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An Update on Top Quark Physics

An Update on Top Quark Physics. Robert Roser Fermilab. PASCOS06 Sept 10 th 2006 (the day after “the game”). Physics of the Top Quark. Top physics is still one of the more sexy things to study at the Tevatron…. Tevatron ~ 800. LHC. Number of Physicists.

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An Update on Top Quark Physics

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  1. An Update on Top Quark Physics Robert Roser Fermilab PASCOS06Sept 10th 2006 (the day after “the game”)

  2. Physics of the Top Quark Top physics is still one of the more sexy things to study at the Tevatron…

  3. Tevatron ~ 800 LHC Number of Physicists Year Discovered # of Physicists for Particle Discovery

  4. Top Quark Discovery: 1995 The search for top lasted almost two decades. It’s unexpectedly heavy mass delayed discovery. April 1994: “Evidence for top production at the Tevatron” CDF • PRD 50, 2966 (1994).... lum = 19 pb-1 • 150 pages ! ..... 2.8 s excess • Mtop = 174 (16) GeV & s(tt) = 14 (6) pb • March 2nd, 1995: CDF and D0 announce simultaneously the discovery of the Top Quark !!! • CDF: PRL 74, 2626 (1995) .... 67pb-1 • D0: PRL 74, 2632 (1995).... 50pb-1 Top Physics begins…

  5. For You Bean Counters… • Run 1 duration: • ~ 2 years of data taking • Number of collisions: • ~ 5 x 1012 ! • Event size: • 180 Kbytes (DST)... 32 Kbytes (PAD) • Number of events on tape: • ~ 50 millions • Number of tapes: • ~ 1800 • Number of people/expt: • ~ 400 physicists • Number of events with a high Pt, isolated lepton: • approx 200,000 e & µ • Number of top event candidates • ~50 ! Cost is (if 1/2 billion $ spent): ~ 1 million dollars/evt. Generated ~ 5 million dollars/evt. detected

  6. Tevatron Physics Trigger is key in reducing the huge amount of interactions producing “something” in the final state (trigger efficiency for high pT leptons ~90% for both CDF and D0) One top pair each 1010 inelastic collisions at s = 1.96 TeV

  7. Why Study Top? Top pairs: (tt) ~7 pb • Top production rate • Mass of top • W helicity in top events • QCD tests • New physics in X tt • Anomalous couplings, • new particles Single top: (tb) ~3 pb • |Vtb| • QCD tests • New physics? Great Place to test S.M. and Search for…

  8. Why Is Top So Interesting? • Well, top physics is different! • Top quark lifetime is short: decays before hadronizing • No spectroscopy like other heavy flavor • Top momentum and spin transferred to decay products • Probes physics at higher scales than other known fermions • Top (or heavy top) very hip in many EWSB models: Higgs, Top Color, Little Higgs, SUSY mirror models • Is it only Top?

  9. Experimental Landscape: The Tevatron Collider • Beam energy =980 GeV • 36x36 bunches, 396 ns coll. sep. • Recycler and e-cooling in use • Pbar “stashes” >350e9 in recycler

  10. Tevatron Luminosity Today’s Presentation: 300 pb-1 ~ 1 fb-1 Peak Luminosity Integ. Lum. (delivered) / Experiment 2002 2003 2004 2005 2002 2003 2004 2005 • Peak luminosity record: 2.2 x1032 cm-2 s-1 • Integrated luminosity • Weekly record: 33 pb-1 /week/expt • Total delivered: 1.7 fb-1 /expt. Total recorded: 1.5 fb-1 /expt • Doubling time: ~1 year • Future: ~2 fb-1by 2006, ~4 fb-1by 2007, ~6-8 fb-1by 2009

  11. We Are Here What Can We Expect? Ldt/expt Date We Can Do It!!!

  12. The CDF Detector

  13. The DØ Detector

  14. Producing Top at Hadron Colliders 15% 85% • Top produced strongly in pairs • Typical S/BG 0.5 at TeV, 2.5 at LHC • σ (tt)LHC~100xσ(tt)TEV • Fraction of qq vs gg is inverted

  15. How Else is Top Produced? Not Yet Observed p t X Resonance Production? Top Color-Assisted Technicolor OR ????? Not Yet Observed p t Standard Model Tevatron Single Top Production

  16. How Does Top Decay? Standard Model: tWb ~ 100% Main “usable” top event topologies: • tt  llbbdi-lepton5% e+ • tt  lqqbblepton+jets30% e+ • tt  qqqqbball hadronic45%

  17. What do we look for in top events? • Electrons • Muons • Neutrinos • Quarks (Jets) • b Quark Jets “All Hadronic Channel” W qq, W qq~45% “Lepton + Jets Channel” W  l, W qq~30% “Di-Lepton Channel” W l, W l~5% Identifying Top Quarks => Signature-Based Analyses!

  18. Particle Signatures • Electrons - deposit all their energy in electromagnetic calorimeter which can be matched to a track • Photons - no track • Muons: Match signal in muon chambers to track

  19. Particle Signatures (2) • Quarks - fragment into many particles to form a jet • Leave energy in both calorimeters • Neutrinos - pass through all material • Measured indirectly by imbalance of transverse energy in calorimeters

  20. CDF Double-Tagged Event Impact parameter resolution for high-pT tracks ~18m b-jet taggers provide clean samples of single and double b-tagged events, useful for single top, top properties, and searches such as for WH.

  21. Lepton+Jets Cross Section • This is the golden channel for its high yield and relative purity (after b-tag)! • Used in top property measurements, single top and Higgs searches. has single best results in 750 pb-1 has results up to 370 pb-1 with 1 and 2 btags

  22. Dilepton Cross Section Signal to background already good enough with l+MET+>=2 jets Even more purity with b-tagging

  23. All Hadronic Cross Section • Combine topological selection and b-tag • Lot’s of data to model background ! • Start from a sample >=6 jets (special trigger). Still overwhelmed by QCD multijets background. (360 pb-1) requires 2 btags and fits the dijet (no btag) and trijet (1 btag) distribution (310 pb-1) requires at least 1 btag and anti-MET cut.

  24. Cross Section Summary • Different channels and techniques all in agreement • Precision at 14%, No combined result as of yet Tevatron goal: 10% uncertainty/experiment with 2 fb-1

  25. Top Mass • Fundamental Parameter of the Standard Model • This summer’s Tevatron Combination yields • Based on that, EWK fit results • Run II TDR Predictions– W Mass Width of 25 MeV and top mass uncertainty of 2 GeV • 35% constraint on the Higgs Mass

  26. Introduction to the Top Mass • It is the same for top events with the following complication • t1 Wb  jet – jet – jet • t2 Wb  lep –  – jet • Each event has 2 top quarks • Two chances to measure its mass in each event • We don’t know which decay products belong to which object

  27. W+ b-jet n X t t jet W- jet b-jet Appreciating the Top Mass Measurement Why is it so hard?? • Life ain’t pretty… • Combinatorics of assigning jets to W/t • ISR/FSR jets

  28. Template Method Evaluate event-by-event best “reconstructed mass”, Mrec, by using observed kinematics of ttbar event (e.g.: c2 fitter) Create “templates”, i.e. MC predictions for Mrec using different true masses , Mtop. 1 2 DATA Measure top mass with likelihood fit of data Mrec to signal + background template. Improved jet energy scale by simultaneous fit in 4 samples to top mass and jet energy scale using Wjj decays 3 Top Template Bckg Template 4

  29. Jet Energy Scale • JES uncertainties are the largest source of systematics: • Fit simultaneously for MW->jj and Mbjj using 2D templates of true Mtop and σJES (680 pb-1) achieves world single best measurement and improves JES systematics by 40% by using in-situ best measurement (370 pb-1) uses a ME method with simultaneous JES fit

  30. Top Mass in Dilepton Under constrained system: two neutrinos but only one MET measurement. Both experiments use Matrix Element Technique (Calculate event-by-event signal probability curve (rather than single Mrec) using decay matrix element and transfer functions. 78 Evts 27.8 bckg 1 fb-1 0.37fb-1

  31. Summary of Top Mass Results Weight (%) Combined Average:

  32. Top Charge and Lifetime Top quark in SM has charge 2/3e. Some models propose an exotic 4th quark with Q=4/3e (365 pb-1) in l +4jets (2 btag) use a jet charge algorithm to discriminate between b and bbar. Top in SM has very short lifetime (SM c ~ 3x10-10m) ~10-24 seconds (320 pb-1) look for anomalous lifetime by fitting impact parameter of lepton in l+jets events When paired tol , top charge is inferred. Data excludes Q = 4/3e @94% C.L.

  33. Search for Single Top s-channel production (W*) Single top is produced via weak interaction at a rate ~1/3 that of top. Allows direct measurement of Vtb. • Kinematically wedged between non-top and top signal, plus high backgrounds (S/B~1/20) require very sophisticated analysis techniques. • Use l +MET+2jet (>=1 btag) events: same signature as • s and t-channel searched jointly and separately (have different sensitivity to new physics).

  34. Single Top Limits • (695 pb-1) has 2 analysis: • Neural Network • Multivariate Likelihood function 370 pb-1Uses a likelihood discriminant 95% observed (expected) exclusion limit getting close to SM expectations! • Projections • (ignoring syst): • 2.4 s excess with 1 fb-1 • 3 s excess around 1.5 fb-1 Based on SM single top XS Stat error only

  35. Does something New Decay to ttbar? Resonance production Look for bumps in the ttbar invariant mass spectrum

  36. ttbar Resonance search • looks for generic spin 1 resonance (X0) • Assume ГX0 = 1.2%´MX0 • Test masses between 450 GeV and 900 GeV in 50 GeV increments. (680 pb-1) Set 95% confidence level limit for σX0 at each mass. Exclude leptophobic Z’ with Mz’ < 725 GeV.

  37. Another Search in Top Sample search for something with “top-like” signature, t’, fit HT vs MWq (Ht is the total energy in the event) (760 pb-1)

  38. TeV Measurements will continue to improve • Ultimate systematics at TeV or LHC will likely be energy-scale related • Will help calibrate LHC detectors The Future of Top <1 week of data taking at 1033 cm-2 s-1

  39. Summary • The Tevatron Top program is well underway. • We will exceed initial expectations on what we can achieve in measuring both the top mass and cross section. • We may soon discover whether the top quark plays a special role in the Standard Model. • The top sector will continue to be a hunting ground for hints of new physics at the Tevatron, and at the LHC • With Increasing Luminosity, the program of top properties measurements will soon tell us how “top” like this particle really is. • It is an exciting time to be a physicist

  40. History: Top Mass Publications

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