Data-driven estimation of the non-prompt background in same charged $W^\pm W^\pm$ scattering within the ATLAS experiment

The scattering of vector bosons (VBS) offers a unique opportunity to study the electroweak sector of the Standard Model, the Higgs mechanism and furthermore physics beyond the Standard Model. A very promising channel to investigate vector boson scattering at the LHC is the scattering of same charged $W^\pm$ bosons due to its comparatively high cross-section of the electroweak processes. The second largest background of this $W^\pm W^\pm jj$-EW signal originates from misidentified leptons (non-prompt leptons). This thesis aims to improve the data-driven method used so far to estimate this non-prompt background. In order to avoid large extrapolation factors from the control region used for the data-driven method to the signal region, a new control region using dilepton events is defined. This dilepton control region is kinematically closer to the signal region than the dijet control region used in the previous publications. The data-driven method is adapted to the dilepton control region and thoroughly tested with Monte Carlo simulated events. The data studied in this thesis were measured with the ATLAS experiment at a collision energy of 13 TeV with an integrated luminosity of 138.7 $\text{fb}^{-1}$. Since the signal region of the analysis is blinded, the validity of the data-driven method is proven by using the low dijet invariant mass validation region, which is kinematically very close to the signal region. The data in this validation region is sufficiently well modeled by the sum of the data-driven estimated non-prompt background and the prompt and charge flip contribution estimated by Monte Carlo simulations. Therefore, the data-driven estimation of non-prompt background described in this thesis is expected to have only a small extrapolation to the signal region and thus provides a valuable contribution to the current $W^\pm W^\pm jj$-EW analysis.