In the first part of the thesis, a performance study in full simulation in the electromagnetic barrel (EMB) and the electromagnetic end cap (EMEC) of the ATLAS detector using different Geant4 software versions (10.1 vs 10.6) is investigated.  Using a particle gun option in Geant4, which enables users to shoot a particle in complex ATLAS geometry, unconverted photon shower shapes are studied to fine-tune the optimal setting that would be useful for Run-3 Monte Carlo Campaign in ATLAS.  Using four calorimetric variables for photon identification and reconstruction, the outcome of each configuration is checked with the Run-2 ATLAS data for physics precision.  Combining the overall results, the EMY option in Geant4 10.6, seems to be the best option for photon transport simulation in the EMB and EMEC.

The second part involves searching for the production of a top quark pair in association with a W boson and at least one extra jet through electroweak interactions in proton-proton collisions. The data used in the study corresponds to an integrated luminosity of 139 fb-1 at a center-of-mass energy of 13 TeV. The aim of the study is to establish limits on the cross-section of the Standard Model and on the effective field theory operators that can modify the coupling of top quark and electroweak interactions through the contribution of tW scattering. The study employs same-sign pairs of electrons and muons along with jets, with at least one b-tagged jet. The most forward jet's properties are used to distinguish between electroweak ttW+j production and the strong production counterpart. The study also measures the dominating backgrounds in data side-band control regions to decrease modeling dependence.