# Presentation: 108209

## CDF Measurements of Top QuarkCross Section and Mass

- Jason Nielsen
- (LBNL)
- for the CDF Collaboration

## Why Study Top Quark Physics?

- Heavy enough to decay to exotics

- CDF/D0
- 2 fb-1goal

- Fits into third generation - CKM

- Large mass interesting in itself

- On-shell charged Higgs, SUSY

- Mass near EWSB scale

- Central role in these theories
- Corrections to MW  mt2, ln(MH)

- Just lighter than tungsten nucleus

- Lake Louise Winter Institute, Feb. 2004

## Top Quark Production and Decay

- Top event signatures (from W bosons)

- Expect 2 b jets from top pair production

- Pair production:

- Cross section increases ≈30% with Tevatron √s increase to 1.96 TeV

- Central, spherical events
- Large transverse energy

- (Single top production is factor of 2 smaller)

- Lake Louise Winter Institute, Feb. 2004

## Tevatron Run 2 at Fermilab

- Record luminosity 5.9E31
- 3pb-1 integrated in a single store
- First store w/ antiprotons from Recycler

- Tevatron successes in early 2004:

- New CDF results use 200pb-1
- collected through Oct. 2003
- (cf. 110 pb-1 from Run 1)

- Lake Louise Winter Institute, Feb. 2004

## CDF II Detector at the Tevatron

- Continuing work to incorporate upgraded detectors in data analysis
- Accurate detector simulation vital to precision physics measurements

- Lake Louise Winter Institute, Feb. 2004

## Top Data Samples

- Define samples counting leptons and jets
- Establish component contributions to samples

- Optimize event selections for
- top physics and new physics

- Cross section results validate top-enriched samples

- can also point toward new physics

- In both cases, the sample
- composition is important

- Develop tools to increase purity

- Lake Louise Winter Institute, Feb. 2004

## Top Dilepton Cross Section

- 2 lepton + 2 jets sample is small but very clean for top signal

- Lepton + track sample has looser ID requirements for second lepton

- Sensitive also to  lepton final states

- Tight e/ selection complemented by e/ + tracks selection

- Lake Louise Winter Institute, Feb. 2004

## Tagging Tools: Vertexing and Soft Muons

- B hadrons in top signal events

**are long-lived and massive**

**may decay semileptonically**

**Top Event Tag Efficiency**
**False Tag Rate (QCD jets)**

**55%**
**0.5%**

**15%**
**3.6%**

- Identify low-pt muon from decay

- Vertex of displaced tracks

- Lake Louise Winter Institute, Feb. 2004

## Cross Section Results using Tagging

- Estimate backgrounds in the lepton + jets sample from first principles:
- Using data as much as possible (non-W QCD, fake tags)
- Some MC calculations for diboson and W + heavy flavor backgrounds

- Counting experiments with vertex tag and soft muon tag in 3,4-jet bins

- Lake Louise Winter Institute, Feb. 2004

## Kinematic Fits to Lepton + Jets Sample

- Vertex-tagged sample

- Provides another way of estimating the background contribution

- Sample before tagging

- Lake Louise Winter Institute, Feb. 2004

## CDF Top Cross Section Summary

- 4.7 ± 1.6(stat.) ± 1.8 (syst.)

- 7.6 ± 3.4(stat.) ± 1.5 (syst.)

*108 pb**-1*

*126 pb**-1*

*108 pb**-1*

*195 pb**-1*

*202 pb**-1*

*126 pb**-1*

- Lake Louise Winter Institute, Feb. 2004

## Comparison with Theory

- Lake Louise Winter Institute, Feb. 2004

## Search for Single Top Quark Production

- Search strategy employs

- Likelihood fit to HT (combined search)
- Likelihood fit to b Q*(t-channel only)

- 0.880.11 pb

- Best hope for constraining Vtb at the Tevatron

- 1.980.24 pb

- Final state is W + exactly 2 jets

- Tag one b jet

- Q of lepton,  of light quark jet

- Lake Louise Winter Institute, Feb. 2004

## Single Top Search Results

|  | combined | t-channel |
| --- | --- | --- |
| t-channel | 2.39±0.56 | 2.34 ±0.54 |
| s-channel | 1.19 ±0.25 | 1.16 ±0.24 |
| tt | 3.47 ±1.04 | 3.39 ±1.02 |
| non-top | 20.7 ±4.1 | 17.4 ±3.3 |
| expected | 27.8 ±4.3 | 24.3 ±3.5 |
| observed | 28 | 25 |

- Cross section limits at 95% confidence level:

- Combined search:  < 13.7 pb (a priori sensitivity 14.1 pb)
- T-channel search:  < 8.5 pb (a priori sensitivity 11.3 pb)

- Sample composition

- Lake Louise Winter Institute, Feb. 2004

## Top Mass Measurement Challenges

**Many combinations of leptons and jets: which one is correct?**

- Choose assignment kinematically most consistent with top
- Use all combinations, but weight them

**Link observables to parton-level energies**

- Detector - Generation -Simulation
- Largest uncertainties come from this
-   difficult relation

- Derive mass templates from top MC and fit data to most likely template
- Add event kinematic information, possibly including matrix element

**Final likelihood fit methods**

- Lake Louise Winter Institute, Feb. 2004

## Top Mass Results: Dilepton Reconstruction

- Likelihood fit result: mtop = 175  17(stat.) 8(syst.) GeV/c2

- For each lepton-b pair assignment

- 6 dilepton candidates in 125.8 pb-1

- Kinematic fit is underconstrained

- Calculate best raw mass, from
-   most probable combination

- Fit to signal and background templates

- Lake Louise Winter Institute, Feb. 2004

## Top Mass Results: Vertex-Tagged Events

**   **

-  22 vertex-tagged events from lepton+4 jet sample

- Choose single combination with best 2 from over-constrained fit

- Compare to templates from

- Top signal MC
- Backgrounds (fixed relative)

- Reconstruct mt via kinematic fit

- Lake Louise Winter Institute, Feb. 2004

## Summary

- Quality top physics results arriving now
- Early CDF passes on Run 2 data complete
- No unexpected physics seen so far...
- Uncertainties will decrease; no surprises here
- Tevatron success - more data - precise results!
- The near future is rich with opportunity.
- Improvements in current measurements
- Plenty of new measurements and searches
- W helicity, top branching ratios, top resonances, all-hadronic channels

- Lake Louise Winter Institute, Feb. 2004