# Presentation: 685342

## Top Pair Production Measurements at s1/2 = 2 TeV

**Robert Kehoe **
**Southern Methodist University**
**Department of Physics**
- October 27, 2005** **

**the newest quark**
**what are modes of production and Decay?**
**what are the final state kinematics?**
**is it really *****a la***** Electroweak theory?**
**a very large mass...**
** ****	****	**
**	****	****...****and a suspicious Yukawa ****	****	****	****coupling ~1**** !**
**need to convincingly find top with understood backgrounds**
**Then can measure it’s mass and other properties**

- Bob Kehoe - SMU

## Production and Signatures

- Even though the top mass is high, pair production wins over single top because it occurs strongly: 85% qqbar, 15% gluon fusion @ LO
- Theoretical cross section @ 1.8 TeV = 5.2 pb			        			        @ 1.96 TeV = 6.7 pb +/- 0.42 pb
- Expect a 30% cross-section increase
- Final states dictated by W decays:

**M. Cacciari et al. JHEP 0404:068 (2004) **
**N. Kidonakis and R. Vogt, Phys. Rev. D 68 114014 (2003) **

- All-jets
- (background: QCD)
- Single lepton 	(background: W+jets, QCD)
- Dilepton
- (background: Z/g, WW, W+jets)
- Keep track of ****** *********e*****,********

- W

- W

- Bob Kehoe - SMU

## Lepton+Track - CDF

- Avoid full inefficiency of lepton identification
- In 360 pb-1
- 46 observed, 14.6 ± 3.6 background

**1 lepton, 1 track**
**2 jets**
**Etmiss**

- mt = 178 GeV

- Bob Kehoe - SMU

## Dilepton - DØ

- Kinematics-based selection
- 13 evts, background 3.2 ± 0.7
- 28 evts, background 6.8+2.6-1.8

** **

** **

**  **

** **

**  **

** **

**   **

** **

**2 leptons**
**2 jets**
**Etmiss, High pT **

- mt = 175 GeV

- Bob Kehoe - SMU

## b-tagging

- Secondary vertex tagging
- Find primary vertex for event
- Reconstruct vertex from tracks in jet
- Select jets with positive tags
- based on decay length significance
- CDF has new tagger, more efficient
- Jet probability tagging
- Know track resolutions
- Calculate probability that tracks in jet originate from event primary vertex
- Select jets with low probability

- Bob Kehoe - SMU

## Single lepton - DØ 

- In 230 pb-1
- 370 pb-1 (prelim.)

- mt = 175 GeV

**1 lepton**
**3 or 4 jets**
**Etmiss**

- Bob Kehoe - SMU

## Single Lepton - CDF

- jet probability tag
- Look at single, double tags
- Pjet < 1%, 5%
- Very consistent results
- More top needed
- SecVtx tag
-  		           - Two taggers
-  		           - Compatible results

- mt = 178 GeV

- 318 pb-1

- Bob Kehoe - SMU

## Multi-Jets

- No required leptons, need to tag b-jets
- All-jets
- MEt + jets
-  		     enhance W

- Bob Kehoe - SMU

## Breakdown of Uncertainties

- Systematic uncertainties
- beginning to dominate lepton+jets
- Significant fraction of dilepton, multijet channel results

|  | D0 dilepton | D0 l+jets/ 2nd vtx | CDF l+jets/ 2nd vtx | CDF l+jets/ jet prob tag |
| --- | --- | --- | --- | --- |
| Tagging | --- | 0.4 pb | 5.2% | 7.2% |
| Lepton ID/ isolation | 0.4 pb | --- | 5.0% | 4.9% |
| Jet energy scale | 0.5 pb | 0.2 pb | 4.3% | 4.2% |
| Background model | --- | 0.4 pb | 4.4% | --- |
| Jet ID/ reconstruction | +0.5 - 0.4 pb | 0.2 pb | --- | --- |
| triggering | +0.7 - 0.4 pb | 0.3 pb | --- | --- |

- Bob Kehoe - SMU

## Summary Results

- Have gone from ~200 pb-1 to almost 400 pb-1 in current analyses

- Bob Kehoe - SMU