Search for Single Top Quark Production in Association with a $W$ and a $Z$ Boson in the 3 Lepton Final State with the ATLAS Experiment at 13 TeV

The dominant mode of top-quark production in proton-proton collisions is the pair production. The single top quark production is an electroweak process with a small rate, providing an additional source for top-quark physics. Associated production of single top quarks are among the rare processes predicted by the Standard Model. A search for associated production of a top quark with a $W$ and a $Z$ gauge boson in the trileptonic final state is presented in this thesis. The study has been done using the data collected with the ATLAS detector at the Large Hadron Collider during the years 2015 and 2016. This dataset corresponds to proton-proton collisions at a centre-of-mass energy of $ \sqrt{s}~$=$~13~$TeV and a total integrated luminosity of 36.1$~\text{fb}^{-1}$. Multivariate techniques are used to enhance signal and background separation. An upper limit of 6.6 $($Expected$: 4.6^{+7.6}_{-3.3})$ times the Standard Model prediction on the cross section at the 95% confidence level has been set using the $\text{CL}_{\text{s}}$ method. No significant deviation from the Standard Model has been observed.