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Measurement of the production of the standard Higgs boson associated to a pair of top and antitop quarks and search for a charged Higgs boson decaying into a pair of t and b quarks with the ATLAS detector at LHC

Abstract : The Higgs field plays an important role in the Standard Model (SM). It is not only responsible for elementary particles generation but also the only scalar field in the SM. Hence study of the Higgs boson becomes one of the most popular topics at LHC. It provides a great opportunity to probe the SM predictions and search for beyond SM evidence. New physics beyond the SM usually has a more complicated Higgs scenario and predict more than one Higgs boson, such as a charged Higgs boson. Searching for these new particles will give us a direct hint of new physics.In this thesis, two main contributions are presented, searching for a heavy charged Higgs boson (H +) decaying to t t H and searching for H + production in multilepton final states. The search for H + is performed with data collected by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb-1. There is no significant excess of H + signal over the SM backgrounds observed in the mass range from 200 GeV to 2000 GeV. The largest deviation from the SM hypothesis obtained from the fit is observed at 300 GeV, corresponding to a local p0 de 1,13%. The search for t t H prodcution in multilepton final state is performed with data collected during 2015-2017, corresponding to an integrated luminosity of 79.9 fb-1. The Yukawa coupling of the Higgs boson and top quark can be directly determined from the measurement of t t H production cross section. This analysis aims at observation of t t H production using only multilepton final states. There are six final states defined by the number of light leptons (electrons or muons) and hadronic taus, and 25 event categories to search for t t H signal and estimate fake leptons simultaneously. The studies in the final state with two same-sign leptons and one hadronic tau is discussed in detail. An excess of t t H events over other SM backgrounds is found with an observed (expected) significance of 1,8 σ (3,1 σ). The measured t t H production cross section is 294 (+ 182 / -162) fb, which is consistent with the SM prediction.
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Xuan Yang. Measurement of the production of the standard Higgs boson associated to a pair of top and antitop quarks and search for a charged Higgs boson decaying into a pair of t and b quarks with the ATLAS detector at LHC. High Energy Physics - Experiment [hep-ex]. Université Grenoble Alpes [2020-..]; Shandong University (Jinan, Chine), 2020. English. ⟨NNT : 2020GRALY015⟩. ⟨tel-03100122⟩

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