Event reconstruction of $t\bar{t}$ and $t\bar{t}H$ using the Kinematic Likelihood Fitter in final states with two same-sign electric charge leptons.

One of the important tests for the validity of the Standard Model of particle physics is the measurement of the top quark Yukawa coupling, which can be directly measured in the production of the top-quark pair in association with a Higgs boson, $t\bar{t}H$. In this thesis, a likelihood based fitting tool, known as KLFitter, is implemented to improve the separation power between the $t\bar{t}H$ signal and the dominant background contribution arising from the top quark pair production, $t\bar{t}$, in the final states containing two same-sign electric charge leptons. NLO Monte Carlo samples of the ATLAS detector at $√s = 13~$TeV is used to test the fit. Improvement of the separation power of the fitter is studied by modifying parameters in the likelihood definition of the fit. Different cases have been analyzed and a better discriminating variable, after modifications to the original KLFitter, has been achieved regarding the performance of the fit.