To the bottom of the stop

In the first part of this thesis, the results of a calibration of bottom quark jets identification algorithms are reported. The analysis is performed with 5 fb$^{-1}$ of proton-proton collisions at 7 TeV centre-of-mass energy recorded by the ATLAS detector at the LHC. A b-jet enriched sample from fully reconstructed $B^{\pm}\rightarrow J/\Psi K^{\pm}$ was used to validate the Monte Carlo simulation of b-jets properties and identification algorithms to the experimental data. In the second part of the thesis, the results of a search for top squark pair production in final states with one isolated lepton, jets, and missing transverse momentum are reported. The search is based on 20 fb$^{-1}$ of pp collisions at 8 TeV centre-of-mass energy recorded by the ATLAS detector. No significant excess over the Standard Model prediction is observed. The results were interpreted in terms of exclusion limits on a wide range of scenarios with different decay modes of the top squark and mass assumptions for the top squark and the other particles considered in the decay chain. Finally, the search was interpreted in terms of dark matter plus top pairs associate production. Limits were derived on dark matter nucleon scattering cross section for two contact operators (D1 and C1) in the context of dark matter effective field theory.