Study of the associated production of the Higgs boson with a top quark pair in a boosted regime in the ATLAS experiment at LHC.

The measurements of the $pp \to t\bar{t}H+ X$ signal strength ($\mu = \sigma_{obs}/ \sigma_{t\bar{t}H (SM)}$ ) and its upper limit at a center-of-mass energy of $pp$ collision of 13 TeV are presented in this thesis. The $pp$ data collected with the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.5 $fb^{-1}$, have been submitted to a detailed analysis. The $t\bar{t}H$ signal strength and its upper limit have been measured searching for the decays $H \to b\bar{b}$ and $t\bar{t} \to l\nu b q \bar{q}'\bar{b}$. Two different analysis approaches are used. In the first (namely "resolved"), the procedure uses standard identification and reconstruction algorithms. In the second (namely "combined"), the events are separated in two exclusive regimes, including events containing the hadronically decaying top quark with a low transverse momentum ($p_{\mathrm{T}}$ < 250 GeV) and the boosted ones with the opposite requirement, where the partially overlapped jets coming from high $p_{\mathrm{T}}$ tops are suitably identified, reconstructed and tagged. The boosted sample is sensitive to a significantly different kinematic region and provides additional information. The measured signal strength is $1.4 \pm 0.5$ and $1.2 \pm 0.5$ respectively for the resolved and the combined analysis. A signal strength larger than 2.3 and 2.0 can be excluded at the 95% confidence level respectively for the resolved and the combined analysis. Both results are compatible with the prediction of the Standard Model ($\mu = 1$).