A measurement of the top quark mass from dilepton decay channels is presented, using approximately 360 pb-1 of data colleced by the D0 experiment at Fermilab. The mass is measured from a total of 21 candidate dilepton events, using the neutrino weighting scheme. The measured mass is found to be 175.6+/-10.7(stat.)+/-6.0(syst.) GeV. This result is in good agreement with the current world average of the top quark mass.

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This thesis re-examines soft dilepton emission from a weakly coupled Quark-Gluon Plasma. It shows that Braaten, Pisarski, and Yuan's result, that the dilepton rate rises as 1/q40 (and the spectral weight scales as 1/q0) at small energy q0 ≪ gT is correct, but that the coefficient they found for this behavior is not correct, because their analysis was incomplete. At still smaller scales, the behavior moderates to ∝ 1/q20 for q0 ≲ g4T, consistent with a finite electrical conductivity. An evaluation of the spectral weight in the q0 ~ g4 T region by kinetic theory techniques is carried out. The spectral weight is then shown to satisfy a sum rule, which makes the determination of electrical conductivity from the Euclidean correlation function very challenging.

Thesis (Ph.D.)--Tufts University, 1999. / Adviser: Krzysztof Sliwa. Submitted to the Dept. of Physics. Includes bibliographical references (leaves 136-141). Access restricted to members of the Tufts University community. Also available via the World Wide Web;

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No description available.

Using 1.0 fb^{-1} of data collected by the Collider Detector at Fermilab (CDF) from Run II of the Fermilab Tevatron, we measure the top-antitop production cross-section in events with two leptons, significant missing transverse energy, and at least jets, at least one of which is identified as a b-jet. As the Run II dataset grows, more stringent tests of Standard Model predictions for the top quark sector are becoming possible. The dilepton channel, where both top quarks decay t-> W b ->l nu b, is of particular interest due to its high purity. Use of an isolated track as the second lepton significantly increases the dilepton acceptance, at the price of some increase in background, particularly from W + jets events where one of the jets is identified as a lepton. To control the increase in background we add to the event selection the requirement that at least one of the jets be identified as a b-jet, reducing the background contribution from all sources. Assuming a branching ratio of BR(W->l nu) = 10.8% and a top mass of m_top = 175 GeV/c^{2} the measured cross-section is sigma = (10.5 +/- 1.8 stat. +/- 0.8 syst. +/- 0.6 lumi.) pb.

Using 1.0 fb^{-1} of data collected by the Collider Detector at Fermilab (CDF) from Run II of the Fermilab Tevatron, we measure the top-antitop production cross-section in events with two leptons, significant missing transverse energy, and at least jets, at least one of which is identified as a b-jet. As the Run II dataset grows, more stringent tests of Standard Model predictions for the top quark sector are becoming possible. The dilepton channel, where both top quarks decay t-> W b ->l nu b, is of particular interest due to its high purity. Use of an isolated track as the second lepton significantly increases the dilepton acceptance, at the price of some increase in background, particularly from W + jets events where one of the jets is identified as a lepton. To control the increase in background we add to the event selection the requirement that at least one of the jets be identified as a b-jet, reducing the background contribution from all sources. Assuming a branching ratio of BR(W->l nu) = 10.8% and a top mass of m_top = 175 GeV/c^{2} the measured cross-section is sigma = (10.5 +/- 1.8 stat. +/- 0.8 syst. +/- 0.6 lumi.) pb.

A search is presented for vector-like quarks using 20.3 fb⁻¹ of proton-proton collisions at a center-of-mass energy of √S = 8 TeV collected with the ATLAS detector in 2012 at the CERN Large Hadron Collider. Vector-like quarks are predicted to exist in many theories beyond the Standard Model of particle physics that attempt to resolve the hierarchy problem. Events are selected containing jets including at least one b-jet, sizable missing transverse momentum, large scalar sum of the jet and lepton transverse momenta, and either three leptons or two leptons with the same electric charge. Standard Model processes rarely produce this final state and production of vector-like quarks would lead to an enhanced rate of such events. The data are interpreted in the context of a variety of models including pair production of vector-like quarks T and B, which have the same electric charge as the corresponding Standard Model quarks and can appear in either SU(2) weak isospin singlets, doublets or triplets. In addition single and pair production of the vector-like quark T₅/₃ which appears in an SU(2) weak isospin doublet or triplet and has electric charge 5/3, is considered. A moderate excess of data above the SM background expectation is observed with a significance of less than two standard deviations. The data are used to set limits at 95% Confidence Level (CL) on the new heavy quark mass for the various VLQ models considered using the CL_S method. The vector-like quarks T and B in the singlet model are excluded at 95% CL below a mass of 0.59 TeV and 0.62 TeV respectively. The T₅/₃ is excluded at 95% CL below a mass of 0.74 TeV when only pair production is considered and below 0.75 TeV when both pair and single production are considered.

Thesis (M.Sc.). / Written for the Dept. of Physics. Title from title page of PDF (viewed 2009/13/07). Includes bibliographical references.