Tau Lepton: as a Tool for Hunting Standard Model and Beyond Standard Model Higgs

This thesis presents a study of the Standard Model (SM) Higgs boson in the decay channel of $\tau$$\tau$ $\rightarrow$ $e$$\mu$, as well as a search for neutral Minimal Supersymmetric Standard Model (MSSM) Higgs boson in the same decay channel, using data corresponding to an integrated luminosity of 35.9 fb$^{-1}$ in proton-proton collisions with a center-of-mass energy of 13 TeV, collected by the CMS detector at the Large Hadron Collider. The results of $e$$\mu$ channel in both analyses are combined with other channels of Higgs to $\tau$$\tau$ decay: $e$$\tau$$_{h}$ $\mu$$\tau$$_{h}$ and $\tau$$_{h}$$\tau$$_{h}$. An observation of the SM Higgs boson to $\tau$$\tau$ is established with a significance of 4.9 standard deviations, to be compared to an expected significance of 4.7 standard deviations. The product of the observed SM Higgs boson to $\tau$ production cross section times the branching ratio is 1.09$_{-0.26}^{+0.27}$ times the SM prediction. This is the first observation of Higgs boson decays to a pair of $\tau$ leptons by a single experiment. A search for neutral MSSM Higgs boson is performed in a mass range of 90 GeV to 3.2TeV. No significant deviations from the SM background expectation are observed. Model independent upper limits on the production cross section times branching ratio are set. The results are interpreted into selected MSSM benchmark scenarios.