Scalar boson decays to tau leptons: in the standard model and beyond

This thesis presents a study of the scalar sector in the standard model (SM), as well asdifferent searches for an extended scalar sector in theories beyond the standard model(BSM). All analyses have in common the fact that at least one scalar boson decays to apair of tau leptons. The results exploit the data collected by the CMS detector duringLHC Run-1, in proton-proton collisions with a center-of-mass energy of 7 or 8 TeV.The particle discovered in 2012, H, looks compatible with a SM Brout-Englert-Higgsboson, but this statement is driven by the H → γγ and H → ZZ decay modes. TheH → τ + τ − decay mode is the most sensitive fermionic decay channel, and allows to testthe Yukawa couplings of the new particle. The search for the SM scalar boson decaying totau leptons, and produced in association with a massive vector boson W or Z, is describedin this thesis. Even though a good background rejection can be achieved by selecting theleptons originating from the vector boson, Run-1 data are not sensitive to the small production cross sections predicted in the SM for the scalar boson. The combination withthe gluon-gluon fusion and vector boson fusion production searches leads to an evidencefor the decay of the H boson to tau leptons.Many BSM models, such as the minimal supersymmetric SM (MSSM) or models withtwo scalar doublets (2HDM), predict the existence of several scalar bosons. The decaysof these bosons to tau leptons can be enhanced in some scenarios depending on the modelparameters, which makes the di-tau decay mode powerful to discover BSM physics. Foursearches for an extended scalar sector are detailed in this thesis. The first analysis searchesfor a pseudoscalar boson with a mass between 220 and 350 GeV, decaying to an SM-likescalar boson and a Z boson, in the final state with two light leptons and two tau leptons.Second, a search for the exotic decay of the new particle H to a pair of light pseudoscalarbosons, which is still allowed by all measurements made up to now, in the final state withtwo muons and two tau leptons is performed. Third, a mass region almost never exploredat the LHC is probed by the search of a light pseudoscalar, with a mass between 25 and80 GeV, decaying to tau leptons and produced in association with b quarks. The lastanalysis describes the search for a heavy resonance in the MSSM, decaying to a pair oftau leptons. None of these analyses has found any hint of new physics beyond the SM,but stringent limits on the cross section of such signals could be set.