A search for the Higgs boson produced in association with top quarks in multilepton final states at ATLAS

This thesis presents preliminary results of a search for Higgs boson production in association with top quarks in multilepton final states. The search was conducted in the 2012 dataset of $\sqrt{s}=8$ TeV proton-proton collisions delivered by the CERN Large Hadron Collider and collected by the ATLAS experiment. The dataset corresponds to an integrated luminosity of 20.3 $\text{fb}^{-1}$. The analysis is conducted by measuring event counts in signal regions distinguished by the number of leptons (2 same-sign, 3, and 4), jets and b-tagged jets present in the reconstructed events. The observed events in the signal regions constitute an excess over the expected number of background events. The results are evaluated using a frequentist statistical model. The observed exclusion upper limit at the 95% confidence level is 5.50 times the predicted Standard Model $ttH$ production cross section. The fitted value of the ratio of the observed production rate to the expected Standard Model $ttH$ production rate is 2.83 $+$ 1.58 $-$ 1.35.