Measurement of the Top Quark Mass The Low Bias Template Method using Lepton jets events

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Measurement of the Top Quark Mass The Low Bias
Template Method using Lepton jets events

• 4 high PT Jets
• 2 from the b-quarks
• 2 from hadronically decaying W
• 1 Isolated high PT lepton
• Muon or electron
• Tau not used in this analysis
• Neutrino

• Kevin Black
• Meenakshi Narain
• Boston University

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Backgrounds

• WJets
• Real high PT lepton
• Neutrino
• Multiple Jets
• ? 12 pb 25
• Multijet Events
• One Jet fakes the signature of an isolated lepton
• Mismeasured transverse momentum

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Event Selection
?jets
From Cross-Section Measurement

• At least 4 jets with
• PT gt 20 GeV , ? lt2.5
• Isolated Muon or Electron with PT gt 20 GeV
• Muon ? lt2.0
• Electron ? lt1.1
• Missing ETgt 20 GeV

ejets
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Topological Variables

• Form log likelihood of signal over background
  for each variable and combine
• Low Bias (LB) discriminant is built from four
  topological variables to give extra
  discrimination between top and background

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Topological Variables
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Topological Likelihood
Using each topological variable separately there
is some separation between signal and
background however combining them in a
likelihood gives optimal separation
Discriminant

• LB disc gt 0.4 (optimal cut)
• Remove 50 background
• Retain 85 Top

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Identifying b-quark Jets

• Each top event has two b-quarks
• travel a few mm and decay
• Search for jets which point back to secondary
  vertex
• Largest backgrounds for top do not have heavy
  flavor
• Same event selection, except
• Jet Pt gt 15 GeV
• Require one or more tagged jets

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Kinematic Fitting

• Kinematic Fit
• 1 unknown longitudinal momentum of the neutrino
• 3 constraints
• Top quark and antiTop quark have the same mass
• 2 jets have the invariant mass of the W
• leptonneutrino have invariant mass of the W
• 2C fit for top mass
• Experimentally see Jets not partons ! try out
  every permutation

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Kinematic Fitting Complications
Final stage gluon radiation - extra jet, splits
the energy from one parton-gt2 jets
Neutrino - 2 possible solutions - different
starting points for fit
g
g
Initial state gluon radiation - extra jet, not
from the top-antitop decay
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Performance of the Fit
In parton matched correct solution, RMS 18 GeV,
Lowest ?2 all events RMS 30 GeV
In parton matched events, the lowest ?2 solution
is correct 37 of the time
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Effect of b-tagging

• Requiring one or more b-tagged jets increases the
  expected S/B dramatically
• without tag 1/2.6
• with b-tagged 3/1 (also lower jet pT
  requirement)
• Reduction of incorrect permutations
• 12 untagged, 6 single tag, 2 double tag
• Higher Probability of getting correct solution !
• better resolution

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Method to measure the top quark mass

• Make Templates for signal events
• 9 different mass points between 150-200 GeV
• Background Templates from Wjets MC
• Use simple Poisson likelihood for the number of
  signal and background compared to data
• constrain the number of background events to the
  expected.

Signal 175 GeV
Background
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Multijet Distributions
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Event Yield - Topological Analysis
ejets mujets
Preselection 87 80
?fit2 lt10 78 68
DLB gt0.4 49 45
ejets mujets
top 27.5 2.2 20.4 0.9
WJets 9.5 2.2 22.0 2.6
Multijet 12.0 0.55 2.6 0.5
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Event Yield b-tagged Analysis
ejets single tag ejets double tag mujets single tag mujets double tag
Preselection 38 9 24 5
Fit Convergence 36 6 22 5
ejets mujets
top 30.5 2.4 22.0 1.75
WJets 7.0 0.6 4.3 0.3
Multijet 4.5 0.4 0.7 0.05
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Ensemble Tests
Calibration Consistent With Slope 1 And 0 offset
Pull ? 1 Mean 0
Pull
Mass Calibration
Topological Expected Error 6.1 GeV
Tagged Expected Error 4.2 GeV
Statistical Error (GeV)
Statistical Error (GeV)
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Topological Analysis Results
Red background only Blue background top
Fit 44.2 6.6 ttbar events, expect 47.9 ? 8.8
events
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Tagged Analysis Results
Red background only Blue background top
Fit 49.2 6.3 ttbar events, expect 52.4 ? 4.2
events
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Comparison with Expectation
Tagged Selection
Topological Selection
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Topological Discriminant in Data
Topological Selection
Tagged Selection
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Systematic Errors Jet Energy Scale
Photon Jet Events - electromagnetic scale
more precisely known - Compare the
transverse energy of the jet and photon and
compute ? S - Do the same with Monte
Carlo events and compare the
difference between data and MC
ETJet- ET?
?S
ET?
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Variation of Distributions withthe Jet Energy
Scale
JES up and down for Wjets
JES up and down for top
Perform Ensemble Tests with 1?, -1? compared to
nominal
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Systematic Errors Gluon Radiation
To obtain bounds - consider the variation
seen in events with and without gluon
radiation
g
g
Conservative approach but not much is known
about gluon radiation in top decays!
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Gluon radiation Kinematics of Jets
Matched Matched Tagged Gluon Jet Gluon Jet
Tagged
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Systematic Uncertainty
Source Topological (GeV/c2) b-tagged (GeV/c2)
Statistical 5.8 4.2
Jet Energy Scale 6.8 -6.5 4.7 -5.3
Jet Resolution 0.9 0.9
Gluon Radiation 2.6 2.4
Signal Model 2.3 2.3
Background Model 0.7 0.8
b-tagging 0.7
Calibration (fitting bias) 0.5 0.5
Trigger 0.5 0.5
MC Statistics 0.5 0.5
Total Systematic 7.8 -7.1 6.0
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Cross-Checks
D gt 0.25 170.5 5.5 GeV
D gt 0.3 170.6 5.5 GeV
D gt 0.4 169.9 5.8 GeV
D gt 0.45 170.3 6.5 GeV
D gt 0.5 169.2 5.6 GeV
D gt 0.55 167.0 6.5 GeV

• Vary the Discriminant Cut
• Drop Constraint
• on background

Constrained (Poisson) Unconstrained
Topological 169.9 5.8 GeV 170.7 6.5 GeV
Tagged 170.6 4.2 GeV 171.8 4.8 GeV
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Summary and Outlook

• First measurement of the top quark mass at D?
  using b-tagging
• best for Run II so far
• Result
• Topological analysis
• b-tagged analysis

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Backup Slides
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Event Selection ejets

• 4 jets PT gt 20 GeV, ? lt2.5
• 1 electron PT gt 20 GeV, ? lt1.1
• EM Fraction gt 0.9
• Isolation lt 0.15
• Hmatrix lt 50 (shower shape)
• EM Likelihood gt 0.85 (multivariate)
• Track Match
• ET gt 20 GeV, lepton ?, ET ? triangle cuts
• Second high PT isolated lepton veto
• Primary Vertex in SMT fiducial range, with at
  least 3 tracks
• Primary Vertex within 1 cm of z position of
  electron track

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Muon Jets

• 4 jets PT gt 20 GeV, ? lt2.5
• 1 muon PT gt 20 GeV, ? lt2.0
• Isolated from jets
• ET gt 20 GeV, lepton ?, ET ? triangle cuts
• Second high PT isolated lepton veto
• Primary Vertex in SMT fiducial range, with at
  least 3 tracks
• Primary Vertex within 1 cm in z of muon track

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Jet Corrections
Parton Matched Jets from Top
Parton Matched Jets from W
Parton Matched Jets
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Likelihood Function
L(mt, ns, nb) P(N N(nb))? P(Nj, Nj(ns, nb))
Nj nspsj(mt) nbpjb
The product is over the number of bins The ps and
pb are the fraction of events from that template
that fall into bin j For each template the
likelihood function finds the signal fraction
which minimizes the negative log likelihood and
returns the value of the likelihood, and the fit
number of top events. Repeated through each top
mass template and then a parabola is fit through
the resulting likelihoods
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Discriminant Cut
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Can we combine the two analyses

• Simplest way remove the tagged events from the
  topological analysis
• Find 95 events
• Anti-tag efficiency
• Top 40
• Wjets and QCD 90
• Expect 80 are background after removal of the
  tagged events.
• Redo ensemble tests, expected statistical error
  11.0 GeV
• Combined expected error would be 3.9 (rather
  then 4.2)
• However, systematic dominated so only a few
  percent overall improvement