Search and prospects for BSM Higgs with the ATLAS detector at the LHC

Almost two years ago, ATLAS and CMS announced the observation of a new boson with a mass of approximately 125 GeV. The properties of this newly discovered boson make a convincing case that it is a Higgs boson related to the BEH mechanism of spontaneous breaking of the electroweak (EW) symmetry. Yet, it remains an open question whether it is the Standard Model (SM) Higgs boson, which is a single elementary scalar particle, or one physical state of an extended scalar sector, as predicted by the two-Higgs-doublet-model (2HDM). There are various ways of trying to answer this exciting question: • Looking for additional scalar particles (e.g. charged Higgs bosons - light and heavy @ 7-8 TeV). This is the main subject of my thesis. • Determining the spin/CP quantum numbers. • Measuring precisely the couplings to fermions and vector bosons. • Studying additional production mechanisms which are not possible in the SM (e.g. from CP-odd decay). During my PhD research, I was involved with each of these topics, some of which will only be resolved in a few years time, once ATLAS will collect enough data. In this thesis I review only analyses in which I took a major part, from analyzing the 7 TeV data searching for new particles, to studies which make the case for the upgrade of the LHC in general and ATLAS in particular.