Top Turns Ten March 2 nd, 2005. Measurement of the Top Quark Mass The Low Bias Template Method using...
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Transcript of Top Turns Ten March 2 nd, 2005. Measurement of the Top Quark Mass The Low Bias Template Method using...
Top Turns TenMarch 2nd, 2005
Measurement of the Top Quark Mass The Low Bias Template Method
using Lepton + jets eventsKevin Black, Meenakshi Narain
Boston University
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
Backgrounds
W+Jets Real high PT lepton Neutrino Multiple Jets ~12 pb ± 25%
Multijet Events One Jet fakes the signature of
an isolated lepton Mismeasured transverse
momentum
Event Selection
At least 4 jets with
PT > 20 GeV , || <2.5
Isolated Muon or Electron with PT > 20 GeV Muon || <2.0 Electron || <1.1
Missing ET> 20 GeV
+jets
e+jets
From Cross-Section Measurement
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
ETA ´ 3
2£Smallest eigenvalue of P
KTmin ´(min¢ R i j ) _E
j 2T
E TW
P i j ´Pa pa i pa jPa j~pa j
2
Ltb=P j =4i=4L tbi (f i )
DL B = 11+1:25e¡ L t b
HT 2 ´H T 2H k
Topological Variables
Topological Likelihood
Discriminant
LB disc > 0.4 (optimal cut)● Remove ~50% background● Retain ~ 85% Top
Using each topological variable separately there is some separation between signal and background
however combining them in a likelihood gives optimal separation
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 > 15 GeV Require one or more
tagged jets
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
lepton+neutrino have invariant mass of the W
2C fit for top mass Experimentally see Jets not
partons ! try out every permutation
Kinematic Fitting Complications
g
g
Initial state gluon radiation - extra jet, not from the top-antitop decay
Final stage gluon radiation - extra jet, splits the energy from one parton->2 jets
Neutrino: - 2 possible solutions - different starting points for fit
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
Lowest chisq solutionParton matched correct solution
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
0 Tags2 Tags
Method to measure the top quark mass
Make Templates for signal events 9 different mass points between 150-200 GeV
Background Templates from W+jets 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
Multijet Distributions
W+jets
Multijet data
Discriminant
Event Yield - Topological Analysis
e+jets mu+jets
top 27.5 ± 2.2 20.4 ± 0.9
W+Jets 9.5 ± 2.2 22.0 ± 2.6
Multijet 12.0 ± 0.55 2.6 ± 0.5
e+jets mu+jets
Preselection 87 80
+fit2 <10 78 68
+DLB >0.4 49 45
b
b
p p
E T
jet
jet
jet
Event Yield – b-tagged Analysis
e+jets single tag
e+jets double tag
mu+jets single tag
mu+jets double tag
Preselection 38 9 24 5
Fit Convergence
36 6 22 5
e+jets mu+jets
top 30.5 ± 2.4 22.0 ± 1.75
W+Jets 7.0 ± 0.6 4.3 ± 0.3
Multijet 4.5 ± 0.4 0.7 ± 0.05
Ensemble TestsPull: ~ 1Mean ~0
CalibrationConsistent With Slope 1And 0 offset
Statistical Error (GeV) Statistical Error (GeV)
Mass CalibrationPull
Topological:Expected Error~6.1 GeV
Tagged:Expected Error~4.2 GeV
Topological Analysis Results:
Red: background onlyBlue: background + top
Fit 44.2§ 6.6 ttbar events, expect 47.9 8.8 events
Tagged Analysis Results
Red: background onlyBlue: background + top
Fit 49.2§ 6.3 ttbar events, expect 52.4 4.2 events
Comparison with Expectation
Tagged Selection Topological Selection
Topological Discriminant in DataTagged Selection Topological Selection
Red: background only Blue: background + top
Discriminant
Dis
crim
inan
t
Dis
crim
inan
t
Mass (GeV)Mass (GeV)
Systematic Errors: Jet Energy Scale
S =ET
Jet- ET
ET
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
Variation of Distributions withthe Jet Energy Scale
Perform Ensemble Tests with +1, -1 compared to nominal
JES up and down for top JES up and down for Wjets
Ensemble tests+6.8 -6.5 GeV topological+4.7 -5.3 GeV tagged
Systematic Errors: Gluon Radiation
g
g
To obtain bounds: - consider the variation seen in events with and without gluon radiation
Conservative approachbut not much is known about gluon radiation intop decays!
Gluon radiation: Kinematics of Jets
MatchedMatched –TaggedGluon JetGluon Jet –Tagged
4 jets, 1 not matched4 jets, all matched
Ensemble tests§2.6 GeV topological§2.4 GeV tagged
Signal Model
Compare the nominal sample ALPGEN/PYTHIA ttbar vs tt+1j
ttbar + 0jttbar + 1 jttbar +2 j
Ensemble tests+2.3 GeV topological+2.3 GeV tagged
Systematic Uncertainty
Source Topological (GeV/c2) b-tagged (GeV/c2)
Statistical
Jet Energy Scale
Jet Resolution
Gluon Radiation
Signal Model
Background Model
b-tagging
Calibration (fitting bias)
Trigger
MC Statistics
Total Systematic
Cross-Checks 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
D > 0.25 170.5 ± 5.5 GeV
D > 0.3 170.6 ± 5.5 GeV
D > 0.4 169.9 ± 5.8 GeV
D > 0.45 170.3 ± 6.5 GeV
D > 0.5 169.2 ± 5.6 GeV
D > 0.55 167.0 ± 6.5 GeV
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
Backup Slides
Event Selection: e+jets
4 jets PT > 20 GeV, || <2.5 1 electron PT > 20 GeV, || <1.1
EM Fraction > 0.9 Isolation < 0.15 Hmatrix < 50 (shower shape) EM Likelihood > 0.85 (multivariate) Track Match
ET > 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
Muon +Jets
4 jets PT > 20 GeV, || <2.5
1 muon PT > 20 GeV, || <2.0 Isolated from jets
ET > 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
Jet CorrectionsParton Matched Jets from W Parton Matched Jets from Top
Parton Matched Jets Z ! b¹b
Discriminant Cut
Can we combine the two analyses Simplest way – remove the tagged
events from the topological analysis Find 95 events Anti-tag efficiency
Top ~40% W+jets 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