Abstract

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 of 
σ_ttbar =  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 be
σ_ttbar = 6.89 ± 0.41(stat) ^+0.41_-0.37(sys) ± 0.14 (theory) pb.
The total 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.

Event Selection

We use the standard CDF lepton + jets selection

• Central Lepton (electron or muon) p_T >= 20 GeV/c;
• Missing transeverse energy >= 20 GeV;
• At least 3 jets with E_T >= 20 GeV. 

 We apply additional cuts

• Tigher missing transeverse energy cut at 35 GeV;
• Tighter leading jet E_T cut at 35 GeV.

The tighter cuts were optimised to remove the most QCD background 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.
 
Note: We do not require any b-tagging.

Signal and Background Modeling

• The ttbar signal is modeled from Pythia Monte Carlo with an assumed 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 dijet events 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 hidden node.
The NN is trained to separate W+4p from ttbar Monte Carlo.
The input variables are
Global event variables

• Σ_jets E_T of all jets excluding two leading jets
• H_T of the event (sum of transverse energy of all reconstucted objects)
• Aplanarity of the event

Variables related to the 3 leading jets

• Σ_jets p_z / Σ_jets E_T
• Mininum dijet mass
• Mininum dijet separation
• Maximum jet E_T

Ratio 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 cross section.
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
σ_ttbar = 6.97 ^+0.42_-0.41 (stat) ^+0.40_-0.42 (sys) ± 0.40 (lumi) pb.
The Z cross section is measured to be
σ_z =  253.27 ± style="font-weight: bold;"> 1.01(stat) ^+4.4_-4.6 (sys) ^+16.63_-13.71 (lumi) pb.
The ratio of the ttbar to Z cross section is computed, taking into account the correlations between the systematics
1/R = σ_Z /σ_ttbar =  36.47 ^+2.06-2.29 (stat) ^+1.88_-1.96(sys).
Multiplying the ratio R by the theoretical Z cross section
σ_Z^theory = 251.3 ± 5.0 (sys) pb,
We get a final result for the measured top pair production cross section of
_{The 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!!!}

_{Final Fits to NN Output Distribution}

<sub>Fits for the 2.8 fb^-1 data sample follow bellow.

The input QCD normalisation is obtained from a fit to the missing ET distribution before the missing ET cut is applied.
The uncertainty on the constraint is set to 50% in the final fit.
The ttbar and W+jets shapes float freely in the fit.</sub>  

_{Plots of NN Input Variables}

_{These plots are shown using the signal and background normalisations obtained from the final fit.}

	Σ ET of jets excluding 2 leading jets
	HT of the event
	Σ pz / Σ ET of 3 leading jets
	Aplanarity of the Event
	Minimum dijet separation of 3 leading jets
	Minimum dijet mass of 3 leading jets
	Maximum jet eta of 3 leading jets

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_{Last modified on 10/21/08 by Alison Lister}