Search for HIGGS Bosons in the Standard Model and Beyond

The detailed study of the Higgs sector is a crucial milestone for the comprehensive understanding of the latest discovered part in the Standard Model. Using the data collected with the ATLAS experiment at CERN's Large Hadron Collider, two searches are performed, probing the Standard Model Higgs Boson and beyond it. A search for a heavy charged Higgs boson (mH± -> mtop ), which decays predominantly into a top and bottom quarks, is performed based on ATLAS Run 2 sqrt(s)=13 TeV pp collisions dataset, corresponding to an integrated luminosity of 36/fb, collected during 2015 to 2016. Different final states with one or two leptons (electrons or muons) are studied, and exclusion limits are set. With increasing and more sophisticated data-taking, ATLAS is becoming more sensitive to new decay channels. A Standard Model Higgs boson with a mass of 125 GeV is predicted to decay to a pair of charm quarks around 2.9% of the time. This thesis describes a search for the decay of the Higgs boson to charm quarks, based on the full ATLAS Run 2 sqrt(s)=13 TeV pp collisions dataset, corresponding to an integrated luminosity of 139/fb, collected during 2015 to 2018. The H -> cc decay candidates are reconstructed from a pair of jets identified by a c-tagging algorithm. The expected upper limit is 26× SM prediction. In 2024 the LHC will be upgraded into a High Luminosity LHC (HL-LHC). The HL-LHC will deliver about 300/fb of data per year. This puts challenging requirements on the trigger algorithms. Novel techniques using Deep Neural Networks are presented in this thesis to increase the detector's ability to identify a tau-lepton correctly. These techniques exhibit a promising ability to explore new (low pT) regions never been analysed before.