Measurement of the ttbar Cross Section in the Lepton + Jets Channel Using Event Kinematics and NN
(Update to 2.8 fb-1)


J. Conway, R. Erbacher, A. Lister 
UC Davis

R. Hughes, K. Lannon,  B. Winer
The Ohio State University



CDF public conference note

Final Results for 2.8 fb-1:


We update the measurement of the ppbar -> ttbar cross section in the lepton+jets channel using kinematics. As in the previous versions of this analysis, a neural network having kinematic and event shape variables as inputs is used to distinguish the top signal from the W+jets background. For this update, we retrain the neural networks with the new ALPGEN W+jets samples, modify the QCD removal cuts and add more data. Moreover we modify the fitting code to fit directly to the top cross-section (instead of the number of top events). This analysis provides a technique to measure the top cross section independent from the traditional b-tag analysis with comparable sensitivity. From a sample of data corresponding to 2.8 fb-1 of integrated luminosity, the ttbar cross section is measured to be
6.80 ± 0.38 (stat) ± 0.61 (syst) ± 0.39 (lumi) pb
in the W+≥ 3 jet mode. In the W+≥ 4 jet mode, the cross section is measured to be
6.50 ± 0.39 (stat) ± 1.02 (syst) &pm 0.38 (lumi) pb.

Final Fits to NN Output Shape:

Fits for the 2.8 fb-1 data sample follow bellow.  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. A systematic is taken to account for this. .  Electrons and muons have been combined for the fits.  The QCD shape is taken from the non-isolated leptons (electrons and muons) and constrainted to be the number of events from our fit to the Missing ET distributions. The uncertainty on the constraint is set to 50%   The ttbar and W-like shapes float freely in the fits.  Based on the previous iteration of this analysis, our primary result is for the 3 or more jet bin.

W + 3 or More Jet Sample

W + 4 or More Jet Sample
W+ >= 3 jets fit
Fit to the NN-output shape for at least 3 jets, showing shapes of contributing components (EPS)
W+ >= 4 jets fit
Fit to the NN-output shape for at least 4 jets, showing shapes of contributing components (EPS)

Other Plots

Other plots can be found at the website for the previous blessed version of this analysis.
Last modified on 08/04/08 by Alison Lister