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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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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Top Production and Decay

p-pp

t

t-X

b

b-

W-

W+

l-

q

q’-

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

(tt ) 6.7 0.90.7 pb

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

SM:

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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%

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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: 2ndary vertex tagger

Optimize S:B by cuts on Mt>20 GeV (leptonic W) and Ht>200 GeV (scalar sum of total energy in event)

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Results

≥1 b-tag ≥2 b-tags

tt (≥1 b-tag) 7.9 ± 0.9 (stat) ± 0.9 (syst) pb

tt (≥2 b-tags) 8.7 ± 1.7 (stat) ± 1.5 (syst) pb

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

signal region

signal region

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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

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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 floatQCD fixed to 4.6%

vs mtop

(hep-ex/0504053, submitted to PRD)

evolution of result w/ mt

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

leadJet 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 )(68.0 14.0

16.0 stat

pb )(2.1)(6.10.6)( syststattt

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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)

pb )()(2.5)( 3.10.1

9.29.1 syststattt

signal region

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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:

pb )()(8.5)( 3.13.1

3.12.1 syststattt

signal region

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Dilepton Channel:Counting Experiments

(Published in PRL 93, 142001)

pb )(4.0)()(0.7)( 6.11.1

4.21.2 lumsyststattt

200 pb-1

signal regionsignal

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

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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, Zee, Z, bb, W,

W+jets

(tt ) 8.6 2.42.5(stat) 1.1(syst) pb

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All-Hadronic Channel: 2ndary Vertex b-tag

165 pb-1

pb )()(5.28.7)( 7.43.2 syststattt

Use dedicated trigger: 4 jets w/ Et>15 GeV and Et

event>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)

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All CDF RunII results

Observed cross sections consistent with each other and

with the SM prediction

Working on combination

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tt Cross Section vs Energy

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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