Observation of the Standard Model Higgs boson and search for an additional scalar in the $\ell^+\ell^−\ell^+\ell^−$ final state with the ATLAS detector

This thesis presents the observation of a Standard Model Higgs boson ($h$) and a search for an additional scalar ($H$) in the $h/H \to ZZ^{(∗)} \to \ell^+\ell^−\ell^+\ell^−$ channel (four-lepton channel), where l stands for either an electron or a muon. The observation of a Standard Model Higgs boson in the four-lepton channel uses the proton–proton collision data collected with the ATLAS detector during 2011 and 2012 at center- of-mass energies of 7 and 8 TeV. An excess with a local significance of 8.1 standard deviation is observed in the four-lepton invariant mass spectrum around 125 GeV. The mass of the excess measured in $ \ell^+\ell^−\ell^+\ell^−$ channel is 125.51 $\pm$ 0.52 GeV. Further measurements on the production cross section times branching ratio of the excess agree well with the Standard Model predictions within the experimental uncertainties. The search for an additional scalar uses an integrated luminosity of 36.1 fb$^{−1}$ proton–proton collision data at a center-of-mass energy of 13 TeV collected with the ATLAS detector during 2015 and 2016. The results of the search are interpreted as upper limits on the production cross section of a spin-0 resonance. The mass range of the hypothesized additional scalar considered is between 200 GeV and 1.2 TeV. The upper limits for the spin-0 resonance are translated to exclusion contours in the content of Type-I and Type-II two-Higgs-doublet models. This analysis is combined with the same search in the $H\to ZZ\to \ell^+\ell^−\nu\nu$ channel to enhance the search sensitivities in the high mass region.