Search for extended scenarios of electroweak symmetry breaking with the ATLAS experiment at LHC

As part of the search for extended scenarios of electroweak symmetry breaking the thesis presents development of phenomenological tools and analysis techniques used in the search for charged Higgs bosons with the ATLAS experiment. This thesis is divided in two parts. The first part is devoted to development of tools used for high precision predictions for processes containing τ leptons in the final state, including signal, background and their interference in the Standard Model (SM) and Beyond the Standard Model (BSM) electroweak symmetry breaking scenarios. The studies concern development of the TauSpinner program. TauSpinner is a software for modifying the physics model of Monte Carlo generated samples in case of the changed assumptions about the event production dynamics without needing to re-generating events. With the help of weights the τ lepton production or decay processes can be modified according to a new physics model. A new version of TauSpinner is presented which includes a mechanism for introducing non-standard states and couplings and studying their effects in vector-boson-fusion processes. Spin correlations of τ lepton pair in processes where final states include also two hard jets were also investigated. The second part of the thesis presents a search for charged Higgs bosons decaying to a τ lepton and its neutrino with the ATLAS experiment at the Large Hadron Collider (LHC). The experimental observation of charged Higgs bosons , which are predicted by several models with an extended Higgs sector, would indicate physics beyond the SM. This part of the thesis is dedicated to the search for charged Higgs using tau+jets and tau+lepton topologies in 2015+2016 and full Run-II (2015-2018) ATLAS data-sets. In this part, focus is on multivariate analysis techniques used for setting up the final signal discriminant and the data-driven technique to estimate backgrounds arising from jets misidentified as hadronically decaying τ leptons.