Search for New Heavy Charged Bosons and Singly Produced Leptoquarks with the ATLAS Experiment

In the years 2015 to 2018 the Large Hadron Collider (LHC) at CERN collided protons at a center of mass energy of $\rm \sqrt{s} = 13\,{\rm TeV}$ while operating at a very high instantaneous luminosity. This energy range has never been reached by accelerator experiments before. This is the reason why the data set, collected by each of the four big experiments which are located at an interaction point of the LHC, gives a great opportunity to precisely test Standard Model (SM) predictions and explore new physics phenomena on the TeV scale. Two direct searches for new particles in the data, which was collected by the ATLAS experiment during this period, are presented in this thesis. A signal in each probed final state can be created by a variety of SM extensions. These are postulated to address its conceptual problems and explain observations, which are inconsistent with SM predictions. The sensitive variable is designed with the aim to be highly correlated to the invariant mass of the new particles' decay products in both analyses. Background contributions are estimated with Monte Carlo simulations and data driven techniques. Resonant signals in final states with a charged {(anti-)lepton} (lepton meaning electron or muon) and large missing transverse energy, that is created by the corresponding {(anti-)neutrino}, are probed in the first analysis. The considered signal characteristic is introduced by a variety of new theories, which is the reason why this search is designed with the aim to make no strong model assumptions. For example the existence of a new heavy charged gauge boson - a so-called $W'$ - creates this signature. The analysis is conducted on the data set recorded in 2015 and 2016 by ATLAS, which corresponds to an integrated luminosity of $36\,{\rm fb^{-1}}$. In the second search the existence of new particles, which carry a lepton and a baryon number simultaneously - the so-called Leptoquarks (LQ) - is investigated. Specifically their single production is explored, which allows to probe higher LQ mass hypotheses at the cost of making stronger model assumptions compared to their pair production process. This leads to final states with a particle anti-particle pair of charged leptons (lepton meaning electron or muon) and at least one high energy jet. For this the full Run 2 data set of proton-proton collisions, which corresponds to $139\,{\rm fb^{-1}}$, is probed. No significant deviations from the SM prediction are found in either of the searches. This observation is quantified by likelihood ratio tests. In the end 95% confidence level (CL) upper limits on the signal cross-section times branching ratio are determined. These are then interpreted as lower limits on the new particles' mass. When probing the Sequential Standard Model, new heavy charged gauge bosons are excluded up to masses of $5.43\,{\rm TeV}$ at 95% CL, which is an improvement by more than $2\,{\rm TeV}$ compared to Run 1 results published by ATLAS and CMS. For the determination of the 95% CL lower mass exclusion limits, the LQ-lepton-quark coupling is assumed to be one. Results are in the range of $1.2\,{\rm TeV}$ to $2.8\,{\rm TeV}$. Under these model specific assumptions, mass limits determined for the LQ decay into up quarks are the strongest to date - with $2.3\,{\rm TeV}$ in the electron and $2.8\,{\rm TeV}$ in the muon channel.