Top Quark Properties

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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

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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
• Leptonjets, dilepton, all hadronic

• 5 Tau dilepton e?, ??
• 5 Dilepton (ee, ??, e?)
• 30 Leptonjets (ej, ?j)
• 45 All hadronic

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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

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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 lt 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.

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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-gttt)/?(qq-gttt)lt0.38 at 95 C.L.
• ?(gg-gttt)/?(qq-gttt)0.07-0.070.15

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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)
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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
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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

MZlt760 GeV for ?Z/MZ 1.2
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tt Resonance Searches at CDF

• CDF latest result in search of G?tt

• Measurement of the tt Differential Cross Section,
  ds/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 G/M 0.5
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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 lt 284GeV at 95C.L.
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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
stop mass     175  175  175  160  160  145    GeV
?1 mass     135  120  105  120  105  105    GeV
s(t1 t1) lt     5.6  6.6  5.6  7.5  9.7  12.3    pb  (95 CL)
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Scalar Top Search at CDF

• Search for tt pair production
• Dilepton channel
• Assumptions

Theoretical cross sections
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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
ljets
Phys Rev Lett 100, 062004 (2008)
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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)
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FCNC search

• Flavor changing Neutral Currents
• Highly suppressed in SM
• Fit to a mass ?2
• Tag and anti-tag(zero-tag)

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Charged Higgs Searches at D0

• Standard Model predicts
• R? ?(tt-gtX)ljets / ?(tt-gtX)ll 1
• D0 measurement
• R?1.210.27-0.26 (statsyst)
• In the context of a general multi-Higgs-doublet
  model in which H ? cs only
• ?(tt-gtX)ljets enhanced, ?(tt-gtX)ll stays the
  same
• Expect BR(t ? Hb) lt0.25 at 95C.L.
• Measure BR(t ? Hb) 0.130.12-0.11
• Observe BR(t ? Hb) lt 0.35 at 95C.L.

Feldman Cousins contours For R? and BR(t ?
Hb) Assumptions MH 80 GeV and H?cs exclusive
decays
BR(t ? Hb)
R?
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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
• ?R lt 1.0
• No signal observed
• Exclusion contour BR versus MH

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Other Measurements

• Simultaneous measurement of R and ttbar
  cross-section
• RBR(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 (statsyst) ,   R gt
0.79  (95 CL)Vtb gt 0.89  (95 CL)  (if 3x3
CKM is unitary)
?(tt)8.180.90-0.84 0.5 (lumi) For Mtop175 GeV
Phys Rev Lett 100 , 192003 (2008 )
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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

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Top Width

• Check the consistency of our data with MC
  generated with different top widths (but same
  mass Mt 175 GeV)

?t lt 12.7 _at_ 95 CL

• Parameterize the reconstructed top mass
  distribution as function of width. Perform
  pseudoexperiments to check sensitivity.