Top Quark Properties
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Transcript of Top Quark Properties
C. Ciobanu, page 1
Top Quark Properties
Catalin CiobanuLPNHE, Pierre and Marie Curie University / IN2P3-CNRS
for the CDF and D Collaborations
Hadron Collider Physics Symposium 2008, Galena ILMay 27, 2008
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Top Quarks Strong (pair) production – main
channel (ttX) ~ 7 pb
qqtt :85%
gg tt :15%
Top decay: SM: t Wb almost 100% of the time Classified according to W decays
Lepton+jets, dilepton, all hadronic
e-e (1/ 81)
mu-mu (1/ 81)
tau-tau (1/ 81)
e -mu (2/ 81)
e -tau (2/ 81)
mu-tau (2/ 81)
e+j ets (12/ 81)
mu+j ets (12/ 81)
tau+j ets (12/ 81)
j ets (36/ 81)
5% Tau dilepton e, 5% Dilepton (ee, , e) 30% Lepton+jets (e+j, +j) 45% All hadronic
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Top Properties2. Top Production• Mechanism• Forward-backward asymmetry• Resonances decaying to top• stop production
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
3. Top Decays• W helicity (V-A)• Branching ratios
•FCNC• Top to Higgs
1. Top Properties• Top Quark mass (prev)
• Charge, width
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 < 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: qqtt 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
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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’
)(
)(
ttpp
ttZppf
Phys. Rev. Lett. 100, 142002 (2008)
> >
00
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AFB measured in both the parton rest frame and the lab rest frame AFB
tt ~ 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
Forward-Backward Asymmetry in Top Production at CDF
)( 04.0)( 07.017.0, syststatA ppobsFB
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
MZ’<760 GeV for Z’/MZ’ =1.2%
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tt Resonance Searches at CDF
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
CDF latest result in search of Gtt
Test:0.05≤ Γ/M ≤0.5
Dynamical Likelihood Method used for the Mt
measurement
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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’ < 284GeV
at 95%C.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
stop mass 175 175 175 160 160 145 GeV
χ1± mass 135 120 105 120 105 105 GeV
σ(t1~ t1~) < 5.6 6.6 5.6 7.5 9.7 12.3 pb (95% CL)
+jets +jets
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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
Fit for f- and f+ gives:F0 = 0.42 0.17(stat) 0.10(sys)
F+= 0.12 0.09(stat) 0.05(sys)
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+
Phys Rev Lett 100, 062004 (2008)
* angle between lepton and top in W rest frame
dilepton
l+jets
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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 methodF0 = 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
R
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
-0.11Observe BR(t Hb) < 0.35 at 95%C.L.
BR(t Hb)
Feldman Cousins contours
For R and BR(t Hb)
Assumptions: MH+ = 80 GeV
and Hcs exclusive decays
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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
ΔR < 1.0
No signal observed: Exclusion contour BR versus MH+
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Other Measurements Simultaneous measurement of R and
ttbar cross-section R=BR(tWb)/B(t Wq) with q = d,s,b Extract (tt) without setting BR(tWb)=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 )
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Conclusions Presented several top quark results from 1-2.3 fb-1
Properties (charge) Production mechanism:
gg/qqZ’/G searchesAFB 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 quarksW 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)
Parameterize the reconstructed top mass distribution as function of width. Perform pseudoexperiments to check sensitivity.
t < 12.7 @ 95% CLt < 12.7 @ 95% CL