Study of the Higgs boson production in association with a top quark pair with the ATLAS experiment at the LHC

The scientific work presented in this thesis is based on proton proton collision produced, at 13 TeV in the center of mass, created by the Large Hadron Collider (LHC) and recorded between 2015 and 2017 by the ATLAS detector. After a short introduction on the physics program at LHC, the first chapter gives a comprehensive overview of the theoretical context and motivation, respectively the theory of the Standard Model (SM) and beyond, with an emphasis on the physics of the Higgs boson and top quark, the particles which are studied in the two analysis detailed in this thesis. Then a complete description of the ATLAS detector and more generally the experimental setup is given with a highlight on the different physics objects used in this research. The third chapter is dealing with a measure of the performances of the ATLAS electromagnetic calorimeter, namely the study of the electron identification efficiencies using W decaying into electrons and neutrinos events. The Run 2 data set taken in 2015 and 2016 provides new experimental challenges due to the increase in collision energy and peak luminosity, hence higher particle pileup. Therefore, the performance measurements are critical to be improved and aligned with the upgrades procedures that the detector constantly undertakes. In this chapter, a W Tag and Probe (T&P) study made on 2015-2017 data is presented with a complete description of the methodology, as well as the background and systematic studies. Then the results on the efficiency and Scale Factors (SF) are given and compared to the Z → ee Tag and Probe method. Finally, a brief overview on the Run 2 W T&P analysis status and of the 2017-2018 trigger performances, developed during the thesis, is given. In the following chapters, the core of this thesis work, the search and study of the associated Higgs boson production with top quark pair, so-called ttH production, within the Standard Model schema and beyond is treated. The observation for ttH production with the ATLAS detector recently announced in 2018 represents a significant milestone for the High-Energy Physics field. The new Run 2 LHC era is therefore the main gate towards precise studies, improvement on past discoveries and moreover a chance to look for events never seen before thanks to the higher available energy and upgraded procedures. A chapter is dedicated to this ttH search. After a short introduction, the baseline selection is presented with a highlight on the different analysis regions, signal regions and control regions. The study of the signal optimization is then described, with a focus on the 4L channel, followed by the estimation of the fake background and the control of the systematics. Finally the obtained result for the 4L channel only of a signa intensity of -0.5+1,3 -0,9, is presented and put in perspective with the full combination leading to an observed significance of 4.2 sigma for an expected value of 3.8. In order to broaden the spectrum of this ttH analysis beyond the SM, this 4L channel final states was also studied in the context of the search for doubly charged Higgs boson decaying into W±W±, H±±→W±W±, dominant decay channel in the phenomenology framework of the doubly-charged scalar bosons H±±, in the case of a disfavored coupling to leptons and the decays to SM charged vector bosons. This additional study is presented in the following chapter, with first an introduction of the phenomenology, then a description of the baseline selection and the regions under control, the cut-based optimization studies, the fake background and systematic estimates and finally the results with in particular an exclusion limit at 95% confidence level for H±± bosons of masses between 200 and 220 GeV and the expected future plans. In conclusions, future prospects in Standard Model (SM) as well as Beyond SM (BSM) scientific results that could be reached using the full Run1 and Run2 combined available statistics are given.