Search for additional Higgs bosons decaying to tau leptons and measurement of the $CP$ properties of the Higgs Yukawa coupling to top quarks using the CMS detector

In 2012 the CMS and ATLAS experiments announced the discovery of a boson with mass around 125 GeV and properties consistent with that of the Higgs boson predicted by the standard model (SM). Despite this success, many observations about the universe appear to be inconsistent with the SM phenomenology, implying the need for beyond the standard model (BSM) physics. Many BSM scenarios require extended Higgs sectors, which usually include additional Higgs bosons and predict different properties of the 125 GeV state. Two approaches may be used to test such scenarios: search for the additional bosons, or measure the properties of the 125 GeV state to test the comparability with the SM. This thesis explores both of these options. A search for additional Higgs bosons in the mass region 90 GeV–3.2 TeV decaying to pairs of tau leptons is presented. This analysis uses the dataset collected by CMS in 2016, which corresponds to an integrated luminosity of 35.9 fb$^{−1}$. No statistically significant excesses are observed. Exclusion limits are set on the cross sections times branching fractions. The results are also interpreted in the context of the minimal supersymmetric standard model (MSSM). A measurement of the $CP$-properties of the Higgs Yukawa coupling to top quarks is also performed using Higgs boson production via gluon fusion in association with two jets. The analysis of the azimuthal angular correlations between the jets is sensitive to the $CP$ mixing angle α$_{hgg}$. This analysis uses the datasets collected by CMS in 2016 and 2017 which correspond to an integrated luminosity of 71.8 fb$^{−1}$. The best-fit value of α$_{hgg}$ is measured to be −59$^{◦}$$_{-50◦}^{+47◦}$. This constitutes the first direct measurement of the $CP$-properties of a fermionic Higgs coupling.