Recent top quark results from D

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Recent top quark results from DØ

• Amnon Harelaharel_at_fnal.gov
• for the DØ Collaboration

International Conference on Particles and
Nuclei Eilat, Israel, November 9th November
24th 2008
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The top quark

• Extremely massive mt gold atom
• Discovered in 1995 by CDF and DØ at the Fermilab
  Tevatron
• Only o(104) were ever produced
• Only quark that decays before hadronizing
• Can measure bare quark properties

• Two such measurements by the DØ Collaboration
• Measurement of W helicity in t?Wb
• Measurement of top mass using the matrix element
  method

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The experiment
CDF
DØ
Tevatron
Highest energy in the world ! Highest
instantaneous luminosity in the world !
Enjoying it while it lasts
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Top quark measurements at DØ
Top Pair Production
µ
production cross section production
asymmetry resonant production? ttH production?
production?
?µ
Branching ratios non SM decays, CKM matrix
b
W
top mass top charge
(1/6)
W Helicity
Single Top Production
Production Cross SectionAnomalous Couplings W
Search
t
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Data samples
Run IIb,dilepton New 1.7 fb-1
Run IIb,ljets New 1.2 fb-1
Run IIa, Published 1 fb-1
DØ upgrade
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A model-independent measurement ofW helicityin
top decays
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W helicity
So far measurements support the SM prediction
f(t?Wb) 100 Breaking it down by W helicity
states
Longitudinal?0 SM 69.6
Right handed?1 SM 0.1
SM uncertainties ltlt Experimental uncertainties ?
cant constrain SM parameters Firm SM prediction,
in particular tiny f ? looking for new physics
Distinguish between helicity states by
reconstructing cos ??
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ljets sample
W helicity
isolated, pTgt20 GeV, ?lt1.1(e) / 2.0(µ)
METgt20GeV, triangle cut on ?F(l,MET)
note any b-tags
4 Jets (pTgt20 GeV, ?lt2.5)

• Signal and Wjets templates from MC.
• Matched ALPGEN Pythia
• VA and V-A signal MC reweighted to yield desired
  cos ? distributions
• Data and MC are compared in control samples
  corrections applied for residual discrepancies
• Their amounts from fit to data sample.

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ljets reconstruction
W helicity
Partons
Observed objects
Parton-level
Particle-level
Detector-level
QCDSimulation
Experimental resolutions b-ID probabilities

• fit partons to the measured objects, minimizing
  a ? 2
• Constraints mt1 mt2 172.5 GeV mW1 mW2
  80.4 GeV
• Do the fit for every combination of assigning a
  jet to a parton

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ljets reconstruction results
W helicity
Parton level
Excellent cos ? reconstruction!
Fitting f0, and f rather than V-A vs. VA ? Can
also use the hadronic W to fit f0
Reconstructed
Leptonic W
Hadronic W
Cant distinguish up and down type quarks
Acceptance
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eµ sample
W helicity
Isolated µ, pTgt15 GeV, ?lt2.0
2 Jets (pTgt20 GeV, ?lt2.5)maybe a couple of
b-tags
Discriminant construction and fit procedures
similar to those in ljets
Isolated e, pTgt15 GeV, ?lt1.1 / 1.5lt?lt2.5
Events

• A strong experimental signature
• no MET requirements
• looser lepton ID requirements

cut value
Discriminant
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W helicity
eµ reconstruction

• With two ?s, reconstruction is harder.
• resolution sampling
• smear objects within their resolution
• 500 times per event
• for each b-jet l combination and smearing,
  solve algebraically for cos?
• use the 2 MET components 4 mass constraints
• 0-8 solutions
• average all solutions

Parton level
Reconstructed
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Results
W helicity

• Consistencies
• first 1fb-1 vs. newer data 49
• ejets vs. µjets 12
• ljets vs. di-lepton 1.6
• data vs. SM 23

• Dominant systematics
• Signal modeling
• underlying event
• additional collisions
• MC generator
• Background modeling
• shape and yield in low discriminant sample

Longitudinal f0 0.490 0.106(stat.)
0.085(syst.) Right handed f 0.110
0.059(stat.) 0.052(syst.)
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A measurement oftop mass
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Analysis basics
mtop

• ljets selection similar to W helicity
  measurement, but
• exactly 4 jets
• at least 1 b-tagged jet (70 efficient for top
  pair signal)
• Base observable 3 jet invariant mass
• very sensitive to JES uncertainties
• ? hence, use the W peak to constraint JES

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Matrix element method
mtop

• Developed for ljets by DØ, yields the most
  precise measurements
• Now used everywhere (dileptons, single top,
  Higgs search)

Goal Use all the measured 4-vectors, x, in each
event
ds
Partonic differential Cross Section, based on LO
Matrix Element
Normalization acceptance efficiency
Initial Statemomentum fractions of incoming
quarks
Transfer Functions probability to measure x from
parton-level y
for a particular assignment which jet goes with
which quark
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Matrix element method II
mtop
Combine events by multiplying event probabilities
extract the most likely mass value


Calibrate method with ensemble of simulated
datasets

• account for approximations made

with 2.2 fb -1
mt 172.2 1.0(stat) 1.4(syst) GeV
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In closing

• Presented two recent top quark measurements from
  DØ
• To see more http//www-d0.fnal.gov/Run2Physics/WW
  W/results/top.htm
• Data sets are well understood and large enough to
  probe to new physics and to measure SM parameters
• so far all measurements are consistent with the
  SM
• Top mass already measured to a precision
  exceeding expectations
• Looking forward to results with bigger data sets

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• Back up slides

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ljets sample composition
W helicity
Source Fit to preselected Fit to preselected Final sample Final sample
Source ejets µjets ejets µjets
Data 577 498 251 247
Signal 192.2 17.4 186.2 17.3 171.3 4.2 162.7 5.1
Wjets 285.0 23.9 301.9 22.4 55.2 3.4 75.6 4.7
Multijet 111.2 9.6 10.7 10.0 35.5 2.9 5.0 2.2
Cut value 0.5 0.2 N/A N/A
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Dilepton sample composition
W helicity
Source Preselected Final
Exp. signal 50.5 2.6 49.0 2.6
Z/??tt 17.7 4.2 5.0 2.9
Fake l 12.5 4.4 4.5 2.9
Dilepton 3.6 0.7 1.9 0.8
Observed 100 77
Total background normalized to fit results.
(N1.05, and uncertainties inflated)
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Fit to data
W helicity
Comparing the global best-fit model to data in
ljets
and in di-leptons
These plots are for the subset of data since the
1fb-1 PRLPRL 100, 062004 (2008)
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Top quark mass
mtop
EW fit CDF DØ World Average

• Fundamental parameter of the standard model
• Implications on mH MW

Quigg hep-ph/0404228
Update of hep-ex/0612034
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World average mtop
mtop
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• Even moreback up slides

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Why is the Top Quark Interesting?
t quark

• Its heavy so it decays so quickly that the
  strong force does not confine it? It is the
  only bare quark.

Standard Model

• Its heavy effects the mass and decays of other
  particles (in particular, well talk about
  implications for mH)

• Its heavy we already know a lot about whats
  happening at low energies. ? Look for new
  physics at high energies.

New Physics

• Its heavy Yukawa coupling to Higgs ?t 1
• Does it play a special role in EW symmetry
  breaking?

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The DØ Detector
Apparatus
Toroid
Endcapcalorimeter
MuonDete-ctors
All used to measure top quarks Tracking and
vertexing (momenta, b-jet ID) Calorimetry (jets,
electrons, pT imbalance) Muon detectors
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The channels
Lepton (isolated, pTgt20 GeV, ?lt1.1/2.0) MET
(gt20GeV, triangle cut on ?F(l,MET) )
4 Jets (pTgt20 GeV, ?lt2,5) maybe a couple of
b-tags
Electron (isolated, ?lt1.1 or 1.5lt?lt2.0, pTgt15
GeV)
2 Jets (pTgt20 GeV, ?lt2,5) maybe a couple of
b-tags
Muon (isolated, ?lt2.0, pTgt15 GeV)
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T
A