Higgs Boson and New Physics Searches in Events with Same-sign Lepton Pairs at the LHC

The physics of elementary particles is well described by the Standard Model of particle physics. This theory predicts with great accuracy the electroweak and strong interactions of elementary particles. The introduction of the Brout-Englert-Higgs mechanism in Standard Model has also given justification to the non-null mass of the elementary particles. This mechanism has been validated by the discovery of the Higgs boson and the measurement of its properties. Despite the enormous and broad success of the Standard Model, a few observations cannot be explained by this theory. For instance, neutrino oscillations would require the neutrinos to have non-null mass in contrast with the Standard Model prediction. Several solutions to this problem have been proposed, but none has been validated yet. Some of these theories have additional modes to produce lepton pairs with same electric charge (same-sign lepton pairs). This production is also expected to be enhanced in other theories beyond the Standard Model. The focus of this thesis is the inclusive search for anomalous production of same-sign lepton pairs in proton-proton collisions. The specific case of resonant production, typical of the Type-II see-saw and Zee-Babu models of neutrino mass generation, is also considered. 20.3 fb$^{-1}$ of data collected by the ATLAS experiment at a centre-of-mass energy of 8 TeV is analysed. No significant production above the Standard Model prediction is found, thus the results are used to set upper limits on the cross-section of processes beyond the Standard Model. Limits are placed as a function of the di-lepton invariant mass within a fiducial region defined according to the event selection criteria. Exclusion limits are also derived for specific models of resonant production. The production of same-sign lepton pairs from proton-proton collisions occurs rarely in the Standard Model. One process which generates same-sign lepton pairs is the associated production of a Higgs boson and a top anti-top quark pair ($t\bar{t}H$). The cross-section of this production mode is proportional to the Top Yukawa coupling squared, while other processes are only sensitive to the combination of this constant with several other couplings. The measurement of the $t\bar{t}H$ cross-section therefore constitutes an important test of the Standard Model and the Brout-Englert-Higgs mechanism. The $t\bar{t}H$ production has never been observed before, and is studied in this thesis. The analysis uses collisions at a centre-of-mass energy of 8 TeV collected by the ATLAS experiment containing a same-sign lepton pair and jets. The measured production cross-section is $2.8^{+2.1}_{−1.9}$ times the value expected in the Standard Model. This result has been combined with other analyses to obtain a more precise measurement of the $t\bar{t}H$ cross-section and the Top Yukawa coupling.