| Summary: | A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science, 2018 === A search for the resonant and non-resonant Higgs pair production is presented based on events containing WW bosons with the semi-leptonic decay from one Higgs boson and γγ from the other Higgs boson. This search is performed by a proton–proton collision data recorded by the ATLAS detector at the Large Hadron Collider with an integrated luminosity of 36.1 fb−1 at √s = 13 TeV. The observed and expected upper limits of the non-resonant Higgs pair production (pp → hh) at 95% confidence-level on the cross-section are 7.6 pb and 5.3 pb, respectively. While by assuming a narrow-width the observed and expected upper limit for the resonant Higgs pair production at 95% confidence-level on its cross-section times the branching fraction ((pp → X)×Br(X → hh)) are as follow: a range between 37.5 pb to 6.1 pb are observed and 16.9 pb to 4.4 pb are expected for the resonant mass between 260 GeV to 500 GeV. In addition, a study of multilepton signatures arising from two new scalar bosons H and S at the Large Hadron Collider is carried out. These two new bosons are an extension of the Standard Model and they interact with the Higgs boson. We consider two production modes for H, one being gluon fusion and the other being in association with top quarks. The H → Sh decay mode is considered, where leptonic final states are studied. The nature of the S boson is considered in two separate contexts. Firstly, in a simplified model, it is considered to have Higgs-like couplings. Secondly, we consider a heavy neutrino model and its interactions with the Z,W± and S bosons. In Refs. [1, 2] an excess in di-lepton plus jets and missing transverse energy is predicted. The available data form the ATLAS and CMS experiments displays this excess. Assuming a simplified model where all the signal comes from H → Sh, with mH = 270 GeV and mS = 150 GeV, the significance of the excess is 3.2 standard deviations. This result is combined with other excesses predicted by the model, yielding β2 g = 1.38±0.22, or 6.3σ, where βg is the strength of the Yukawa coupling of H to top quarks. A number of regions of the phase-space is suggested to the experiments for further exploration. === XL2019
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