Event Kinematics Only
W + >= 3
= 7.08 ± 0.38 (stat) ± 0.36 (syst) ±
Result shown at
ICHEP and used in the CDF combination
= 6.80 ± 0.38 (stat)
(syst) ± 0.39 (lumi) pb
σttbar = 6.81
(stat) ± 1.01 (syst) ± 0.39 (lumi) pb
Result shown at
= 6.50 ± 0.39 (stat)
(syst) ± 0.38 (lumi) pb
We present a measurement of the top pair production cross
section in ppbar collisions at 1.96 TeV, with an integrated
luminosity of 2.8 fb-1 at the CDF
experiment on the Fermilab
Tevatron. We use a neural network technique to discriminate between top
pair production and background processes in a sample of events with an
isolated, energetic lepton, large missing transverse energy and three
or more energetic jets. We measure a top pair production cross section
σttbar = 7.08 ±
0.38 (stat) ± 0.36 (sys)
± 0.41 (lumi) pb for a top mass of 175 GeV/c2.
We then significantly reduce the dependence on the luminosity
measurement and its associated large systematic uncertainty. We compute
the ratio of the ttbar to Z cross section, measured using the same
triggers and dataset, and then multiplying this ratio by the
theoretical Z cross section. The final ttbar cross section, assuming a
top mass of 175 GeV/c2, is measured to
σttbar = 6.89
± 0.41(stat) +0.41-0.37(sys)
± 0.14 (theory) pb.
uncertainty is 8.2%, greatly surpassing the Tevatron Run II goal of
10%, and now as precise as the best theoretical calculations.
For this analysis the signal to background ratio is about 1:4.5 after
final event selection.
We use the standard CDF lepton + jets selection
- Central Lepton (electron or muon) pT
- Missing transeverse energy >= 20 GeV;
- At least 3 jets with ET
We apply additional cuts
- Tigher missing transeverse energy cut at 35
- Tighter leading jet ET
cut at 35 GeV.
The tighter cuts were optimised to remove the most QCD
while mainting reasonable efficiency for the ttbar signal. As the
statistical uncertainty is not our dominant uncertainty, we can affort
to cut quite hard. These cuts remove ~85% of the QCD, ~40% of the
W+jets while maintining ~80% efficiency for the ttbar signal.
Both central electrons and muons have been used
for the final fit.
: We do
not require any b-tagging.
Signal and Background Modeling
- The ttbar signal is modeled from Pythia Monte
Carlo with an
mass of 175 GeV/c2.
- The W+jets background is used to model all EWK
backgrounds, as the previous analysis showed that the effect of
including all backgrounds was very small and such a difference is
included as a source of systematic uncertainty.
- The W+jets background us modeled from
ALPGEN+Pythia MC. The
W+jets sample is obtained from a combination of W+0p, W+1p, W+2p, W+3p
exclusive as well as W+4p inclusive
- The QCD shape is modeled from a sample of
which pass our event selection criteria. These events are dominated by
jets that fake electrons. The QCD contamination in the electron sample
is significantly larger than in the muon sample
Neural Network Input Variables
The Neural Network uses 7 kinematic distributions as
inputs, with 1
The NN is trained to separate W+4p from ttbar Monte Carlo.
The input variables are
Global event variables
- Σjets ET
excluding two leading jets
- HT of the event (sum of
energy of all
- Aplanarity of the event
Variables related to the 3 leading jets
- Mininum dijet mass
- Mininum dijet separation
- Maximum jet ET
over the Z Cross Section
The dominant systematic uncertainty, 5.8%, is is the luminosity
measurement due to the detector used to measure the inelastic ppbar
We can almost entirely cancel this uncertainty by considering the ratio
of the ttbar to the Z cross sections.
The Z cross section is measured using the central electron and muon
triggers and the same data sample as the Z cross section.
For this measurement, the signal MC for ttbar and Z is re-weighted to
the CTEQ6.6 PDF (with its associated uncertainties).
The meausred ttbar cross section is found to be
σttbar = 6.97 +0.42-0.41
+0.40-0.42 (sys) ± 0.40 (lumi)
The Z cross section is measured to be
The ratio of the ttbar to Z cross section is computed, taking into
account the correlations between the systematics
= σZ /σttbar
+2.06-2.29 (stat) +1.88-1.96(sys).
Multiplying the ratio R by the theoretical Z cross section
= 251.3 ± 5.0 (sys) pb,
We get a final result for the measured top pair production cross
= 6.89 ± 0.41 (stat)+0.41-0.37
(sys)± 0.14 (theory) pb.
total uncertainty is 8.2%, a
significant reduction on the 9.2% obtained using the kinematic fit only.
The total uncertainty is decreased by 10% by taking the ratio of the
top pair to the Z cross sections!!!