# Presentation: 474462

## tt Production Cross Section at CDF

- Petra Merkel – Purdue University
- For the CDF Collaboration
- Frontiers in Contemporary Physics III
- Vanderbilt University – 05/26/05

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## Top @ Tevatron

- Top is the only quark at weak scale
-  window to EWSB ?
- New physics may appear in production (e.g. topcolor) or in decay (e.g. charged Higgs)
- Top mainly produced in pairs at Tevatron
- (top pairs) ~ 6.7 pb : 85% qq annihilation + 15% gg fusion
- cf (single top) ~ 3 pb
- Extremely rare process (smallest cross section measured so far)
- About 1 tt event in 1010 inelastic pp collisions !

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- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## CDF @ Tevatron

- 1.96 TeV pp collisions are collected for top physics by triggering on one high-Pt lepton, on jets or on missing Et
- Presenting measurements with data collected up to 09/2004, corresponding to up to ~350 pb-1
- (cf ~108 pb-1 in RunI)

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- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## Top Production and Decay

- Final States
- Dileptons:
- BR=11% (ee/e/ only = 5%) cleanest sample, lowest statistics
-  Lepton+Jets:
- BR=44%  (e/ + jets = 29%) golden channel w/ high statistics and reasonable S/B
- All-Hadronic:
- BR=44%,  most challenging channel w/ high statistics but large backgrounds

- Cacciari *et al., JHEP* **0404**, 068 (2004)
- Kidonakis and Vogt, *Phys. Rev*. **D68**, 114014 (2004)

- SM:

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## B Jet Tagging Algorithms

- Displaced vertex
- reconstruction with
- silicon detector:
- B hadrons travel ~3mm
- before decay w/
- large track multiplicity

- 2ndary Vertex Tag

- Soft Muon Tag

- Jet Probability Tag

- Top Event Tag Efficiency	58-63%
- False Tag Rate (QCD jets)	0.8%

- look for relatively low
- momentum m in or
- near the jets that
- result from
- semileptonic decays
- of B hadrons

- Top Event Tag Efficiency	15%
- False Tag Rate (QCD jets)	3.6%

- Determine probability
- for jet to originate from
- b or c or light quarks
- based on signed
- impact parameter
- distribution of all tracks
- in the jet

- Top Event Tag Efficiency	57%
- False Tag Rate (QCD jets)   1.2%

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## Lepton+Jets Channel:2ndary Vertex b-tag

- NEW !

- 318 pb-1

- 1 isolated high-Pt e or  (Et(Pt)>20 GeV)
- Missing Et > 20 GeV accounting for escaping 
- ≥3 jets (Et>15 GeV) w/ ≥1 b-tags
- b-tag: *2**nd**ary vertex tagger*
- Optimize S:B by cuts on Mt>20 GeV (leptonic W) and Ht>200 GeV (scalar sum of total energy in event)

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## Results

- ≥1 b-tag

- ≥2 b-tags

- (previously published with 162 pb-1 in PRD 71, 052003)

- signal region

- signal region

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## Lepton+Jets Channel:Event Kinematics + Neural Net Fit

- Independent of b-tagging
- Select events w/ 1 e or t and 3 jets
- Neural Net: feed forward, 7 input variables, 7 hidden nodes, 1 output node
- Input variables: Aplanarity, Max-Jet-Rapidity, Min-Dijet-Separation, Ht, jet(3,4,5)Et, Pz/Et, Min-Dijet-Mass
-  3-Jet Sample:
- a priori most sensitive
-  4-Jet Sample:
- better S:B  cross check

- NEW !

- 347 pb-1

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## Results

| Sample | Events | Fitted tt | (tt ) |
| --- | --- | --- | --- |
| W +  3 jets | 936 | 148.2  20.6 | 6.0  0.8  1.0 pb |
| W +  4 jets | 210 | 80.9  15.0 | 6.1  1.1  1.4 pb |

- Fit: tt and W-like float
- QCD fixed to 4.6%

-  vs mtop

- (hep-ex/0504053, submitted to PRD)

- evolution of result w/ mt

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## Lepton+Jets Channel: 2ndary Vertex b-tag + EtleadJet Fit

- (Published in PRD 71, 072005)

- 162 pb-1

- Use semileptonic b-tagged tt events with 3 jets
- Take signal and background shapes in leading jet Et from MC and data
- Fit leading jet Et distribution in data to templates:
**signal fraction:                            in 57 events**** **

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## Lepton+Jets Channel:Soft Muon b-tag

- 200 pb-1

- In events w/ 1 e or t and 3 jets require 1 b-tag
- b-tag: *soft muon tag*
- Reduce background by Ht>200 GeV cut (scalar sum of total energy)

- signal region

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## Lepton+Jets Channel:Jet Probability b-tag

- 162 pb-1

- In events w/ 1 e or t and 3 jets require 1 b-tag
- b-tag: *jet probability tag:*

- signal region

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## Dilepton Channel:Counting Experiments

- (Published in PRL 93, 142001)

- 200 pb-1

- signal region

- signal region

- Tight selection
- Two well identified leptons  (e or ): ee, e, 
- Large MEt
- 2 jets, no b-tagging

- Loose selection
- One well identified lepton (e or ) + isolated track: e+track, +track
- Large MEt
- 2 jets, no b-tagging

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## Dilepton Channel:Global Fit

- 193 pb-1

- Global fit to isolated high-Pt dilepton data (ee/e/) in Njet - MEt plane
- Fit data to shapes from templates from SM processes
- Signal(s) floating in fit: tt, WW, Z
- Backgrounds fixed in fit: WZ, ZZ, Zee, Z, bb, W, W+jets

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## All-Hadronic Channel: 2ndary Vertex b-tag 

- 165 pb-1

- Use dedicated trigger: 4 jets w/ Et>15 GeV and Etevent>125 GeV
- S:B of 1:2500 increased to 1:24 with Etjets and topological cuts (aplanarity, centrality)
- Require 6 – 8 jets, and 1 b-tag (using the 2ndary vertex algorithm)

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## All CDF RunII results

- Observed cross sections consistent with each other and with the SM prediction

- Working on combination

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## tt Cross Section vs Energy

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF

## Conclusions & Outlook

- All measurements consistent with SM
- Current measurements will continue to improve
- Doubled data set each year
- Reduction of systematics through better understanding of detector
- New techniques employed
- Expect ~10% precision on (tt) with 2 fb-1 (now ~20%)
- Top is major background for new physics at LHC
- Concise understanding of top sector before LHC turns on is crucial

- P.Merkel - 05/26/05 : TTBAR PRODUCTION CROSS SECTION AT CDF