Search For The Production Of The Higgs Boson Associated With A Pair Of Top Quarks With The ATLAS Detector At The LHC

The production of the Higgs boson associated with a pair of top quarks is still not observed and is one of the most important Higgs boson production modes.Hence,the discovery of this production mode is a very attracting search:not only will it be the first time we can observe this Higgs production mode but also we will be able to measure its Yukawa coupling to the top quark.The measured results can answer the basic question of the Standard Model(SM)and can also search for any hints of new physics by comparing them to the SM prediction.An analysis of searching for the production of the Higgs boson associated with a pair of top quarks in three leptons final state is presented in this thesis.This analysis is performed with the collected data by the ATLAS detector in 2015 and 2016 during the so-called Run 2 campaign corresponding to an integrated luminosity of 36.1 fb-1 at a center of mass energy of(?)= 13 TeV.It uses a boosted decision tree algorithm to discriminate between signal and background.The dominant background of fake leptons is estimated with the data-driven method(Matrix Method).For a Standard Model Higgs boson of a 125 GeV mass,an excess of events over the expected background from other SM processes is found with an observed significance of 2.2 standard deviations,compared to an expectation of 1.5 standard deviations.The best fit for the t(?)H production cross section is 1.5-0.7+0.8 times the SM expectation,consistent with the value of the Yukawa coupling to top quarks in the Standard Model.