Search for new physics processes with same charge leptons in the final state with the ATLAS detector using proton--proton collisions at $\sqrt{s} = 13$ TeV

The thesis presents the advantage of using same-charge leptons in the final state in searches for new physics processes using proton--proton collisions recorded by the ATLAS detector at the LHC. The used data sample at a centre-of-mass energy of 13 TeV corresponds to 36.1 $\mathrm{fb}^{-1}$ of integrated luminosity recorded in 2015 and 2016. Data-driven methods are derived to estimate the Standard Model background in same-charge channels up to transverse momenta of leptons of about $p_{\mathrm{T}}=$ 1 TeV. The Left-Right Symmetric Model is used as the benchmark beyond the Standard Model framework to optimize the analysis and to search for the production of doubly charged Higgs bosons ($H^{\pm\pm}$), heavy neutrinos ($N_{\mathrm{R}}$), and right-handed vector gauge bosons ($W_{\mathrm{R}}$). No significant evidence of a signal is observed and corresponding lower limits on the production cross-section are derived for the pair production of $H^{\pm\pm}$ bosons ($pp \to H^{++}H^{--}$) and for the Keung-Senjanovic process ($pp \to \ell N_{\mathrm{R}} \to \ell \ell W_{\mathrm{R}} \to \ell\ell jj$). The observed limits of the two processes are the strongest to date and a novel method of parametrizing the electron charge misidentification probability is presented and used for the first time in the ATLAS Collaboration.