Tau lepton reconstruction and search for Higgs bosons decaying to tau pairs in the CMS experiment at the LHC

This thesis presents a search for Higgs bosons decaying into tau pairs in the contextof the Standard Model (SM) and its minimal supersymmetric extension (MSSM) withthe Compact Muon Solenoid (CMS) experiment, using the data collected during the firstyears of operation at the LHC.After introducing the theoretical context relevant for the SM and MSSM Higgs bosonssearches, the corresponding phenomenological aspects are discussed. The discovery of ascalar boson by the ATLAS and CMS Collaborations in 2012 is then presented. Thedesign and performance of the CMS experiment are described as well as the identificationand measurements of physical objects needed for the analysis. The accent is put onthe tau lepton reconstruction including in particular a novel tau isolation that uses thelifetime information. We optimized this tool in terms of tau identification e ciency andfake tau rejection, particularly suited for boosted taus. A new technique for rejecting theelectrons faking taus is also presented: a discriminator based on a multivariate analysiswhich considerably reduces the contamination from electrons faking taus while preservingthe e ciency of identification is exposed.The SM and MSSM Higgs boson searches in the di-tau final state are presented. Thepdataset corresponds to an integrated luminosity of 4.9 fb 1 at s = 7 TeV and 19.7 fb 1pat s = 8 TeV recorded by CMS respectively in 2011 and 2012. The focus is put onthe most sensitive semi-leptonic decay channels: H ! ⌧ ⌧ ! l⌧h where l is either anelectron or muon and ⌧h stands for hadronic tau decays. After combining all the decaychannels in SM Higgs boson to di-tau search and in particular using the new anti-electrondiscriminator, an excess of observed events with respect to the background-only hypothesisis found. The statistical significance of this excess corresponds to 3.2 standard deviationsat a mass of 125 GeV. The measured signal strength is compatible with the SM Higgssignal expectation. This result constitutes an evidence for a coupling between the scalarboson and leptons.Finally, a new strategy in the search for the MSSM Higgs bosons is presented in thisthesis. It combines both the novel tau isolation technique and a new refined categorizationbased on the ⌧h transverse momentum. With respect to the previous CMS public result(2013), the sensitivity of the analysis presented in this document is improved by morethan 30%. This is equivalent to an increase of the total integrated luminosity by a factor3 to 4. No significant excess over the Standard Model backgrounds is observed.