Searches for chargino and neutralino production in multileptonic final states using $\sqrt{s}$ = 13 TeV proton-proton collisions with the ATLAS detector

This thesis presents the work of the Candidate during his PhD on three analyses targeting the Electroweak Supersymmetry production of charginos and neutralinos decaying into multilptonic final states. Only electrons and muons are considered along with R-Parity-conserving decays. These analyses all use from $\sqrt{s}$ = 13 TeV proton-proton collisions collected by the ATLAS experiment at the Large Hadron Collider in the 2015-2018 period for a total of 139 $fb^{−1}$. Firstly, the analysis targeting final states with two leptons of the same electrical charge and intermediate $W$ and Higgs on-shell bosons is presented. The results show no significant excess from the Standard Model predictions and new constraints on the masses of charginos and neutralinos are obtained for this model, significantly extending the known bounds from previous searches. Secondly, results of another analysis targeting intermediate states with on-shell $W Z$ and $W h$ bosons and three-lepton final states are also presented. Once again, no significant deviation from the Standard Model prediction is observed and exclusion limits on the masses of charginos and neutralinos are set for these models. Finally, the statistical combination of these searches with those of other analyses targeting the same electroweak Supersymmetry production mechanism with different final states is also presented in this thesis. The final results show how such combination can be used to further improve the constraints on the masses of the charginos and neutralinos. Moreover, the details of a technical task concerning the characterisation of the performance of the Inner Detector Trigger tracking of electrons and muons in the ATLAS experiment through the usage of the Tag-and-Probe technique is also discussed. The final result show a significant improvement in the accuracy of the measured Trigger tracking efficiencies with respect to those obtained by means of the previously used procedure.