The Light, the Massive and the Unattainable: chasing new Physics in the dimuon channel with ATLAS at LHC

The studies presented in this thesis are motivated on new physics models which predict the existence of new particles decaying in a dilepton final state. Searches for new resonant phenomena in the mass range 250 GeV to 6 TeV and non resonant phenomena up to an energy scale of 40 TeV are presented. The data were recorded by the ATLAS experiment in proton-proton collisions at a centre-of-mass energy of 13 TeV during Run 2 of the Large Hadron Collider and correspond to an integrated luminosity of 139 fb−1. Fits of the experimental dilepton invariant mass spectra are performed allowing to determine the significance of possible deviations from the estimated backgrounds. No significant deviation is observed. Upper limits are placed on the fiducial cross-section times branching ratio, as well as on the number of signal events. For a set of benchmark models, the limits are converted into lower limits on the resonance mass and on the contact interaction energy scale. A fundamental role is played by the performance of the ATLAS muon spectrometer in the reconstruction of high-pT muons. This work provides an overview on the muon reconstruction method, defining the requirements for the identification of high-pT muons, together with several innovations. The high-mass analysis is extended to the lower mass (40-73 GeV) searching for a narrow resonance decaying to a pair of muons. Expected model independent upper limits are set on the fiducial cross-section times branching ratio. A reinterpretation of the limits in the context of dark photon model is also presented. Furthermore, the LHC accelerator will be upgraded with an increase of instantaneous and integrated luminosity. The ATLAS goal is to cope with higher particle rate while preserving the current muon detection efficiency. For this purpose the current innermost stations of the Muon Spectrometer end-caps will be replaced with the New Small Wheel (NSW) detector. The NSW will feature two new detector technologies: resistive Micromegas and sTGC chambers. An overview of the design, construction, assembly procedures and validation test of the Micromegas modules is reported. Results and characterization are also presented.