A Precision Measurement Of The Top Quark Mass

This dissertation describes the measurement of the top quark mass using events recorded during a ≈230 pb−1 exposure of the DØ detector to proton-anti-proton ( pp¯) collisions at a center of mass energy of 1.96 TeV. The Standard Model of particle physics predicts that the top quark will decay into a bottom quark and a W boson close to 100% of the time. The bottom quark will hadronize (bind with another quark) and produce a jet of hadronic particles. The W bosons can decay either into a charged lepton and a neutrino or a pair of quarks. This dissertation focuses on the top quark (tt¯) events in which one W decays hadronically and the other decays leptonically. Two methods of identifying tt¯ events from the large number of events produced are used. The first is based on the unique topology of the final state particles of a heavy particle. By using the topological information of the event, the tt¯ events can be efficiently extracted from the background. The second method relies on the identification of the remnants of the long lived bottom quarks that are expected to be produced in the decay of almost every top quark. Because the largest background processes do not contain bottom quarks, this is an extremely efficient way to select the events retaining about 60% of the tt¯ events and removing almost 90% of the background. A kinematic fit to the top quark mass is performed on the tt¯ candidate events using the final state particles that are seen in the detector. A likelihood technique is then used to extract the most likely value of the top quark mass, m t, and signal fraction. The result for the topological selection is mt = 169.9 ± 5.8 (statistical) +8.0-7.8 (systematic) GeV while the results on the sample selected from identification of a b quark in the event is mt = 170.6 ± 4.2 (statistical) +6.3-6.8 (systematic) GeV