# Presentation: 214063

## Top Decay Properties and Search for Single Top Productionat the Tevatron

**in behalf of the CDF and DØ Collaborations**
**Ursula Bassler, LPNHE-Paris**

## Top Quark Properties 

**top decay properties:**
-  W helicity measurements:
- SM excluded right handed W polarization
-  search for tH+b
- prediction of light, charged Higgs in SSM
-  BR(tWb)/BR(tWq)
- contribution from FCNC or  4th generation quark
**single top search:**
-  ultimate measurement of |Vtb|

** ****|V****tb****|**

**W helicity**

## W Helicity in Top Decays

- helicity states of the W:

## W Helicity Measurement

**W****L**

**W****R**

**W****0**

**SM ******** 70%W****0**
**+30% W****L**

**t**

**W polarizations can be disentangled:**
- cos * distribution
- lepton pT distribution
- **longitudinal W-polarization:**
- - leptons are emitted perpendicular
- - harder pT distribution
- **left-handed W: **
- - leptons are emitted opposite
- - soft pT distribution

## Longitudinal W Polarization

**likelihood analysis of p****T**** spectrum **

**combined lepton+jet and di-leptons samples**

**likelihood analysis of cos************* **

**combined lepton+jet sample**

**main systematic uncertainties: background normalization, m****t**

## Limit on Right-Handed W

**topological selection**

**b-tagged selection**

** ****analysis of lepton+jets sample (169 pb****-1****)**
** ****simulate templates for various f****+**** contributions, f****0**** fixed at 70%**
** ****likelihood analysis**

**systematic uncertainties: background normalization, m****t****, jet energy scale**

## MSSM Search for tH+b

- for charged Higgs-bosons with mH< mt , thedecay tH+b modifies tt event topologies according to H decay modes
**large tan********:** 	H 	excess of  decays
**small tan********:** 	H cs	excess in all-hadronic decays
- 		         	H Wbb 	2 extra b jets

**inputs for H ******** limits:**
******(tt********dilepton)**
******(tt********lepton+jets)**
**limit on ********(tt********lepton+********)**

## BR tWb/tWq

- Unitarity constraint on the CKM-matrix with 3 families
- Br(tWb) at nearly 100% in the SM

-  model independent limit on R using tag-rate Rb
- 	for 0-,1- and 2- tagged events:
- 			0=(1-Rb )2  1=2Rb(1-Rb) 2=(Rb)2
- estimate background level in tt-sample
- compare tag rates with expectation
- compute most likely value of Rb and set lower limit on R

- CDF: 0-, 1- and 2-tagged lepton+jets and di-lepton samples
- DØ: 1- and 2-tagged lepton+jets samples with two different b-tagging 		algorithms

## Determination of R

**DØ:**

**tagging probability from simulation ********b****=0.436±0.032**

**CDF:**

**impact parameter algorithm for b-tagging:**

**secondary vertex algorithm for b-tagging:**

******measurements compatible with SM**
******independent cross-section determination**
******main systematic uncertainties: difference of b-tagging probability between the data and the simulation**

## Single Top Production

**Single top quarks produced by weak interaction are a direct probe of top quark’s weak couplings.**
**Measure *****V******tb***** without assuming three-generation unitarity.**
**Cross section is close to top quark pair production cross section (2.9 pb vs. 7.5 pb), but: signal/background is much worse**
**fewer jets, softer kinematics, lower acceptance**
**signature: isolated lepton, missing E****T****, jets********2**
**s-channel: 1 b-tag  ****	****   t-channel: 2 b-tags**
**major backgrounds: W+jets, top-pairs, fake leptons**
******Single top production has not been observed yet!**

*s*-channel
- s ~ 0.9 pb

*t*-channel
- t ~ 2.0 pb

-  new charged particles

-  anomalous couplings, FCNC

## Single Top Search

**CDF:**

***s*****-channel****:***  ****s* < 13.6 pb (95% CL)
**	****expected limit: ***********s***** < ****12.1**** ****pb**

***t*****-channel****:***  ****t* < 10.1 pb (95% CL)
**	****expected limit: ***********s***** < ****11.2**** ****pb**

**combined analysis: *****s+t* < 17.5 pb (95% CL)
**	****expected limit: ***********s***** < ****13.6**** ****pb**
**	****most likely  cross-section:**

-  event shape analysis
-  after selection 42 events observed 38.1 ± 5.9 expected in combined channel
-  QL•b distribution to disentangle s and t channels

## Single Top Search

**DØ: 230 pb**-1
**2 analysis strategies after pre-selection: cut-based analysis and Neural Network analysis in lepton+jets samples**
**detailed study discriminative power of 25 kinematic variables: object kinematics, global event kinematics and angular variables**
**cut-based analysis using the 6-7 most efficient cuts**
**major systematic uncertainties:b-tagging simulation, JES**

## Neural Network Analysis

**DØ: 230 pb**-1
**training of 4 different Neural Networks in order to separate the Wbb and the tt-background in the single and the double **** ****tagged channels separately**

** ****good separation for tt-background, less for Wbb**
** ****2 dimensional kinematic likelihood on tt and Wbb NN-output**

## Single Top Results

**Single Top Results**

- cut based analysis:
*s*-channel:*  **	****s* < 10.6 pb (95% CL)
- 	**expected limits: ***********s***** < ****9.8**** ****pb**
*t*-channel:*  **	****t* < 11.3 pb (95% CL)
**	****expected limits: ***********t***** < ****12.4**** ****pb**
**NN-based analysis:**
*s*-channel:*  **	****s* < 6.4 pb (95% CL)
**	****expected limits: ***********s***** < ****4.5**** ****pb**
*t*-channel:* **	****t* < 5.0 pb (95% CL)
**	****expected limits: ***********t***** < ****5.8**** ****pb**
-  single top production expected with the Tevatron Run II data
-  improvements on acceptance and systematics ongoing in CDF and DØ

- comparison of exclusion limits in the s (tb-muon) and t (tqb-electron) channels
-  close in sensitivity to non SM contributions such as FCNC or 4th quark family

## Summary

** ****Study of top-properties allows to investigate in detail Standard Model prediction**
****** ****increased statistics will allow for precision studies**
** ****No deviations from Standard Model expectations have been observed so far**
****** ****room for new physics**
** ****Substantially improved limits for single top search with respect to Run I results**
******further improvements in analysis will allow to reduce the integrated luminosity necessary for single top observation **