Emanuela Barberis

1
Top Quark Measurements at the Tevatron

• Emanuela Barberis
• Northeastern University, Boston
• for the CDF and DØ collaborations

- Introduction Top quark physics at the
Tevatron - Production of Top (Run II)
- Top mass and properties (Run III) -
Summary and outlook
2
CDF and DØ at the Fermilab Tevatron
Run II proton antiprotons collide_at_
, upgraded detectors, higher
luminosity
Triggering and reconstruction of a top event
involves all aspects of the detector Run II
measurement progress in operation and
understanding of the new detectors
3
Top Quark Production and Decay

• in proton antiproton collisions
• at Tevatron energies, top quarks
• are primarly produced in pairs

• Br(t?Wb)100

Both Ws decay via W?l? (le or ? 5)
dilepton One W decays via W?l? (le or ? 30)
leptonjets Both Ws decay via W?qq (44) all
hadronic
EW single top production not yet observed
4
Top Quark Physics in Run II
Run I discovery

• Run II
• with high precision
• we hope to answer
• questions such as
• Why is top so heavy ?
• Is it or the third generation special ?
• Is top involved with EWSB ?
• Is it connected to new physics ?

5
Production cross section
6
Run I cross section results 100 events
Measured in Run I in all decay channels and using
different techniques b tagging, kinematic
selection, Neural Networks

• Run II top cross section (1.96 TeV)
• 30 higher than Run I (1.8 TeV)

7
in the dilepton channel, Run II
8
Event selection
Backgrounds

• WW/WZ, Z? tt determined with MC
• Z/g from data, QCD from data

79 pb-1
9
Run I
Run II
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dilepton candidate, CDF Run II
??- 2 jets
Lego view
µ2
µ1
Jet1
Jet2

• muon
• electron
• photon

µ1
Primary vertex
µ2

• muon
• electron
• hadron

Jet1
Vertex view
11
DØ Run II preliminary
Event selection
Backgrounds

• WW, Z? tt determined with MC
• Z/g, Wjets and QCD from data

DØ Run II preliminary
mmjets
ET (GeV)
candidates
Mmm (GeV)
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33pb-1
42pb-1
48.2pb-1
Jet1
Tracking view
Jet2
e?- 2 jets
13

• Larger yield, and still relatively clean

jet
Event preselection

• 1 high PT isolated charged lepton (e,m).
• Neutrinos large missing ET
• Large jet multiplicity
• dilepton veto

?
Backgrounds
p
b

• Wjets and fake leptons in QCD

jet
Further selection techniques
jet

• from kinematic
• 4 jets (DØ)
• tag b jets with displaced VTX
• 3 jets, 1 b tag (CDF)
• tag b jets with Soft Lepton Tag
• 3 jets, 1 SLT tag (DØ)

jet
t(?Wb) t(?Wb)
e,m
qq
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, CDF Run II
Event selection
jet
Method
Secondary vtx

• Look for displaced vertices
• ( 2 tracks), jet is tagged as
• b jet if Lxy/?xy gt3

displaced tracks
Lxy
do
Primary vtx

• Efficiency of b tagging a tt event
• measure in tt MC, apply Data/MC scale factor

prompt tracks
?(event tag) 45 ? 1 ? 5
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, CDF Run II
, CDF Run II
Backgrounds

• Mistags
• from tagged jets with
• Lxylt0 in inclusive jet data
• Wheavy flavor
• from Wjets data, b tag
• rate, and Run I flavor
• composition
• Non W
• from data
• WW, WZ, Z?tt
• from MC

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, CDF Run II
57.5 pb-1
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leptonjets candidate, CDF Run II
Jet2
Jet3
? 4 jets
Jet1
Jet4
µ

• muon
• electron
• photon

Lego view

• muon
• electron
• hadron

18
DØ Run II preliminary
Backgrounds (analysis I, topology)

• QCD multijet evaluated from data vs. Njets
• ejets due to fake jets (po and g)
• mjets due to heavy flavor decays
• Estimate real W4 jets with scaling law

Scaling of W for N jets 4 jets
DØ Run II preliminary
19
, DØ Run II
Topological cuts

• 4 jets
• HTgt180 GeV (e)
• Aplanaritygt0.06
• HT(jets,pWT)gt220GeV (m)

Missing ET
Missing E T
ejets candidate (with lifetime tag)
e
e
e
e
e
5 jets, ET 32.1 GeV, HT 422.6 GeV
Aplanarity 0.17, pT(e) 34.2 GeV
40-49.5 pb-1
b-tag jet
SV
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, DØ Run II SLT

• Event selection
• same preselection as kinematic analysis
• 3 jets
• softer topological cuts
• HTgt110 GeV
• Aplanaritygt0.04
• soft muon within jet (b?m, b?c?m)

Backgrounds QCD and Wjets determined from data
DØ Run II preliminary
DØ Run II preliminary
ejets
mjets
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DØ Run II preliminary
leptonjets channels only
all combined
22
Mass and properties
23
The top quark mass
The top quark mass, mt, enters as a parameter in
the calculation of radiative corrections to
other Standard Model observables

• mt can be related, with
• mW, to the Higgs mass

• mt is roughly ½ the
• vacuum expectation
• value of the Higgs field

• better understanding
• of EWSB mechanism?
• to perform more precise
• tests we need more data
• and improved techniques

24
Top quark mass, CDF Run II

• 33 candidates after event selection
• (same as for cross section, no b tags).
• 2C fit, 24 permutation,
• mass constraints
• Maximum likelihood fit

Will improve with detector understanding
mfit (GeV)
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b-tagged Top quark mass, CDF Run II

• Cleaner sample smaller
• combinatorics, smaller
• background
• relaxed 4th jet requirement
• 11 candidates

Work in progress
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Optimized techniques, DØ Run I mt
Likelihood method using most available information
resolutions, reconstruction effects
Measured
to be estimated
Acceptance
Matrix Element
PDFs
LO ME used, 4 jets required exclusively,
additional cut on background probability (to
improve purity) ? 22 events
Likelihood definition estimate signal
and background fractions and mt
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Optimized techniques, DØ Run I mt
mtop
(5.6 GeV from PRD 58 052001,1998)
large improvement on the statistical uncertainty
(2.4? stats)
mW
Expect a substantial improvement in the JES
systematic, and the application of this technique
to many measurements
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Top properties, W-helicity, CDF Run I

• Measurements of top properties test the
• Standard Model nature of top
• CDF new Run I W helicity measurement
• dilepton and leptonjets events
• with 1 and 2 SVX b tagged jets, fit to
• of data to VA and models
• Preliminary (fVA0 in SM fVA1 if all VA)
• expect 0.1stat 0.1sys for Run IIa

M2lb ½ (M2T M2W)(1 cos y )
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Conclusions
Cross section from Run II (CDF), dilepton and
ljets Cross section from Run II (DØ),
combined Mass from Run II (CDF) Improved
mass from Run I (DØ) W helicity from Run I
(CDF)

• Run II has started,
• we re-establishing signals and
• preparing for the top physics of
• large datasets

• Precise knowledge of mt (1 GeV) to
  constrain Higgs/SM extensions.
• EW single top production, top width
• Couplings to W,g,g,Z, new physics