|Distribution of the transverse-energy values for the two jets with the largest transverse energy in each event (black points) for the signal sample. The lower panel shows the observed data yield divided by the predicted yield.|
An important part of the Tevatron’s legacy is the discovery of the top quark and subsequent determinations of its intrinsic properties. Here we make the first measurement of the top-antitop cross section using the full Run II data set. The top-antitop cross sections give the probabilities for its various decay processes to occur and are a function of the particles’ energy. (For the Tevatron the energy in the center-of-mass system is 1.96 TeV, and for the LHC it is currently 8 TeV.)
The top essentially always decays into a W and a b quark, and the W then decays into either two light quarks or into a lepton and a neutrino. In this experiment CDF looked at the decay mode in which the W from the top and the W from the antitop each decays into a lepton plus neutrino. The final state consists of two leptons, two neutrinos, and two b quarks. This is the dilepton decay mode.
Because of the presence of two leptons (electron or muon), a large amount of missing energy carried by the two neutrinos and the presence of four or more jets, this is an easy mode to recognize. However, this is the rarest of the decay modes. Another way of saying that this is an easy mode to recognize is to say that the signal to background is large. In an earlier paper on dilepton cross sections, we reported a signal of about 70 percent; the remaining 30 percent was the background. By requiring a very positive identification of at least one b-tagged jet, we have now improved the percentage of signal to slightly more than 90 percent.
The full sample consists of 246 events, and a typical distribution is shown in the top figure. The result of our measurement for a top mass of 172.5 GeV/c2 is σtt (CDF dilepton) = 7.09 ± 0.84 picobarns. When all CDF and DZero measurements are combined, σtt (CDF + DZero) = 7.60 ± 0.41 picobarns. Theorists’ ability to calculate the Standard Model has also recently improved, and the latest result is σtt (theory) = 7.35 ± 0.24 picobarns. We see that there is very good agreement between the combined Tevatron measurement and the Standard Model prediction.
—edited by Andy Beretvas