| Izvleček: | <p>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.</p>
<p>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/c<sup>2</sup> in the s-channel, a cross section of 1.12<sup>(+0.61)</sup><sub style='position: relative; left: -2.4em;'>-0.57</sub>
(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<<sup>(+0.37)</sup><sub style='position: relative; left: -2.4em;'>-0.32</sub> (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 σ<sub>s</sub> = 1.29<sup>(+0.26)</sup><sub style='position: relative; left: -2.4em;'>-0.24</sub>
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<sup>-10</sup>, corresponding to a significance of 6.3 standard deviation.</p>
<p>The second is the search for <em>W'</em>-like resonances decaying to tb. No significant excess above
the SM prediction is found. Using a benchmark <em>W'</em> → <em>tb</em> left-right symmetric model, 95%
C.L. mass-dependent upper limits are placed on the <em>W'</em> boson production cross section
times branching ratio to tb. Assuming a <em>W'</em> boson with SM-like couplings and allowed
(forbidden) decay to leptons, <em>W'</em> → <em>tb</em>is excluded with 95% C.L. for <em>W'</em> boson masses
below 860 (880) GeV/c<sup>2</sup>. Relaxing the hypothesis on SM-like couplings, we exclude <em>W'</em>
boson coupling strength values as a function of the <em>W'</em> boson mass above 10% of the
SM coupling strength for MW0 = 300 GeV/c<sup>2</sup>. The constraints obtained with the present
analysis are the most stringent for charged resonance masses below 570 GeV/c<sup>2</sup> decaying
to a top and a bottom quark.</p>
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