A search for single top quark production mediated by flavor changing neutral currents in ep collisions is presented. The search is motivated by the observation of events with an isolated lepton, missing transverse momentum and large hadronic transverse momentum in the H1 detector. The rate and topology of these events is found to be unlikely for Standard Model processes. The goal of this thesis is a comprehensive search for top quarks. Decays of top quarks in a b-quark and a W boson are searched for in both the leptonic and the hadronic decay modes of the W. The top signal is discriminated from Standard Model background processes in a multi-variate analysis. The full HERA I data set taken with the H1 detector is used, corresponding to an integrated luminosity of 118 pb^-1. As a result of this analysis, it is found that a part of the observed lepton events is kinematically more consistent with top production than with Standard Model processes. Anomalous top production should also be visible as an increased event rate for hadronic W decays. No significant deviation from the Standard Model background is observed. The combination of all decay channels in a likelihood fit yields a cross-section for single top production of 0.31 +0.16 -0.15 pb at sqrt(s)=320 GeV. This combination gives a much better description of the data than the Standard Model alone. Due to the presently small number of top candidates, also an exclusion limit is set on the single top production cross-section and the flavor changing neutral current coupling. An upper limit on the t-u-gamma-coupling of kappa(t-u-gamma)

The top quark is by far the heaviest known fundamental particle with a mass nearing that of a gold atom. Because of this strikingly high mass, the top quark has several unique properties and might play an important role in electroweak symmetry breaking—the mechanism that gives all elementary particles mass. Creating top quarks requires access to very high energy collisions, and at present only the Tevatron collider at Fermilab is capable of reaching these energies. Until now, top quarks have only been observed produced in pairs via the strong interaction. At hadron colliders, it should also be possible to produce single top quarks via the electroweak interaction. Studies of single top quark production provide opportunities to measure the top quark spin, how top quarks mix with other quarks, and to look for new physics beyond the standard model. Because of these interesting properties, scientists have been looking for single top quarks for more than 15 years. This thesis presents the first discovery of single top quark production. It documents one of the flagship measurements of the D0 experiment, a collaboration of more than 600 physicists from around the world. It describes first observation of a physical process known as “single top quark production”, which had been sought for more than 10 years before its eventual discovery in 2009. Further, his thesis describes, in detail, the innovative approach Dr. Gillberg took to this analysis. Through the use of Boosted Decision Trees, a machine-learning technique, he observed the tiny single top signal within an otherwise overwhelming background. This Doctoral Thesis has been accepted by Simon Fraser University, Burnaby, BC, Canada.

The Lepton-Photon symposiums ? as represented by the contributions in this volume ? are among the most popular conferences in high energy physics since they give an in-depth snapshots of the status of the field as provided by leading experts.The volume covers the latest results on flavor factories, quantum chromodynamics (QCD), electroweak physics, dark matter searches, neutrino physics and cosmology, from a phenomenological point of view. It also offers a glimpse of the immediate future of the field through summaries on the status of the next generation of high energy accelerators and planned facilities for astroparticle physics.The review nature of the articles makes the volume particularly useful to students, as well as being of interest to established researches in high-energy physics and related fields.

The Lepton-Photon symposiums — as represented by the contributions in this volume — are among the most popular conferences in high energy physics since they give an in-depth snapshots of the status of the field as provided by leading experts.The volume covers the latest results on flavor factories, quantum chromodynamics (QCD), electroweak physics, dark matter searches, neutrino physics and cosmology, from a phenomenological point of view. It also offers a glimpse of the immediate future of the field through summaries on the status of the next generation of high energy accelerators and planned facilities for astroparticle physics.The review nature of the articles makes the volume particularly useful to students, as well as being of interest to established researches in high-energy physics and related fields.