Asymmetry in Muon PseudoRapidity and the Search for Single Top Quarks at the Tevatron

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Asymmetryin Muon Pseudo-Rapidity and the
Search for Single Top Quarks at the Tevatron

• Amanda Gray
• Undergraduate Research Project
• with Thomas Gadfort and Gordon Watts
• On behalf of the D0 Collaboration

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The Tevatron
P-Pbar collider 980 GeV protons and
anti-protons L 1032 cm-2 s-1
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Top Quark Discovery

• Pair Production, a strong interactionstt 7pb
  (pb 10-40 m²)

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Why Do We Care About Single Top Quark Production?

• Rare electroweak process, verify Standard Model
• Direct measurement of Vtb, verify CKM unitarity
• s ? Vtb2
• CKM matrix is empirical 3x3 unitary matrix, each
  element Vij is relative amplitude for quark-quark
  coupling via W-boson
• Precision measurement of top mass, properties
• Background for Beyond the Standard Model processes

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Single Top Quark Production
S-Channel st 0.88 0.14 pb
T-Channel st 1.98 0.30 pb
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Standard Model Background
S-Channel W-boson Production W Plus
Jets sWJets 1000 pb
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Signal Drowned in Noise

• Single Top signature
• one isolated lepton (the muon)
• missing energy (the neutrino)
• two b-jets or one b-jet and a light jet
  (produced by the decay of the down quark)
• Signature of W-plus (2) jets
• one isolated lepton
• missing energy
• 2 jets

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Signal Drowned in Noise

• Single Top signature
• one isolated lepton (the muon)
• missing energy (the neutrino)
• two b-jets or one b-jet and a light jet
  (produced by the decay of the down quark)
• Signature of W-plus (2) jets
• one isolated lepton
• missing energy
• 2 jets

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Signal Drowned in Noise

• Single Top signature
• one isolated lepton (the muon)
• missing energy (the neutrino)
• two b-jets or one b-jet and a light jet
  (produced by the decay of the down quark)
• Signature of W-plus (2) jets
• one isolated lepton
• missing energy
• 2 jets

st 2.9 pb
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Signal Drowned in Noise

• Single Top signature
• one isolated lepton (the muon)
• missing energy (the neutrino)
• two b-jets or one b-jet and a light jet
  (produced by the decay of the down quark)
• Signature of W-plus (2) jets
• one isolated lepton
• missing energy
• 2 jets

st 2.9 pb
sw 1000 pb
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Single Top Production

• Traditional straight cut-based analysis
• Doesnt work very well here, because we cant
  isolate the signal based on event signature
• Shape-based analysis
• Model the background and signal sufficiently well
  to build a comprehensive simulation that matches
  data (verify ratios of signal to background
  events)
• How well do we understand the kinematics of both
  background and signal?

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The Project

• Analyze Asymmetry in Muon Pseudo-Rapidity.
• Explore different methods of measuring asymmetry
• Qualitative shape
• Quantitative bin-by-bin counting, skewness,
  kurtosis

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Psuedo-Rapidity
? 1
? 2
transverse axis
? 3
?
-

beamline
antiprotons
protons
Psuedo-rapidity is boost-invariant
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Asymmetry in Pseudo-Rapidity
Muons tend to move in the direction of the
initial-state up quark.

• S-channel Single-Top ProductionParton
  Distribution FunctionsUp has more momentum than
  Down.
• T-channel Single-Top Production Straightforward
  kinematics valence Up quark has more momentum
  than sea gluon.
• W plus Jets background ??

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Quantitative Measures(bin-by-bin)
Asymmetry
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Quantitative Measures(distribution)
Skewness lack of symmetryof distribution Kurtos
is peakedness of distribution where is the
mean, s is the standard deviation, and N is the
number of data points
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• D0 Run II Data
• collected through June 2005, 320 pb-1
• p14Ipanema WJets dataskim
• Preselection
• Tight Isolation, High MET
• Cuts
• 2 good jets
• (excludes WJets bins with TTbar events)
• 1 loose good muon, 0 electrons
• Primary vertex central, ntracks gt 2
• MET with 20 lt PT lt 200 GeV

Muon Pseudo-Rapidity
Asymmetry in Pseudo-Rapidity
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S-Channel MC tb Single Top MonteCarlo
CompHEP Additional Cuts NO BAD JETS Asymmetry
is driven by kinematics of initial state partons
due to PDFs, Up quark has a greater fraction of
hadrons momentum than Down quark.
Muon Pseudo-Rapidity
Asymmetry in Pseudo-Rapidity
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T-Channel MC tqb Single Top MonteCarlo
CompHEP Additional Cuts NO BAD JETS Asymmetry
is driven by kinematics of initial state partons
valence quark has 1/3 momentum of hdaron sea
gluon tends to have little momentum.
Muon Pseudo-Rapidity
Asymmetry in Pseudo-Rapidity
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W plus Jets MC MonteCarlo Alpgen Additional
Cuts NO BAD JETS
Muon Pseudo-Rapidity
Asymmetry in Pseudo-Rapidity
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Skewness
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Kurtosis
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Conclusions

• Asymmetry of Single Top S- and T-channels is
  markedly different.
• WJets is very similar to S-channel Single Top
  Production in asymmetry.
• Skewness may be a more useful measure than
  kurtosis.
• Big statistical errors at this point, perform
  analysis again with 4 fb-1.

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Acknowledgements References

• Collaborators Thomas Gadfort Gordon Watts
• T Gadfort, A Gray, G. Watts, Asymmetry in Muon
  Pseudo-Rapidity and the Search for Single Top
  Quarks, D0 Note xxxx, (Feb 2006)
• M. T. Bowen, S. D. Ellis and M. J. Strassler,
  In Search of Lonely top Quarks at the
  Tevatron, arXivhep-ph/0504186v1.
• S. Sengupta, The W Boson Production Charge
  Asymmetry in the Muon Channel at vs 1.96 TeV,
  D0 Note 4855, (June 2005).

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