Measurements of Top Quarks Produced in Pairs at D0

Top quarks are produced in pairs via the strong interaction. At the Tevatron, operating at 1.96 TeV in Run II, the expected production rate of tt pairs is 6.8±0.6 pb @ mt=175 GeV/c . Roughly 85% arise from qq interactions, while the remaining 15% are created by gluon fusion. As of February 4, 2007, 2.05 fb of data have been recorded by the D0 experiment. To study the properties of the top quark, events are separated according to the types of decays of the tt pairs. Each top quark decays almost exclusively to a W boson and a b quark. The W boson then decays either to a lepton and neutrino or to two quarks. Classification of events is done according to the particles in the final state: dilepton, all jets, or lepton+jets. “All jets” events make up approximately 54% of tt events. Top quark measurements benefit from the larger statistics of this channel, but, unfortunately, the QCD multijet background is also very large. Jet energy scale uncertainties are also largest for this channel, making the all jets channel difficult for top quark measurements. Dilepton events have fewer jets in the final state and an easily identifiable Z+jets background, but much smaller statistics – only 7% of tt events are dilepton events. We find the best combination of statistical and systematic uncertainties for top quark measurements in lepton+jets events.