Search for the neutral heavy Higgs bosons decaying to hadronic ditau in proton-proton collisions at \sqrt{s} = 13 TeV

After the discovery of the scalar Higgs boson that is predicted in the Standard Model of the particle physics (SM) with a mass of 125 GeV, the remaining issues in particle physics, such as dark matter in the universe and hierarchy problem of the Higgs boson, are to be solved by an extension of the SM. A boson-fermion symmetry, which is known as a “supersymmetry”, is one of the most motivated extensions of the SM as it might address the missing of dark matter candidates, the hierarchy problem of Higgs boson mass, and realization of the grand unification, simultaneously. The supersymmetry requires the Higgs sector of the SM to be extended with at least one additional Higgs doublets, resulting in five physical Higgs states; two neutral CP-even Higgs states ($h$, $H$), one CP-odd Higgs state ($A$), and two charged Higgs states ($H$±). One of the two neutral CP-even Higgs states is considered to be the discovered 125 GeV boson ($h$), and the other will be an additional heavy Higgs boson ($H$). An observation of the new heavy Higgs boson ($H$ and $A$) can provide important insight into the nature of the supersymmetry. The coupling constants of the new heavy Higgs bosons are expected to depend on the ratio of the vacuum expectation values of the two Higgs doublets (tan β), and the coupling constants to τ leptons are enhanced for a large tanβ scenario. This thesis presents a search for the neutral heavy Higgs boson ($H$ or $A$) decaying into a pair of tau leptons ($H$/$A$ → τ+τ−), which can provide the best sensitivity for a large tanβ scenario of the Higgs sector extension. Hadronic decay mode of τ leptons (τ → hadrons + $v$) is considered in the identification of the tau leptons. This analysis uses 139 fb−1 of proton-proton collisions recorded by the energy frontier ATLAS experiment at the Large Hadron Collider with a center-of-mass energy of $\sqrt{s}$ = 13 TeV. This analysis applies an optimal set of event selection criteria to enhance $H$/$A$ → τ+τ− signals. Possible backgrounds remaining after the selection are estimated with data-driven techniques, exploiting control regions, and with the assistance of the simulation of the background processes. New analysis techniques have been developed to maximize the search sensitivity. An improved τ selection and categorization in the signal extraction are used in the $H$/$A$ → ττ search for the first time in this analysis. The result confirms no significant excess from SM prediction in the search for $H$/$A$ → τ τ . An exclusion limit in the context of the hMSSM scenario is presented, and the analysis excludes tan β > 23 at M$_{H/A$ = 1500 GeV at 95% confidence level. This analysis allows setting the most stringent limit for the production of the heavy neutral Higgs boson.