Search for a Scalar Partner of the Top Quark in the Jets+MET Final State with the ATLAS detector

This dissertation presents searches for direct pair production of a scalar partner of the top quark in events with only jets and \acrlong{met} in proton--proton collisions recorded during LHC Run 1 and Run 2 with the ATLAS detector. In the supersymmetry scenario, the partner is called top squark or stop. The stop ($\stop$) is assumed to decay via $\stop \to t \ninoone$, $\stop\to b\chinoonepm \to b W^{\left(\ast\right)} \ninoone$, or $\stop\ra bW\ninoone$, where $\ninoone$ ($\chinoonepm$) denotes the lightest neutralino (chargino). Exclusion limits are reported in terms of the stop and neutralino masses. The LHC Run 1 analysis uses an integrated luminosity of 20.1~{\ifb} at $\acrshort{sqrts}=8~\tev$ to exclude top squark masses in the range $270$--$645~\GeV$ for $\ninoone$ masses below $30~\GeV$, assuming a 100\% $\stop \to t \ninoone$ \gls{br}. For a \gls{br} of $50\%$ to either $\stop \to t \ninoone$ or $\stop\to b\chinoonepm$, and assuming $m_{\chinoonepm} = 2 m_{\ninoone}$, stop masses in the range $250$--$550~\GeV$ are excluded for $\ninoone$ masses below $60~\GeV$. The LHC Run 2 analysis uses an integrated luminosity of 13.3~{\ifb} at $\acrshort{sqrts}=13~\tev$. Assuming a 100\% $\stop \to t \ninoone$ \gls{br}, stop masses in the range $310$--$820~\GeV$ are excluded for $\ninoone$ masses below $160~\GeV$. For $\mstop\sim m_t+\mLSP$ scenario, the search excludes stop masses between $23$--$380~\GeV$. Additionally, scenarios where stops are produced indirectly through gluino decay but have very low $p_T$ signature due to a very small $\Delta (\mstop, \mLSP)$, have been considered. The result is interpreted as an upper limit for the cross section in terms of the gluino and stop masses. This excludes all models considered which include $m_{\gluino}$ up to 1600~\GeV\ with $\mLSP<560~\GeV$ at 95\% CL. Finally, the analysis strategy from the LHC Run 1 search is applied in the broader scope of supersymmetry called \gls{pmssm}. This dissertation presents a summary of the results that related to the stop search.