# Presentation: 346175

## Top Physics: CDF Results

- Ken Bloom
- University of Michigan
- for the CDF Collaboration

- Moriond EWK
- March 23, 2004

## Why care about top?

**Quantum numbers?**
**Production properties?**
**Decay properties?**

**Relation to other particles?**
**Is it special?**
**Clues to new physics?**

- CDF/D0
- 2 fb-1
- goal!

**1) Identify top events  2) Measure top properties**

## SM top-physics overview

- Top-antitop pair production via strong interaction, assume
- B(t→Wb) = 100%, label final states by W decays – dilepton, lepton + jets, all-hadronic.

- EW single-top production, x2 smaller rate, not (yet) seen

- b quarks identified
- by displaced vertices,
- lepton-enriched jets

## Finding top – decay modes

**dilepton**

**lepton + jets**

- S/B **»** 1.5-3.5, 4-6 events/100 pb-1
- Dominant backgrounds:
- Drell-Yan, fakes

- S/B **»** 0.2-3.0,
- 25-45 events/100 pb-1
- Dominant background: W+jets

## Dilepton-mode results

- 19 events on 7.1 ± 1.2 background
- 11 e-track, 8 -track
-  = 6.9 +2.7-2.4 ± 1.3 pb

- 13 events on 2.4 ± 0.7 background
- 1 ee, 3 , 9 e
-  = 8.7+3.9-2.6 ± 1.5 pb

**lepton-**
**lepton**

**lepton-**
**track**

- Search for two identified leptons, or one lepton plus one isolated track

## Lepton + jets results: counting

- Require ≥ 1 b-tagged jets to reduce background, count ttbar candidate events
- Predict rates for SM non-top processes in tagged W+jets from first principles, excess in ≥3 jets is top
- Use data as much as possible (non-W QCD, fake tags)
- Get help from MC when necessary (diboson, W+heavy flavor)
- SVX = 57 events on 23.4 ± 3.0 bkgnd, SLT = 18 events on 12.9 ± 2.6 bkgnd

- Vertex tag

- Lepton tag

## Lepton + jets results: fitting

- Without b-tag: MC shapes for
- W+jets backgrounds, energy-scale
- and modeling uncertainties dominate
- (13 ± 4)% signal

- With SVX tag: W+jets background shape from non-tagged data, smaller systematics
- (67+13-16)% signal

- Fit kinematic distributions to signal, background shapes

## Lepton + jets: neural nets

- ttbar and W+jets kinematics differ modestly, but do so in many different variables.
- Pick variables that are largely uncorrelated, or correlated differently for signal, background.
- Develop a neural net to use this information optimally.

## Lepton + jets: NN results

- NN gives good separation of signal, background
-   = 6.7 ± 1.1 ± 1.6 pb
- Statistical, systematic uncertainties improved compared to single-variable fit.  No b tagging!

**NEW!**

## Cross-section summary

- 162 pb-1

- 126 pb-1

- 162 pb-1

- 195 pb-1

- 195 pb-1

- 200 pb-1

- 193 pb-1

## Single-top (EW production) search

- 1.980.24 pb

- 0.880.11 pb

- Direct determination of |Vtb|2
- Search for W+2 jets, tag one b
- Likelihood fit to HT (combined),
- Ql*q (t-channel)

- 95% CL limits:
**Combined: ******** < 13.7 pb**
**t-channel: ******** < 8.5 pb**

## Branching fractions I

- Dilepton and lepton-plus-jets cross-section values assume t→Wb always, never t→Xb.
- Ratio of measured ’s (R=ll/lj) tests that assumption:
- Should be unity – tests consistency
- Ratios are good:
- Many systematic uncertainties cancel
- Independent of theory value of  (PDF’s, mt)
- Sensitivity to non-SM decays of top, e.g. t → H+b, different tan  gives different mix of ll/lj

## ratio, non-SM limits

- Create probability distribution for R:
- R = 1.45+0.83-0.55,
- 0.46<R<4.45 @ 95%CL
- Set limits on non-SM BR’s of top assuming “simple” model -- detect non-SM decays with same efficiency as SM:
-  BR < 0.46 for additional t→Xb all-hadronic decay

**Preliminary**

- w/o cancellation

- w/cancelllation

**NEW!**

## Branching fractions II

- Cross section measurements assume t→Wb always, never t→Wq.
- b-tag rates in tt events test that assumption, depend on B(t→Wb) = *b*, tag efficiency = :
- Always measure product *b*: assume  and measure *b*, or vice-versa.

## Measuring b, 

- Find most likely *b* in SVX-tagged sample:
*b* = 0.25+0.22-0.18,
- → *b* = 0.54+0.49-0.39,
-   consistent with measurements in calibration samples
*b* > 0.12 @ 95% CL

**NEW!**

## Top mass: dileptons

- Underconstrained problem – scan over  directions, weight by pT of ttbar system.
- Choose solution with best kinematic consistency.
- 6 events in 126 pb-1
- Systematic uncertainty dominated by jet-energy scale

**m****t**** = 175 ± 17****stat**** ± 8****sys**** GeV**

## Top mass: lepton + jets

- Choose events with 4 jets, ** ̧** 1 vertex tag
- Overconstrained -- select combination of jets most consistent with ttbar hypothesis in kinematic fit
- 22 events in 108 pb-1
- Systematic uncertainty dominated by jet-energy scale.

**m****t**** = 178 ****+13****-9stat**** ± 7****sys**** GeV**

## Outlook -- CDF

- No doubt about Run 2 top-quark signal:
- Seen in multiple channels
- Seen with multiple techniques (e.g. new NN)
- In agreement with theory, each other
- Studies of top-quark properties beginning:
- First Run 2 measurements of branching ratios
- Mass:  adding more data, new techniques, reducing systematics – new results imminently!
- Start of a broad program: W helicity, charged Higgs search, resonant production, spin correlations....
- More data and more news soon!