SUSY and dark matter searches using the ATLAS detector

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Scattering experiments govern our study of natural phenomena. Our eyes make observations via the scattering of light. Understanding its behaviour allowed microscopes and telescopes to be built, which Hooke and Galileo used to unveil then hidden structures of cellular biology and extraterrestrial moons. This set the basis of empirical enquiry: making measurements to probe known phenomena allows the construction of instruments to reveal unknown sectors of nature. The science at the Large Hadron Collider (LHC) is no exception. This modern incarnation of the microscope is the most sophisticated scattering experiment in human history. Situated at CERN on the Swiss-French border, its discoveries could profoundly deepen our understanding of space and time to the matter and forces in our universe, which is the subject of this thesis. The LHC scatters proton beams at the world's highest centre-of-momentum energies of 13 TeV. TheATLAS detector measures the resulting debris using tracking, calorimetry and muon systems based on a variety of solid, liquid and gaseous phase technologies, enabling a rich physics programme. This class of scattering experiment exploits two striking features of relativity and quantum mechanics: 1) matter-energy equivalence enables production of new distinct particles, and 2) particle-wave duality opens non-optical probes of subatomic structures. Decades of experimental and theoretical work reconciled these two pictures, culminating in the Standard Model (SM) of particle physics. This principled and predictive theory continues to describe all observed phenomena at colliders with remarkable success.However, as a fundamental description of nature, the Standard Model has numerous shortcomings. Two of the most pressing addressed in this thesis are a) the fine-tuned structure of fundamental parameters, and b) no viable particle to explain the observed astrophysical dark matter. One class of solutions extends spacetime symmetries of the SM using supersymmetry. This predicts partners of SM particles which differ by half a unit - 1 - of spin and are accessible at the LHC. Two complementary experimental programmes seek evidence for such new phenomena: 1) empirically testing the consistency of relativity and quantum mechanics at the energy frontier, and 2) directly searching for new phenomena to reveal unprobed sectors of nature.