Search with the CMS experiment for a heavy scalar boson decaying into a pair of Standard-Model-like Higgs bosons in the final states $b \bar b \tau^+ \tau^-$ with one $\tau$ decaying hadronically and the other leptonically

This thesis reports a search for a heavy scalar boson $H$ predicted by the Two-Higgs-Doublet Models (2HDM) and, in particular, by the Minimal Supersymmetric Extension of the Standard Model (MSSM). We consider a decay mode where the heavy Higgs decays into a pair of lighter ``Standard-Model-like'' Higgs bosons $h$ with mass 125 \GeV. The final state studied consists of a pair of $\tau$ leptons and a pair of $b$ jets from either of the $h$. One of the $\tau$ leptons decays into an electron or muon, while the second $\tau$ lepton decays hadronically. The search is done using proton--proton collision data collected by the Compact Muon Solenoid experiment in 2012, corresponding to an integrated luminosity of 19.7 \fbinv. No evidence of the heavy boson $H$ was found, and the observed results are compatible with the predictions of the Standard Model. The upper limits on the $H$ production cross section times the branching fraction of $hh \to bb\tau\tau$ in the mass range $260 \leq m_{H} \leq 350\GeV$ and the exclusion limits for the parameter regions of the 2HDM Type 2 and of the MSSM phase space have been set.