The Top Quark

1. The Top Quark • Why? • How? • Recent Results Cecilia E. Gerber University of Illinois at Chicago December 17, 2009

2. Why bother with HEP? • What is the World Made of ? • What are the building blocks of matter? • How do they interact with each other? • Connection with Cosmology?

3. e e u d   c s   t b g W+ Z0 W- g What is the World Made of? Standard Model (~1970) ELEMENTARY CONSTITUENTS Strong 1 Electromagnetic 10-2 INTERACTIONS Higgs Weak 10-6 H Gravity 10-40

4. Rutherford Scattering Alpha particles were allowed to strike a thin gold foil. Surprisingly, alpha particles were found at large deflection angles and ~1 in 8000 were even found to be back-scattered. This experiment showed that the positive matter in atoms was concentrated in an incredibly small volume (10-13cm) and gave birth to the idea of the nuclear atom.

5. Chicago  Booster CDF DØ Tevatron p source Main Injector DØ CDF The Fermilab Tevatron Accelerator p anti-p collider: 1992-96 Run 1: 100pb-1, 1.8TeV 2001-2011? Run 2: 10-12fb-1, 1.96TeV Next in line: CERN LHC ~2010 (pp) 14TeV (2.36)

6. UnderlyingEvent g q u u q d u d u Hard Scatter How do we do research in HEP? • By taking these speeding subatomic particles and smashing them together, we can see what comes flying out. • At the Tevatron, ~2,500,000/sec a proton and an anti-proton cross each other. Only ~1/sec a Hard Scatter occurs • We are probing matter at the 10-17 cm level!

7. Now (13.7 billion years) Stars form (1 billion years) Atoms form (380,000 years) Nuclei form (180 seconds) Nucleons form (10-10 seconds) Quarks differentiate (10-34 seconds?) ??? (Before that)

8. Fermilab 4x10-12 seconds Now (13.7 billion years) Stars form (1 billion years) Atoms form (380,000 years) Nuclei form (180 seconds) Nucleons form (10-10 seconds) Quarks differentiate (10-34 seconds?) ??? (Before that)

9. Now (13.7 billion years) Stars form (1 billion years) Atoms form (380,000 years) Nuclei form (180 seconds) LHC Nucleons form (10-10 seconds) 10-25 seconds Quarks differentiate (10-34 seconds?) ??? (Before that)

10. Proton-anti Proton Collision Small x, products boosted along beam direction • Large x, can create massive objects that decay to secondaries with large momentum component transverse to the beam For every proton there is a probability that a single quark (or gluon) carries a fraction “x” of the proton momentum Good way of telling that a hard collision occurred.

11. neutrinos A generic HEP detector

13. DØ CDF The D0 and CDF detectors at Fermilab

14. Neutrinos do not interact with the detector p _ p hard scattering • total energy:unknown • total longitudinal momentum:unknown • total transverse momentum:zero Identifying Neutrinos Electron We infer the presence of a neutrino from the imbalance in the transverse momentum Neutrino

15. Identifying Quarks Quarks (and Gluons) do not exist as free particles q-anti q pairs are pulled from the vacuum to produce stable particles : mesons, baryons Quarks “hadronize’’ single quark appears as a “Jet” (spray) of hadrons in the detector. Jets originating from the quarks of the first two generations (u,d,s,c) cannot be separated from each other.

16. B Decay Products Flight Length ~ few mm Collision Decay Vertex Impact Parameter Identifying b-quarks: lifetime tag life time  1.5 ps  c  0.5 mm (short, but not too short) precise tracking close to primary collision point achieved with silicon microstrip detectors

17. Lepton + jets with lifetime tag Top candidate

18. Why Study the Top Quark? • Predicted by the SM and discovered in • 1995 by CDF and DØ • mt=173.1 ± 0.6 ± 1.1 GeV • Couples strongly to the Higgs • ─ may help identify the mechanism of mass generation • ─ may serve as a window to new • physics that might couple • preferentially to top • Successful Tevatron top quark program • High precision measurements for the top quark mass, top pair production cross section and decay properties • Some basic quantities still unmeasured: spin, width, lifetime • Single top quark production predicted by the SM, has been observed • in March 2009, 14 years after the pairs observation.

19. Top-anti Top quark production Top anti-Top pair production (via strong interaction) (qq annihilation) (gluon fusion) Run1(1.8TeV)Run2(2TeV)LHC(14TeV) 90%85% 5% 10%15% 95% 5.47 800 x-sec(pb)

20. Single Top quark production Single Top production (via electroweak interaction) (s-channel) (t-channel) Run1(1.8TeV)Run2(2TEV)LHC(14TeV) 0.71 10 1.72 250 x-sec(pb)

21. Top-quark decay • ~100% of the time, a top quark decays into a bottom quark and a W boson. • The W boson can decay into two quarks or into a charged lepton and a neutrino. • A Top-anti Top event should therefore have either: • 6 quarks • 4 quarks, 1 charged lepton and 1 neutrino • 2 quarks, 2 charged leptons and 2 neutrinos In all cases, 2 b-quarks are present in the event

22. e,m n b-jet b-jet jet jet e,m n jet jet MET MET b-jet b-jet b-jet b-jet n e,m jet jet All-hadronic (BR~46%, huge bckg) Lepton+jets (BR~30%, moderate bckg) Dilepton (BR~5%, low bckg) Top Quark Decay Modes “Lepton”: electron or muon

23. ET emTop candidate m- jet e+ jet

25. Top quark events are rare! • Top production is a rare process: about one collision in every 11010 produces a Top-anti Top quark pair. • Small cross sections require high luminosity, and the ability to detect and filter out • Top-anti Top events from a large number of other processes with the same final states (backgrounds)

26. Single Top is even rarer

27. And has a less populated final state

28. Experimentally very challenging

29. But important for Higgs observation • Same final state as WH • Backgrounds are the same • Test of techniques to extract small signal from a large background

30. Recipe to measure a x-section Number of events that pass selection cuts Number of events from processes other than top Measured x-sec in channel x Integrated Luminosity: a measure of amount of data Efficiency for top events

31. Top-anti Top x-section Measurement • Measure in different channels • Measure with different techniques • b-tagging method assumes Br(t→Wb)=1 • Kinematic fit methods are free of this assumption Test of pQCD at high Q2 Sensitive to new physics: Expect higher x-sec if resonant or non-SM production occurs Experimental uncertainties reaching precision in theoretical prediction.

32. Single Top Production signal • Simple counting experiment cannot extract the signal from the background • Need advanced techniques • Multiple methods per experiment • Serve as cross check • Combination adds power

33. Top Quark Pair Production Mechanisms Gluon Fusion Fraction Extracted from a fit to the azimuthal correlation between leptons (dilepton events) Uncertainty dominated by statistics Search for ttbar resonances ≥4jets, 1b-tag Study invariant mass spectrum of l+j events No evidence for narrow resonance decaying into ttbar

34. W helicity Top Mass l+ Top Width Anomalous Couplings Production cross-section Top Spin W+ CP violation Top Charge Resonant production p n t b Production kinematics _ b X _ Top Spin Polarization _ q’ t q Rare/non SM Decays W- _ p Branching Ratios |Vtb| Top quark Properties b-tagging provides pure sample of top quarks for properties measurements 2 b-tags l+j 1 b-tag dileptons

35. Top quark Mass Best results (errors ~ 1%) obtained by ME Method: - Event by event weight calculated according to quality of agreement with SM top and background differential cross-sections - Product of all event probabilities gives the most likely mass - JES constrained in-situ by the hadronic decay of the W→jj Dominated by systematics M(top)=173.1±0.6±1.1 GeV

36. SM Constraints on the Higgs Light Higgs preferred by the SM with latest top and W mass Plots from LEP/TEV EW working group

37. Top Mass from x-section Assuming production is governed by SM, top quark mass can be extracted comparing the measured x-sec with theory Measurement has different experimental and theoretical uncertainties than direct measurements. Both direct mass measurement and extraction from cross-section measurmement agree within errors.

38. Top Anti-top Mass Difference CPT invariance requires that the mass of particles and corresponding anti-particles be identical. Difficult to test with quarks because they hadronize before decaying. Not the case for top quarks. Measured Mass Difference = 2.2±2.2%, consistent with zero

39. Decay Properties: W Helicity θ* b W+ ℓ+ DØ: 2-parameter fit for fraction of longitudinal (f0) and right-handed (f+) polarized W bosons in top decays Statistically limited: consistent at the 23% level with the SM prediction.

40. W charge * bJet Charge Top Quark Charge • Fundamental property of particle • has not been determined yet • One possible scenario Phys Rev D59, 091503 (1999): • The discovered top quark is an exotic quark of charge - 4e/3 • The top quark with charge 2e/3, mass 270GeV not observed yet • Model accounts for precision Z data (including Rb and AFBb) Statistically Limited! PRL 98, 041801 (2007) Both CDF & DØ Data strongly favor the SM over XM

41. Top Quark Spin • Use Dilepton Events ee, em, mm • Angular distribution of leptons w.r.t. the beam axis sensitive to correlation Measured correlation agrees with the SM within 2SD • GtSM  1.4 GeV at mt = 175 GeV • Top decaysbefore hadronization, transferring spin and kinematics to the final state • SM predicts the spin of the top and the anti-top are correlated

42. MANY Searches for BSM effects • Using top pair final states: • SM H→b anti-b in association with top (Htt-bar) • Top decay to charged Higgs B(t→H+b) • Scalar Top pair production • Using single top final states • H+→tb search • Anomalous Wtb couplings • W'→tb search • FCNC search All results agree with the SM expectations…

43. After ~15 years of studies • mt=173.1±1.3 GeV • σ(tt)=7.84±0.95pb (for mt=175GeV) • σ(t)=3.76+0.58-0.47pb (for mt=170GeV) • |Vtbf1L| = 0.88 ± 0.07 • Charge: -4/3 excluded @ 92% CL • Longitudinally polarized W: f0=0.49±0.14 [f0(SM)=0.7] • Δm = mt−mt = 3.8±3.7 GeV • Γt < 13.1 GeV @ 95% CL [Γt(SM) = 1.4 GeV] • plus MANY limits on new physics http://www-cdf.fnal.gov/physics/new/top/top.html http://www-d0.fnal.gov/Run2Physics/top/top_public_web_pages/top_public.html

44. Conclusions • Tevatron Run2 is an ongoing success… 7pb-1 delivered and ~12pb-1 expected (2011) • LHC startup imminent but at low energy • Tevatron is the only place for the next year to study the properties of the Top quark and search for the Higgs Boson • The next few years promise to be a very exciting time in the field of high energy particle physics.