Searches for supersymmetric partners of the bottom and top quarks with the ATLAS detector

"Supersymmetry is a promising candidate theory that could solve the hierarchy problem and explain the dark matter density in the Universe. The ATLAS experiment at the Large Hadron Collider is sensitive to a variety of such supersymmetric models. This thesis reports on a search for pair production of the supersymmetric scalar partners of bottom and top quarks in 20.1 fb$^{-1}$ of $pp$ collisions at a centre-of-mass energy of 8 TeV using the ATLAS experiment. The study focuses on final states with large missing transverse momentum, no electrons or muons and two jets identified as originating from a $b$-quark. This final state can be produced in a $R$-parity conserving minimal supersymmetric scenario, assuming that the scalar bottom decays exclusively to a bottom quark and a neutralino and the scalar top decays to a bottom quark and a chargino, with a small mass difference with the neutralino. As no signal is observed above the Standard Model expectation, competitive exclusion limits are set on scalar bottom and top production, surpassing previously existing limits. Sbottom masses up to 640 GeV are excluded at 95% CLs for neutralino masses of up to 150 GeV. Differences in mass between $\tilde{b}_1$ and $\tilde{\chi}_{1}^{0}$ larger than 50 GeV are excluded up to sbottom masses of 300 GeV. In the case of stop pair production and decay $\tilde{t}_{1} \rightarrow b + \tilde{\chi}_{1}^{\pm}$ and $\tilde{\chi}_{1}^{\pm} \rightarrow \tilde{\chi}_{1}^{0} + W^{*}$ with mass differences $\Delta m = m_{\tilde{\chi}_1^{\pm}} - m_{\tilde{\chi}_1^{0}} = 5~\text{GeV} (20~\text{GeV})$, stop masses up to 580 GeV (440 GeV) are excluded for $m_{\tilde{\chi}_1^{0}} = 100~\text{GeV}$. Neutralino masses up to 280 GeV (230 GeV) are excluded for $m_{\tilde{t}_1} = 420~\text{GeV}$ for $\Delta m = 5~\text{GeV} (20~\text{GeV})$. In an extension of this analysis, sbottom quarks cascade-decaying to at least a Higgs boson are searched for in final states with large missing transverse momentum, at least $3~b$-tagged jets and no electrons or muons, using neural network discriminants."