We report the first observation of single-top-quark production in the s channel through the combination of the CDF and D0 measurements of the cross section in proton-antiproton collisions at a center-of-mass energy of 1.96 TeV. The data correspond to total integrated luminosities of up to 9.7 fb-1 per experiment. The measured cross section is $\sigma_s = 1.29^{+0.26}_{-0.24}$ pb. The probability of observing a statistical fluctuation of the background to a cross section of the observed size or larger is $1.8 \times 10^{-10}$, corresponding to a significance of 6.3 standard deviations for the presence of an s-channel contribution to the production of single-top quarks.

First observation of single-top-quark production in the s channel is reported. The result is based on the combination of the CDF and D0 measurements of the cross section in proton-antiproton collisions at a center-of-mass energy of 1.96 TeV. A summary of other recent single-top-quark results are also included.

Testing the Standard Model (SM) and looking for new phenomena have been the focus of generations of particle physicists in the last decades. Following this spirit, this thesis presents two searches. The first is the search for single top quark production from the exchange of an s-channel virtual W boson using events with an imbalance in the total transverse energy, b-tagged jets, and no identified leptons. Assuming the electroweak production of top quarks of mass 172.5 GeV/c2 in the s-channel, a cross section of 1.12+0.61 -0.57 (stat+syst) pb, with a significance of 1.9 standard deviations, is measured. This measurement is combined with the result obtained from events with an imbalance in total transverse momentum, b-tagged jets, and exactly one identified lepton, yielding a cross section of 1.36+0.37 -0.322 (stat+syst) pb, with a significance of 4.2 standard deviations. The first observation of single-top-quark production in the s channel through the combination of the CDF and D0 measurements is also reported. The measured cross section is [sigma]s = 1.29+0.26 -0.244 pb. The probability of observing a statistical fluctuation of the background to a cross section of the observed size or larger is 1.8 10-10, corresponding to a significance of 6.3 standard deviation. The second is the search for W'-like resonances decaying to tb. No significant excess above the SM prediction is found. Using a benchmark W' → tb left-right symmetric model, 95% C.L. mass-dependent upper limits are placed on the W0 boson production cross section times branching ratio to tb. Assuming a W' boson with SM-like couplings and allowed (forbidden) decay to leptons, W' → tb is excluded with 95% C.L. for W' boson masses below 860 (880) GeV/c2. Relaxing the hypothesis on SM-like couplings, we exclude W' boson coupling strength values as a function of the W' boson mass above 10% of the SM coupling strength for MW' = 300 GeV/c2. The constraints obtained with the present analysis are the most stringent for charged resonance masses below 570 GeV/c2 decaying to a top and a bottom quark.

The production of single-top quarks occurs via the weak interaction at the Fermilab Tevatron proton-antiproton collider. Single top quark events are selected in the lepton+jets final state by CDF and D0 and in the missing transverse energy plus jets final state by CDF. Multivariate classifiers separate the s-channel and t-channel single-top signals from the large backgrounds. The combination of CDF and D0 results leads to the first observation of the s-channel mode of single top quark production. The t-channel and single top combined cross sections have also been measured.

The top quark is the heaviest known fundamental particle, with a mass of 172.0+0.9-1.3GeV. This is nearly twice the mass of the second heaviest known particle, the Z boson, and roughly the mass of a gold atom. Because of its unusually large mass, studying the top quark may provide insight into the Higgs mechanism and other beyond the standard model physics. Only two accelerators in the world are powerful enough to produce top quarks. The Tevatron, which first accelerated protons in 1983, has produced almost 400,000 top quarks, roughly half at each of its two detectors: DO and CDF. The LHC is a much newer accelerator which currently has accumulated about 0.5% as much data as the Tevatron. However, when running at full luminosity, the LHC is capable of producing a top quark about once every second and will quickly surpass the Tevatron as the leading producer of top quarks. This analysis uses data from the DØ detector at the Tevatron, which are described in chapter 3. Top quarks are produced most often in pairs of top and anti-top quarks through an interaction of the strong force. This production mode was first observed in 1995 at the Tevatron. However, top quarks can also be produced though an electroweak interaction, which produces just one top quark. This production mode was first observed at the Tevatron in 2008. Single top quark production can occur in different channels. In this analysis, a measurement of the cross section of the t-channel production mode is performed. This measurement uses 5.4 fb-1 of data and uses the technique of boosted decision trees in order to separate signal from background events. The t-channel cross section is measured to be: ?(p$ar{p}$ → tqb + X) = 3.03+0.78-0.66 pb (0.0.1). Additional cross section measurements were also performed for the s-channel as well as the s + t-channel. The measurement of each one of these three cross sections was repeated three times using different techniques, and all three methods were combined into a 'super-method' which achieves the best performance. The details of these additional measurements are shown in appendix A.

The author reports on the observation of electroweak production of single top quarks in p{bar p} collisions at (square root)s = 1.96 Tev using 2.3 fb−1 of data collected with the D0 detector at the fermilab Tevatron Collider. Using events containing an isolated electron or muon, missing transverse energy, two, three or four jets, with one or two of them identified as originating from the fragmentation of a b quark, the measured cross section for the process p{bar p} --> tb + X, tqb + X is 3.94 ± 0.88 pb (for a top quark mass of 170 GeV). the probability to measure a cross section at this value or higher in the absence of signal is 2.5 x 10−7, corresponding to a 5.0 standard deviation significance. Using the same dataset, the measured cross sections for the t- and the s-channel processes when determined simultaneously with no assumption on their relative production rate are 3.14{sub -0.80}{sup +0.94} pb and 1.05 ± 0.81 pb respectively, consistent with standard model expectations. The measured t-channel cross section has a significance of 4.8 standard deviations, representing the first evidence for the production of an individual single top process to be detected.