# Presentation: 544529

## 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%
- 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

## 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

## 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.

## 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

## 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)***** **

## 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

- 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

- Forward-Backward Asymmetry in Top Production at CDF

- CDF II

- tt frame

- pp frame

## 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%

## 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 Gtt

- Test:
- 0.05≤ Γ/M ≤0.5

- Dynamical Likelihood Method used for the Mt measurement

## 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.

## 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

## Scalar Top Search at CDF

- Search for tt pair production
- Dilepton channel
- Assumptions:

- Theoretical cross sections

## 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

## 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)

## FCNC search

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

## 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+0.12-0.11
- Observe 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 Hcs exclusive decays

## 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+

## 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 )

## 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

## 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% CL