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Top Quark Properties

Top Quark Properties. Catalin Ciobanu LPNHE, Pierre and Marie Curie University / IN2P3-CNRS for the CDF and D  Collaborations Hadron Collider Physics Symposium 2008, Galena IL May 27, 2008. Top Quarks. Strong (pair) production – main channel (ttX) ~ 7 pb qqtt :85% gg tt :15%

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Top Quark Properties

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  1. Top Quark Properties Catalin Ciobanu LPNHE, Pierre and Marie Curie University / IN2P3-CNRS for the CDF and D Collaborations Hadron Collider Physics Symposium 2008, Galena IL May 27, 2008

  2. Top Quarks • Strong (pair) production – main channel • (ttX) ~ 7 pb • qqtt :85% • gg tt :15% • Top decay: • SM: t Wb almost 100% of the time • Classified according to W decays • Lepton+jets, dilepton, all hadronic • 5% Tau dilepton e,  • 5% Dilepton (ee, , e) • 30% Lepton+jets (e+j, +j) • 45% All hadronic

  3. Top Properties • 2. Top Production • Mechanism • Forward-backward asymmetry • Resonances decaying to top • stop production • 3. Top Decays • W helicity (V-A) • Branching ratios • FCNC • Top to Higgs • 1. Top Properties • Top Quark mass (prev) • Charge, width

  4. 1. Top Properties: Charge • Standard Model: Qtop = 2/3 e • Exotic model: doublet (–1/3e,–4/3e) ? • D. Chang et al., Phys. Rev. D59 (1999) 091503 • D0 Phys Rev Lett. 98, 041801 (2007) • Fraction of exotic quark pairs < 0.80  (90% CL) • CDF result with 1.5/fb • Hypotheses tested: 2/3e (null) or 4/3e (test) • P-value 0.31 • Exotic charge model XM of -4/3 excluded with 87% C.L.

  5. 2. Top Production Mechanism • Top pair production via: qqtt and gg tt (theoretical uncertainties ~10%) • Other production mechanisms? • Two approaches to test this at CDF: • Using a neural network based on event kinematics • Using multiplicity of low PT tracks • Both approaches statistics limited • Result: • (gg->tt)/(qq->tt)<0.38 at 95% C.L. • (gg->tt)/(qq->tt)=0.07-0.07+0.15

  6. > 0 Forward-Backward Asymmetry in Top Production at D0 • Test of discrete symmetries of strong interaction at high PT • Asymmetry: • LO – no asymmetry • At NLO, a few percent (4-7%) asymmetry predicted • Higher order corrections may be important • AFB measured in parton rest frame: • AFB = 0.12 ± 0.08 (stat) ± 0.01(syst) • Consistent with expectation Also probing new physics; leptophobic Z’ Phys. Rev. Lett. 100, 142002 (2008)

  7. Forward-Backward Asymmetry in Top Production at CDF • AFB measured in both the parton rest frame and the lab rest frame • AFBtt ~ 1.3 AFBpp • Unfolding to go from reconstructed to parton level • Results: • AFBtt = 0.24± 0.13(stat) ± 0.04(syst) • AFBpp = 0.17± 0.07(stat) ± 0.04(syst) • AFB Higher , but consistent with expectation CDF II tt frame pp frame

  8. tt Resonance Searches at D0 • New heavy particles could couple strongly with 3rd generation fermions • A narrow-width leptophobic Z’ is such a case • technicolor model (Hill and Parke, Phys. Rev. D 49 (1994) 4454) • Analyze the high-mass region of the reconstructed Mtt spectrum MZ’<760 GeV for Z’/MZ’ =1.2%

  9. tt Resonance Searches at CDF • CDF latest result in search of Gtt • Measurement of the tt Differential Cross Section, dσ/dMtt • Lepton+4jets sample • Measure top pair cross-section in bins of Mtt • Indirectly probing for new phyiscs • Data consistent with SM • P-value = 0.45 Dynamical Likelihood Method used for the Mt measurement Test: 0.05≤ Γ/M ≤0.5

  10. t’ Resonance Search • Fourth generation heavy quark: • Pair-produced via strong interaction • Heavier than the top • Decays to Wb, Ws, Wd • Two-variable search: • Reconstructed Mt’ • HT (total transverse energy) • P-value 2.8% Mt’ < 284GeV at 95%C.L.

  11. Scalar top search at D0 • SUSY searches? • Search for stop pair production • If the stop is lighter than top • Final state similar to the SM tt • Likelihood discriminant: • 11 variables, depending on M(t) point • First search of this kind at the Tevatron • Limits ~7-12 theory values +jets +jets

  12. Scalar Top Search at CDF • Search for tt pair production • Dilepton channel • Assumptions: Theoretical cross sections

  13. 3. Top Decays - W Helicity at D0 • The V-A nature of the weak current tWb requires the following helicity fractions: • F0 = 0.7, F = 0.3, F+ = 0.0 (longitudinal, left-handed, right-handed • One can simultaneously fit for F0 and F+ Fit for f- and f+ gives: F0 = 0.42 0.17(stat) 0.10(sys) F+= 0.12  0.09(stat) 0.05(sys) dilepton * angle between lepton and top in W rest frame l+jets Phys Rev Lett 100, 062004 (2008)

  14. W Helicity at CDF • CDF used three approaches for this measurement • Using the fully reconstructed decay chain: • F0= 0.38 ± 0.21(stat) ± 0.07(syst) • F+= 0.15 ± 0.10(stat) ± 0.04(syst) • Template method with similar results • Using a matrix element-based technique • Likelihood based on differential cross-sections of signal and background Matrix Element - based method F0 = 0.64 ± 0.08(stat)± 0.07(syst)

  15. FCNC search • Flavor changing Neutral Currents • Highly suppressed in SM • Fit to a mass 2 • Tag and anti-tag(zero-tag)

  16. Charged Higgs Searches at D0 • Standard Model predicts: • R = (tt->X)l+jets / (tt->X)ll = 1 • D0 measurement: • R=1.21+0.27-0.26 (stat+syst) • In the context of a general multi-Higgs-doublet model in which H±  cs only: • (tt->X)l+jets enhanced, (tt->X)ll stays the same • Expect BR(t  Hb) <0.25 at 95%C.L. • Measure BR(t  Hb) = 0.13+0.12-0.11 • Observe BR(t  Hb) < 0.35 at 95%C.L. Feldman Cousins contours For R and BR(t  Hb) Assumptions: MH+ = 80 GeV and Hcs exclusive decays BR(t  Hb) R

  17. Charged Higgs Searches at CDF • In the context of a general multi-Higgs-doublet model in which H±  cs only • Reconstruct the dijet system • Higher invariant mass than the W  qq • Sharpen peak by adding the FSR jet to mother jet • ΔR < 1.0 • No signal observed: • Exclusion contour BR versus MH+

  18. Other Measurements • Simultaneous measurement of R and ttbar cross-section • R=BR(tWb)/B(t Wq) with q = d,s,b • Extract (tt) without setting BR(tWb)=1 • Simultaneous fit R = 0.97 +0.09 –0.08 (stat+syst) ,   R > 0.79  (95% CL)|Vtb| > 0.89  (95% CL)  (if 3x3 CKM is unitary) (tt)=8.18+0.90-0.84 ± 0.5 (lumi) For Mtop=175 GeV Phys Rev Lett 100 , 192003 (2008 )

  19. Conclusions • Presented several top quark results from 1-2.3 fb-1 • Properties (charge) • Production mechanism: • gg/qq • Z’/G searches • AFB measurement • Other searches in top-related samples: • Differential M(tt) cross-section • Fourth generation t’ • Scalar top quarks • Tevatron top properties sector very rich and mature • No new phenomena uncovered yet, but: • New data crucial, with many New Physics analyses statistics limited • Very dynamic CDF and D0 groups, please visit: • http://www-d0.fnal.gov/Run2Physics/top/top_public_web_pages/top_public.html • http://www-cdf.fnal.gov/physics/new/top/top.html • Decay of top quarks • W helicity • FCNC search • Charged Higgs

  20. Top Width • Check the consistency of our data with MC generated with different top widths (but same mass Mt =175 GeV) t < 12.7 @ 95% CL • Parameterize the reconstructed top mass distribution as function of width. Perform pseudoexperiments to check sensitivity.

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