# Presentation: 707116

## Measurement of 
Top Quark Properties 
at the TeVatron

**Measurement of **
**Top Quark Properties **
**at the TeVatron**

**Jessica Lev****êque**
*University of Arizona*
-  on behalf of the CDF and DØ collaborations

## Top Quark Physics Overview

**Top Quark Physics Overview**

- Since its discovery in 1995 at Fermilab, the top quark has been intensively studied but most of its properties are still poorly understood due to statistically limited samples.
- Direct measurements of the top quark properties are necessary :  the constraints already set by precision electroweak measurements leave plenty of room for exotic behaviors regarding the charge, mass, branching fractions, decay and production modes.

- Top quark properties covered in this talk :

|   | Lifetime | L = 318 pb-1 |
| --- | --- | --- |
|   | Charge | L = 370 pb-1 |
|   | Search for resonant production | L = 682/370 pb-1 |
|   | Search for “heavy” top quark | L = 760 pb-1 |
|   | W boson helicity | L = 370 pb-1 |

## Top Quark at TeVatron

**Top Quark at TeVatron**

- At the TeVatron, the top quark is mainly produced in pairs via strong interaction.  In the following, we will assume :
-  = 6.8 ± 0.8 pb   [*]
- B(t→Wb) = 100%
- mtop =175 GeV/c2

- [*] Phys. Rev. D **68**, 114014

- The top quark properties are measured in events where at least one of the W bosons decays into leptons (e, , → e,)
-  Large branching fraction ~ 40%
-  Limited physics background
- Most of the analyses presented here use  b-jets identification to increase the fraction of signal in the selected data samples :
-  cut on the decay length of the secondary vertex found in jets

## Top Quark Lifetime at CDF

**Top Quark Lifetime at CDF**

- Method :  measure the distance between the initial collision point and the W boson decay vertex via the lepton impact parameter **d****0**
- Signal templates generated for top quark lifetimes c between 0 and 500 mcombined with background distributions and corrected with the detector resolution to model the impact parameter distribution in data.
- Detector resolution  measured with Z/ → e +e-, -
- Impact parameter of the lepton measured by fitting the data with templates using a maximum likelihood method

- Top quark lifetime constrained in Standard Model (SM) to be  less than 10-24 s  (c < 3.10-10 m) in absence of a 4th quark generation.
- First direct measurement of the top quark lifetime:
-   - Confirm the identity of the “top” candidates
-   - Detect unexpected production modes  through  new    long-lived particles

## Top Quark Lifetime at CDF

**Top Quark Lifetime at CDF**

- Maximum likelihood attained
- with SM template c = 0 m
- Limit :   c < 52.5 m   @ 95% C. L.

- For comparison :
-    b-hadrons c ~ 450 m
-    detector resolution ~ 40 m

- Top quark  selection :
-  1 lepton (e or ), 3 or more jets
-  1 b-jet tagged with secondary vertex
-  Missing transverse energy
-   / Ks /  conversions removed
- 97 electron tracks and 60 muon tracks selected

## Top Quark Charge at DØ

**Top Quark Charge at D****Ø**

- [*] Phys. Rev. D **59**, 091503 / **61**, 037301 / **65** 053002

- The SM predicts a top quark charge of +2/3e
-   Exotic model with a 4th generation of quarks and a Higgs triplet predict the existence of an additional heavy quark with a charge -4/3e  [*]

- Top quark selection :
-  1 lepton (e or ), at least 4 jets
-  2 b-jets tagged with secondary vertex
-  Kinematic fit used to reconstruct the top quarks and find the correct (lepton, b-jet) pairing
- Top (anti-top) charge defined as the jet charge plus (minus) the lepton charge
- b-jet charge measured with a jet-charge algorithm using tracks with PT > 0.5 GeV/c
- Jet-charge algorithm calibrated with data using semi-leptonic b-jet decays to determine the charge (corrected for c-jets contamination and B mixing)
-  - 17 top pair candidates selected in data
-  - 2 measurements per event

## Top Quark Charge at DØ

**Top Quark Charge at D****Ø**

- To test the Standard Model, a likelihood ratio is computed from data :
- psm(q i) = probability to observe the quark charge qi in the SM
- pex(qi) = probability to observe the quark charge qi in the exotic scenario
- The value  data = 11.5 is then compared to the expected distributions sm and  ex  derived from Monte-Carlo pseudo-experiments.
- The number of background events is allowed to fluctuate according to a Binomial distribution.

-  Exotic quark with charge 4/3e
- excluded  @ 94% C. L.

## Search for tt resonances

**Search for tt resonances**

- [*] Phys. Rev. D **92** 221801 / Phys. Rev. D  **85** 2065

- The search is model dependant.
- We consider values for the resonance mass MX in the range  [350 - 1000] GeV/c2
- and assume a width x = 0.012 Mx

- Monte Carlo
-  templates

## Search for tt resonances at CDF 

**Search for tt resonances at CDF **

- Top quark selection :
-  1 lepton (e or ), at least 4 jets
-  Large missing transverse energy
- Top quarks reconstructed with matrix element information and “transfer functions” describing the correlation between partons and jet energies.
- Background and top quark pair production normalized to SM expectations. The QCD contribution is estimated from data.

- No excess observed
- Exclude the production of a
- leptophobic Z’ with mass
- Mx < 725 GeV/c2     @ 95% C. L.

## Search for tt resonances at DØ

**Search for tt resonances at D****Ø**

- Top quark  selection :
-  1 lepton (e or ), at least 4 jets
-  At least 1 b-jet tagged with secondary vertex
-  Missing transverse energy
- Use a constrained kinematic fit to reconstruct the top quarks 4-vectors.
- Top quark and background processes normalized to SM predictions. The QCD multijets contribution is estimated from data.

- No excess observed
- Exclude the production of
- a leptophobic Z’ with mass
- Mx < 680 GeV/c2      @ 95% C. L.

## Search for heavy top quark at CDF

**Search for heavy top quark at CDF**

- Event selection :   1 isolated lepton, at least 4 jets, large missing transverse energy
- t’ reconstructed mass determined with the mass template method.
- Signal Monte-Carlo was generated with Mt’ varying in the range  [175 - 400] GeV/c2
- To discriminate the new physics signal from the SM processes, 2 variables are used :
-  Reconstructed quark mass Mreco
-  Total transverse energy in the event HT

- [*] “beautiful mirror model”  hep-ph/0109097

- Looking for a 4th generation of heavy quarks.
- Assumptions : produced in pairs, larger mass than the top quark, decaying promptly into W+b  [*]

## Search for heavy top quark at CDF

**Search for heavy top quark at CDF**

- No evidence for t’ signal observed
- Mt’ <  258 GeV/c2
- excluded @ 95% C. L.

## W boson helicity 

**W boson helicity **

- Assuming a massless bottom quark, the V-A structure of the weak interaction in the SM
- strongly suppress the fraction of right-handed
- W boson produced in t→Wb decay
- A measured value of f+ ≠ 0 would be
- a unambiguous signature of new physics

- Experimentally, the W boson helicity is measured from the cos *  distribution :
- * = angle between the charged lepton and the top quark boost direction measured in the W rest frame.

- SM prediction

## W boson helicity at DØ

**W boson helicity at D****Ø**

- Dilepton selection : 2 oppositely charged leptons (e or ), at least 2 jets, large missing transverse energy.  Additional kinematic cuts to suppress Z→l+l- events.
- Lepton+jets selection : at least one lepton and 4 jets. Cut on likelihood discriminant using topological and b-tagging variables to increase the signal fraction.

- Kinematic fit used to reconstruct the top quark pairs and the W rest-frame, assuming mtop = 175 GeV/c2
- In dilepton events, the system is underconstrained and gives two possible angle measurements : both are used in the fit to increase the statistics. The resulting distribution still shows discrimination between V-A and V+A models.

## Summary

**Summary**

- All top properties measurements consistent within errors with the Standard Model predictions
- The analyses are still statistically limited but were intensively optimized and already reached interesting precisions
- CDF and DØ have now more than
- 1 fb-1 of data recorded on tape
-  currently being analyzed
- Updates expected from both experiments
-  for summer conferences
- If the top quark is hiding something,
- we may figure it out next summer !

| Measurement | Best result |
| --- | --- |
| Charge | Rule out +4/3e model 
@ 94% C. L.
 |
| Lifetime | c < 53 m @ 95% C. L.
 |
| Search for heavy t’ quark | Exclude Mt’ in [196-207] GeV/c2  @ 95% C. L.
 |
| Search for tt resonance | Mx > 725 GeV/c2  
@ 95% C. L.
 |
| W boson helicity | 

 |