Upgrade of the Small Wheels and search for Dark Matter in association with top quarks at the ATLAS experiment

The ATLAS experiment has been successfully taking data since 2009, allowing to deeply investigate the Standard Model (SM) physics at unprecedented precision and to constrain many Beyond the Standard Model (BSM) scenarios. In this thesis a search for Dark Matter (DM) production in association with top quarks based on a Simplified Model is presented. The analysed data have been collected at the ATLAS experiment in 2015 and 2016 for a total integrated luminosity of 36.1 fb$^{-1}$, at $\sqrt{s}=13$ TeV. The investigated DM model describes the coupling between Dark and Standard Model sectors, as mediated by a spin 0 scalar or a pseudoscalar particle. A final state with two leptons, missing transverse momentum and jets is considered. No significant excess is observed with respect to the SM background. Results are interpreted in terms of limits on the model as a function of the mass of the mediator and of the Dark Matter particle, in particular excluding scalar mediators with mass up to 50~GeV for a DM particle of 1~GeV. Limits in the context of a generic BSM signal are also set. The same results are also reinterpreted in terms of a model extending the Simplified one, described above, and foreseeing two Higgs doublets (2HDMa). This model is particularly attractive since it extends the SM, while being compatible with the stringent Higgs physics constraints. \\ In between data taking periods, shutdowns are required to upgrade both the experiments and the LHC machines in order to achieve higher luminosity. One of the most challenging on-going upgrades in ATLAS concerns the replacement of the first muon station in the forward region (New Small Wheels) with novel gaseous detector technologies. The upgrade has entered the mass production of the detectors, with the goal of building all the quadruplets in time for their installation during the LS2, starting in 2019. In the thesis large emphasis is given to the readout Printed Circuit Boards (PCBs) production and to the quality control performed at CERN, focusing on some of the most relevant challenges that have been faced. Additionally, the performance of the Micromegas detectors is evaluated in a dedicated test facility at CERN, in a very high particle rate environment. No performance degradation is observed up to 70 kHz/cm$^2$, four times the maximum rate expected in the New Small Wheels region. The ageing of the detector prototypes is also evaluated by accumulating 0.3 C/cm$^2$, a value largely exceeding what is expected in 10 years of LHC operations. No significant detrimental effects are observed in the detectors.