Continuing the Search for Nothing: Invisible Higgs Boson Decays and High Luminosity Upgrades at the ATLAS Detector

This thesis presents two main projects involving the ATLAS experiment at the Large Hadron Collider: a search for invisible decays of the Higgs boson produced via vector boson fusion, and the design and simulation of an application specific integrated circuit produced for the Inner Tracker Strip detector upgrade project, the Hybrid Controller Chip (HCCStar). The HCCStar will be installed in the ATLAS detector around 2026 for High Luminosity LHC operations, which will see the rate of collisions increased to 200 every 25 ns. Verification of the HCCStar design was performed using cocotb, a Python framework for testing digital logic. The search for invisible Higgs decays was conducted using 139 $fb^{-1}$ of recorded proton-proton collision data with a center-of-mass energy of $\sqrt{s}$ = 13 TeV collected between 2015 and 2018. Observed (expected) upper limits were set on the branching ratio of the Higgs boson to an all-invisible final state at $\mathcal{B}_\text{H to inv.} = 0.15$ $(0.11)$ at a 95% confidence level. No significant disagreement from the Standard Model, which predicts $\mathcal{B}_\text{H to inv.} \sim 1.0 \times 10^{-3}$, was observed. This result is then reinterpreted in the context of Higgs portal dark matter and compared to various direct detection experiments searching for evidence of weakly interacting massive particles (WIMPs).