Abstract Discovery of the Higgs boson, has completed all particles which had predicted by the Standard Model. In addition to the Higgs boson, top quark plays an important role in phenomenological and experimental studies and precision measurements of these two particle properties. Although these studies have provided noteworthy information on the validity of the Standard Model and achieved many successes for this model, it has not been able to describe some phenomena such as neutrino oscillations, dark matter, and the asymmetry between matter and antimatter. Therefore, physicists are looking for an approach to find appropriate answers to these problems by considering models beyond the standard model. In such conditions, a powerful and attractive tool to parametrise any potential deviations from the SM predictions and new physics sings in a model-independent way is the Standard Model effective field theory. Our main aim in this thesis is to constrain the effective field theory describing the top quark couplings through the e^- e^+→tt ̅+jet process that is merged with e^- e^+→tt ̅ using the MLM matching prescription. This study is performed on the benchmark scenarios proposed by the future electron-positron colliders in two different center-of-mass energies of 500 and 3000 GeV. The analysis is carried out considering a multivariate technique with a BDT method to suppress the contributions of the SM backgrounds. The expected limits at 95% CL are derived on the new physics couplings such as tt ̅γ, tt ̅Z, hgtt ̅ and tt ̅g for each benchmark scenario using the dileptonic tt ̅ final state. We show that the 95% CL limits on dimensionless Wilson coefficients ci considered in this analysis could be probed down to 10-4. Our findings are calculated for individual and marginalized signal scenarios and indicate that a future lepton collider operating at a center of mass energy above the tt ̅ threshold would improve the constraints by orders of magnitude with respected to the LHC expectations.

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