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34th International Conference on High Energy Physics, Philadelphia, 2008
Top Quark Pair Production Cross Section and Forward-Backward
Asymmetry at the Tevatron
A. Lister (on behalf of the CDF and DO Collaborations)
University of California at Davis, CA 95616, USA
We present recent results on top quark pair production cross section in several final states and the tt forward-
backward charge asymmetry in the lepton + jets channel at the Tevatron pp collider operating at Va = 1.96 TeV. A
new combination of DO measurements gives a tt production cross section of T,, = 7.83 i. (stat) 0. (syst) +
0.48 (lumi). The new combination of CDF results for tt production yields 7.0 + 0.3 (stat) + 0.4 (syst) + 0.4 (lumi)
pb, which corresponds to a total uncertainty of 9%, very close to the that of the current best theoretical predictions.
It is important to measure the tt cross section in as many different channels as possible as any significant discrepancy
between them could be a sign of new physics. Three new measurements of the forward-backward charge asymmetry
are also presented. The two CDF measurements correct the acceptance and observed asymmetry back to parton level
in order to compare the central values with specific theoretical predictions. The DO measurement does not unfold its
acceptance and therefore does not depend on the specific method used for unfolding.
The study of top quark pair production at the Tevatron is now entering the realm of precision physics. With
2.8 fb-1 of CDF data there are over 1200 reconstructed tt events in the lepton + jets channel. As the integrated
luminosity increases, it is possible to improve not only measurements of the properties of the top quark but also maybe
to search for new physics by comparing the tt cross section measurements in different channels, or by measuring the
forward-backward charge asymmetry.
We present here results of the tt cross section in the dilepton channel, where both of the Ws decay leptonically as
well as in the lepton + jets channel, where one W decays leptonically and the other hadronically. We will also present
combinations of recent CDF and D0 tt cross sections. In addition, we present results from studies of asymmetry in
the centre of mass in the production of top quarks in the lepton + jets tt final states.
2. TOP PAIR PRODUCTION CROSS SECTION
Many new tt cross section measurements were presented at this conference. In the dilepton channel, a new
measurement from CDF  uses the full 2.8fb-1 of data. In this channel, the backgrounds are relatively small, as
illustrated in Fig. 1 (left). The measured cross section is oU1 6.7 0.8 (stat) 0.4 (syst) 0.4 (lumi) pb.
D0 has developed excellent hadronic T identification algorithms that makes use of a series of Neural Networks
to separate out real T leptons from electrons and jets. This can be used to measure the tt cross section in the
dilepton mode, where one of the Ws decays to a r that subsequently decays to a v7 and hadrons . The event
signature consists of a lepton (e or p) and a jet from a r decay, missing transverse energy and at least two other jets.
Although the signal fraction after all selections is relatively small, as can be seen from Fig. 1 (right), it is sufficient for
extracting the tt cross section. The result, using 2.2 fb-I1 of DO data, is Utt 7.3 1.3 (stat) j. (syst) 0.5(lumi)
pb, consistent with that of all other channels, thereby showing so large sign of new physics from this sector.
In the lepton + jets channel, two new CDF measurements were shown that use the full integrated luminosity
available, 2.8 fb-1. The first makes use of the enhanced signal to background fraction when at least one of the
jets is required to have a heavy-flavor tag (using the displaced vertex tagger SecVtx) . Figure 2 (left) shows the
distribution of tt signal and standard model backgrounds as a function of jet multiplicity. In this analysis, the data
is used to constrain the backgrounds that cannot be predicted with great accuracy, such as W+jets. The value of the
tt cross section is scanned until a minimum is found between the expected sum of the backgrounds plus signal and
FERMI LAB-CON F-08-474-E
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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/m1/1/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.