Search for doubly charged Higgs bosons in multi-lepton final states with the ATLAS detector

Standard Model events with prompt, isolated, same-charge high-pT leptons are rare, and therefore events with such final states provide a powerful signature towards new discoveries. This thesis presents two searches for doubly charged Higgs bosons decaying into same-charge lepton pairs using proton–proton collision data recorded by the ATLAS detector at the LHC. Observation of doubly charged Higgs bosons might provide hints about the mechanism for generating small neutrino masses. The first search uses a data sample corresponding to 36.1 fb$^{-1}$ of integrated luminosity recorded in 2015 and 2016, focusing on the final states with electrons and muons. The second analysis extends the search with hadronically decaying taus and the full LHC Run 2 dataset recorded in 2015-2018 at √s = 13 TeV, corresponding to 139 fb$^{-1}$. The searches cover a wide signal mass hypothesis range from 300 GeV to 1.3 TeV, and show excellent understanding between the collision data and the Standard Model background predictions. Despite further optimization of event and physics object selection to enhance the sensitivity to new physics processes, no significant evidence of a signal is observed. Corresponding limits on the production cross-section and on the mass of the doubly charged Higgs bosons are derived for the pair production of the $H$±± bosons at 95% confidence level. As the reconstruction of charged leptons requires excellent momentum resolution and tracking capabilities, the thesis also describes the drift time calibrations of the ATLAS Transition Radiation Tracker (TRT). By extending the lever arm of the Inner Detector, the TRT improves the momentum resolution and the track parameters of the event reconstruction. Preserving track reconstruction performance under challenging experimental conditions despite irreparable gas leaks in the detector is of the utmost importance. Precise calibrations in various gas configurations using both simulated and real data from proton-proton and heavy ion collisions are performed to achieve that goal, and overall good drift time accuracy and data quality are provided by the TRT during the full Run 2.