Supersymmetry search in final states with missing energy and at least three $b$-jets

Supersymmetry (SUSY) is a leading candidate theory for describing physics beyond the Standard Model (SM) as it has the potential for providing a consistent and natural embedding of the SM in a more general theory whose natural scale is the Planck scale ($M_{P}$). The Large Hadron Collider (LHC) and the ATLAS experiment have been designed with the capability to determine whether SUSY is a correct description of Nature. This thesis presents the results of the first search for top and bottom squarks from gluino pair production in events with large missing transverse momentum and at least three jets identified as originating from a $b$-quark. The analysis is performed with a total integrated luminosity corresponding to 4.7 fb$^{-1}$ of proton-proton collisions at center-of-mass energy $\sqrt{s} = 7$ TeV. An exhaustive optimization has been done targeting different topologies involving SUSY particles, from which five enriched signal regions (SR) have been chosen to identify possible SUSY-like event candidates with at least three $b$-jets and missing transverse energy. The results obtained in each of the five SR have been found to be in good agreement with the SM predictions. Therefore, exclusion limits at 95\% confidence level are presented for a variety of gluino-mediated models with gluino masses up to 1.02 TeV excluded. These are the most stringent limits obtained up to now by collider experiments, and impose harsh constraints in several supersymmetric models