Novel searches for top squarks at the LHC

In this thesis, I present two searches for new physics performed analysing the data collected by the ATLAS detector, during the Run 2 of the LHC. The searches are designed to target top squark pair production in Supersymmetric models which escape the traditional search approaches and are particularly sensitive to scenarios with compressed spectra or long decay chains. Both searches are characterised by the presence of multiple charged leptons (electrons or muons) and missing transverse momentum in the final state. The results were found to be in agreement with the Standard Model predictions. The observations were interpreted to set exclusion limits significantly expanding the previous coverage on the masses of the SUSY particles under test. Models featuring direct production of either the lighter top squark ($\tilde{t}_1$) or the heavier top squark ($\tilde{t}_2$) were excluded at 95% confidence level for $\tilde{t}_1$ and $\tilde{t}_2$ masses up to about 1140 and 875 GeV, respectively, as well as neutralino masses up to 520 GeV, depending on the model considered. The challenges that silicon detectors will have to face in the high luminosity environment of the LHC upgrade (HL-LHC) are then presented. In this context, the effect of radiation damage on pixel sensor prototypes, designed for the ATLAS Inner Tracker upgrade (ITk), is studied, focusing on the evolution of the active region of the sensors with the increase of radiation. All the sensors measured after proton irradiation show an increase of depletion depth, reaching a maximum between $1\times 10^{14}$ and $3\times 10^{14}$ 1 MeV $\text{n}_{\text{eq}}/\text{cm}^2$, when irradiated with 16.7 MeV protons, and between $1\times 10^{14} $ and $6\times 10^{14}$ 1 MeV $\text{n}_{\text{eq}}/\text{cm}^2$ when irradiated with 24 GeV protons. These results present strong differences with respect to the ones obtained after neutron irradiation, demonstrating the distinct effect caused by the two particles. In parallel, I investigated the sensitivity for future searches for electroweakino production in nearly mass degenerate scenarios, assuming an integrated luminosity of $3000~\text{fb}^{-1}$ collected during the HL-LHC data taking. This search is based on events with tracks recorded only in the innermost layers of the detector, and it is therefore key for the assessment of the physics performance of the ITk. The discovery potential of the analysis would allow for the discovery of wino-like (higgsino-like) charginos of mass 100 GeV with lifetimes between 20 ps and 700 ns (30 ps and 250 ns), or for a lifetime of 1 ns would allow the discovery of wino-like (higgsino-like) charginos of mass up to 800 GeV (600 GeV).