Search for new physics in events with same sign leptons and missing energy with ATLAS at LHC

The Standard Model of particle physics describes the phenomena in the infinitesimal with great success. However, it suffers from several inadequacies~: it can not explain the neutrino masses, it has no candidate for the dark matter and it has no solution for the gauge hierarchy problem, which all require new physics and beyond the Standard Model theories. Such a theory is Supersymmetry (SUSY) and occupies a primer place in the LHC physics program. Here two proton beams are colliding at energies up to 14 TeV and gigantic detectors were built to reconstruct the collision events. For the presented studies only the data recorded with the ATLAS detector is analyzed. More details on the theoretical framework, the LHC collider and the ATLAS experiment are given in the first part of this thesis. At hadron colliders, the lepton identification and reconstruction are crucial for precise SM cross sections and coupling measurements or for new physics searches. Therefore, the second part of my thesis is dedicated to performance studies : in-situ measurements of electron identification and reconstruction efficiencies. Both the methodology employed to obtain the results and the methods applied to estimate the background are detailed. The search for new physics is presented in the third part of the thesis. For these studies the channel with two same~- sign leptons and missing transverse energy is considered. The results are obtained with $L$ = 20 fb$^{-1}$ of data recorded at $\sqrt s$ = 8 TeV and with the very first data at $\sqrt s$ = 13 TeV. At the end of Run-1 no significant excess in data over the Standard Model prediction is observed and stringent limits on the supersymmetric particle masses are set. With this analysis, the gluino mass is constrained to be above 1 TeV and the sbottom mass should be above 600 GeV when using simplified assumptions. These results provide new and very strong constraints on natural SUSY models. With the two same-sign leptons and jets final state it is also possible to measure directly the top quark Yukawa coupling using the $t \bar t$ + $H$ channel. This coupling is the largest in the Standard Model and it connects two of the heaviest particles predicted by the Standard Model, the top quark and the Higgs boson. At $\sqrt s$ = 8 TeV the results are dominated by the systematic uncertainties and only upper limits can be obtained. In the fourth part of this thesis, a complete optimization is performed to reach the highest sensitivity for this signal in the two same-sign leptons channel at $\sqrt s$ = 13 TeV with 100 fb$^{-1}$ of data, corresponding to the end of Run-2. This prospect study is based on a simplified but realistic framework for systematics and the 8 TeV Monte Carlo samples are PDF re-weighted. Two signal regions are proposed and a precision of 60$\%$ on the signal strength is reached. When combined with other measurements performed in $t \bar t$ + $H$ ($\rightarrow \gamma\gamma$) and $t \bar t$ + $H$ ($\rightarrow b\bar b$) channels the top Yukawa coupling can be measured with a precision below 30$\%$.