different contributions of and • ATLAS observed spin correlations at 5.1 s.d. Spin Correlation

44. Top Coupling with Vector Bosons with and

45. Major background to • Number of b-tagged jets distribution Production

46. Searches with Top Quarks

47. Search for Resonances Decaying into

48. Anomalous Single Top Search for Search for FCNC

49. Top-Higgs coupling almost 1 • Consistent with backgrounds • Cross section limits at Search for

50. Approaching 20 years of rich physics program at hadron colliders with top quark events • Top quark production and properties consistent with SM • Many measurements systematics limited. What can you do with millions of top quark events? Summary and Outlook