Two Searches for Signals of Dark Matter with the ATLAS Detector in 139 $\mathrm{fb^{-1}}$ of LHC $\mathrm{\sqrt{s}}$ = 13 TeV Proton-Proton Collision Data

This dissertation presents two searches for signals of dark matter in an integrated luminosity of 139 $\mathrm{fb^-1}$ of proton-proton collision data collected at a center of mass energy of $\sqrt{s}$ = 13 TeV with the ATLAS detector at the Large Hadron Collider (LHC). The search for direct pair production of the supersymmetric partner to the top quark (the stop) in the all-hadronic $t\bar{t}$ plus missing transverse momentum final state yields no significant excess over the expected Standard Model background and was able to exclude stop masses up to 1.25 TeV for dark matter candidate masses below 200 GeV. The search for dark mesons decaying into top and bottom quarks is sensitive to a proposed strongly coupled dark sector which contains a viable dark matter candidate scalar baryon. This analysis considers the all-hadronic channel of a final state of all top and bottom quarks ($tttb$ or $tbtb$) with no additional missing transverse momentum. No previous LHC searches have considered this dark meson model and we expect to set new limits on dark pion masses of up to 500 GeV.