Of Triggers, Supersymmetry and Photons - Triggers for Dark Matter and a Search for Gauge-Mediated Supersymmetry Breaking with the ATLAS detector at the LHC

This thesis summarises research using the ATLAS experiment to search for Dark Matter. A performance study of the ATLAS Run 2 $E_{\mathrm{T}}^{\mathrm{miss}}$ trigger is presented, introducing relevant performance metrics and the algorithms that have lead to the successful operation of the trigger during LHC Run 2. The trktc algorithm, a candidate algorithm for the Run 3 $E_{\mathrm{T}}^{\mathrm{miss}}$ trigger, and its performance are discussed in detail, motivating the combination of a tracking-based algorithm with a Run 2 algorithm in constructing the Run 3 primary trigger. A search for new physics in $hh$+invisible in $139\,\mathrm{fb}^{-1}$ of $pp$ data collected at $\sqrt{s}=13\,\mathrm{TeV}$ with the ATLAS experiment is presented. Searches are performed for events in which one Higgs boson decays to $\gamma\gamma$ and the other to $b\bar{b}$. The final state of $\gamma\gamma b\bar{b}$+$E_{\mathrm{T}}^{\mathrm{miss}}$ was previously unexplored by the ATLAS experiment. No evidence for physics beyond the Standard Model has been found. Results are interpreted in the context of Gauge-Mediated Supersymmetry Breaking models. Limits are presented assuming a pure Higgsino neutralino, which decays via $\tilde{\chi}_1^0\rightarrow h\tilde{G}$ or $\tilde{\chi}_1^0\rightarrow Z\tilde{G}$. Assuming $\mathrm{BF}(\tilde{\chi}_1^0\rightarrow h\tilde{G}$) = 100%, models with $m(\tilde{\chi}_1^0)\leq300\,\mathrm{GeV}$ are excluded. For lower branching fractions of $\mathrm{BF}(\tilde{\chi}_1^0\rightarrow h\tilde{G}$) = 75%, models with neutralino masses of up to $\sim250\,\mathrm{GeV}$ are excluded. Models with $m(\tilde{\chi}_1^0)=130\,\mathrm{GeV}$ are excluded for branching fractions as low as 40%. The presented search almost completely closes a gap in the ATLAS experiment's sensitivity to Dark Matter models.