Search for Standard Model Higgs bosons produced in association with top-quark pairs in ttH¯ multilepton final states using the ATLAS detector at the LHC.

This thesis describes the search for the associated production of a Higgs boson and a top-quark pair in proton-proton collisions with a center-of-mass energy of 13 TeV. The analysis focuses on events containing two leptons with same-sing electric charge or exactly three leptons in the final state. With these requirements, the analysis is sensitive to the $H$ to $WW$*, $H$ to $ZZ$* and $H$ to $\tau$ $\tau$ decay modes. The used data was recorded with the ATLAS detector at the Large Hadron Collider (LHC) during the 2015-17 period. The primary sources of background were found to be ttW, ttZ, Diboson processes and events containing fake or non-prompt leptons coming from hadron decays, photon conversions and electron charge mis-identication. The estimation of the hadron decay and photon conversion fake backgrounds was performed using a novel semi-data driven technique where Monte Carlo background categories are created for each fake process of interest. A profile likelihood fit to data is then used to extract a normalization factor for each fake category, which are used to correct the Monte Carlo predictions. The ratio of the measured ttH signal cross-section to the Standard Model expectations, ttH, and a normalization factor for the ttW process are extracted simultaneously from the same t. The mu_ttH for a Standard Model Higgs boson of mass 125 GeV was found to be mu_ttH = 0:88^(+0:37)_(-0:36)(stat)^(+0:39)_(-0:36)(syst) with an observed (expected) significance of 1.80 (2.52) standard deviations.