Top Quark Pair Production Cross Section and Forward-Backward Asymmetry at the Tevatron Page: 2 of 4
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34th International Conference on High Energy Physics, Philadelphia, 2008
Pretag Top Candidales With NjeI 2
CDF II Preliminary 2.9 fb-1
s.= 6.7 ph
1 Q. DYEww
100 200 300 400 50D 600 700 BOO
scak urm ot event transverse ergies. GeV
DO Run II Preliminary
L = 1.2 fb~1
- M ultijet
m W - I + jets
-Z -> rr + jets
- WW, WZ, ZZ
tt -- lepton + jets
tt -> dilepton non- z (non-et)
20 40 60 80 100 120 140
Tau ET (GeV), 2 jets
Figure 1: (left) Scalar sum of the transverse energies in the dilepton data compared to the sum of all the backgrounds and tt
signal (yellow) in the >2 jets channel. (right) ET distribution of the r leptons in data compared to the sum of all backgrounds
and tt signal (white).
the observed number of data events. The observed cross section is ot= 7.2 0.4 (stat) 0.5 (syst) 0.4 (lumi)
The other CDF analysis in the lepton + jets channel makes use of the difference in kinematic properties between
the signal and background events . Unlike the other analyses presented in these proceedings, that count the
number of events above the background to obtain the cross section for the signal, this analysis relies on a fit to a
Neural Network (NN) output that is based on inputs from seven kinematic quantities, most of them related to the
characteristics of jets in the events. The distribution of the NN output is shown in Fig. 2 (right); it clearly shows
the signal peaking at high values and the backgrounds, mostly W+jets, peaking at low values. Doing this fit allows
us to remove the need for heavy-flavor tagging which has a relatively low efficiency. This analysis has over 1200
reconstructed tt events, and in effect, trades off additional statistics for uncertainties due to the fit of the NN output.
This results in a statistical uncertainty comparable between the two lepton + jets measurements. The fitted cross
section is otr= 6.8 0.4 (stat) 0.6 (syst) 0.4 (lumi) pb.
2.1. tt Cross Section Combinations
To obtain the most precise tt cross section, we combine the different measurements. A new combination from
CDF was presented at this conference . The five most recent tt cross section results, shown in Fig 3 (left) are
combined. The combined result, also shown on this plot, is U1 7.0 0.3 (stat) 0.4 (syst) 0.4 (lumi). The
total uncertainty is thus of the order of 9%. The improvements on the single best measurement, the b-tagged lepton
+ jets measurement described earlier, is of the order of 16%.
Figure 3 (right) shows a similar plot for the most precise D0 measurements, using up to 1.0 fb-1 of data . The
top line shows a new combination of all analyses, which yields a measured cross section of u1t = 7.83 +0.46 (stat)
1jgs (syst) 0.48 (lumi).
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Lister, Alison & /UC, Davis. Top Quark Pair Production Cross Section and Forward-Backward Asymmetry at the Tevatron, article, October 1, 2008; Batavia, Illinois. (digital.library.unt.edu/ark:/67531/metadc896526/m1/2/: accessed November 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.