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Synopsis

The Higgs boson, with a measured mass of approximately 125 GeV , has been studied extensively since its discovery in 2012 at the Large Hadron Collider. This discovery opens the question of whether the Higgs boson of the Standard Model (SM) is the only scalar particle of the nature or it belongs to a larger scalar sector, as predicted in many Beyond the Standard Model (BSM) theories. Charged Higgs bosons are predicted by several BSM theories and observation of them would indicate new physics-archive beyond the SM.

This thesis presents results of a search for charged Higgs boson in mass range 80 GeV to 3 TeV, through tau-lepton final states. The search is performed using proton-proton collisions data at centre-of-mass energy of 13 TeV, collected with the ATLAS experiment, during 2015 to 2018.  The final results are interpreted in the context of the Minimal SuperSymmetric Standard Model (MSSM) benchmark scenarios. In theses scenarios, charged Higgs bosons coupling to tau-lepton are enhanced for some parts of the search phase space, thus increasing the chance of their discovery.

No significant excess of the events above the expected background from the Standard Model processes is observed. Therefore, upper limits on the cross section times the branching ratio are set at a 95% Confidence Level for two production modes of the charged Higgs bosons: the fully hadronic mode and the semi-leptonic mode. The results are also interpreted in the context of the hMSSM and ³¾³å³ó³¾´Ç»å− benchmark scenarios of the MSSM. Due to the enhancement of the charged Higgs boson coupling to tau-leptons at high values of the tanβ parameter of the MSSM, it is possible to exclude high tanβ region in the mH±–tanβ parameter space. In this work, tanβ values around 60 are excluded up to the charged Higgs boson mass of 1400 GeV. Furthermore, in the low mass region, below 170 GeV, all values of the tanβ in range 1–60 are excluded at 95% confidence level.

Keywords: CERN; ATLAS; Standard Model; Charged Higgs Boson