Higgs boson searches at OPAL (LEP), ATLAS (LHC) and ILC

An impressive effort was done by the four LEP collaborations in the search for the SM and the MSSM Higgs boson. However, very little attention was paid to the possibility of a CP violating Higgs sector and no significant effort to search for the resulting signatures was carried out. Once the CP state of the neutral Higgs bosons is ill defined, new Higgs boson production and decay channels might be opened. One of them, $\epm\ra\Zo\Htwo\ra\Zo\Hone\Hone\ra\nunu\bb\bb$ is analyzed in the scope of this thesis. No significant excess of the data over the expected background is found. The results of this channel are combined with the results of other searches for the MSSM Higgs bosons and interpreted in the framework of a model with CP violation at OPAL. It was found that under certain assumptions (CPX scenario) the region $\tanb < 2.8$ is excluded at 95\% confidence level but no universal mass limit is obtained for either of the Higgs bosons. The combination of the results of the four LEP experiments improves the limit on $\tanb$ to 3.3.\\ The LHC will be complemented by the ILC. In this thesis we consider a scenario where only one Higgs boson is observed by (both) LHC and ILC. We estimate the uncertainties in the indirect determination of the mass of the pseudoscalar Higgs boson, $\mA$. Taking for the first time all experimental and theoretical uncertainties into account, a determination of $M_A$ with an accuracy of about 20\% (30\%) seems to be feasible for $M_A = $ 600 (800) GeV.\\ In the main part of this thesis we develop a completely new algorithmic method for $\tau$-lepton identification within the framework of the fast simulation of the ATLAS detector. We have found out that this method is reproducing quite accurately the full detector simulation Tau ID performance (efficiency and rejection).\\ One of the main goals of the ATLAS experiment is to measure the various Higgs boson couplings as accurate as possible. Such a measurement is mandatory for a full understanding of the Higgs sector. The most challenging measurements of the Higgs boson properties is the determination of the Yukawa coupling to the top quark. We conducted a feasibility study aimed to asses ATLAS sensitivity to this coupling using the the $\ttbar H$ channel followed by $H\ra\tautau$. The signal events were reconstructed using the full and the fast simulation of the ATLAS detector. It is shown that the two methods are in good agreement and that we can use the fast simulation to complete the analysis. We obtain a significance of $1.6\sigma$ for the low luminosity condition (30~$\ifb$) and $m_H=120$~GeV, and $2.0\sigma$ for the high luminosity condition (300~$\ifb$) and $m_H=120$~GeV.\\