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

Abstract : This thesis presents a search for Higgs bosons decaying into tau pairs in the context of the Standard Model (SM) and its minimal supersymmetric extension (MSSM) with the Compact Muon Solenoid (CMS) experiment, using the data collected during the first years of operation at the LHC. After introducing the theoretical context relevant for the SM and MSSM Higgs bosons searches, the corresponding phenomenological aspects are discussed. The discovery of a scalar boson by the ATLAS and CMS Collaborations in 2012 is then presented. The design and performance of the CMS experiment are described as well as the identification and measurements of physical objects needed for the analysis. The accent is put on the tau lepton reconstruction including in particular a novel tau isolation that uses the lifetime information. We optimized this tool in terms of tau identification efficiency and fake tau rejection, particularly suited for boosted taus. A new technique for rejecting the electrons faking taus is also presented: a discriminator based on a multivariate analysis which considerably reduces the contamination from electrons faking taus while preserving the efficiency of identification is exposed.The SM and MSSM Higgs boson searches in the di-tau final state are presented. The dataset corresponds to an integrated luminosity of 4.9 fb-1 at sqrt{s}=7 TeV and 19.7 fb-1 at sqrt{s}=8 TeV recorded by CMS respectively in 2011 and 2012. The focus is put on the most sensitive semi-leptonic decay channels: $H\to\tau\tau\to l\tau_{h}$ where $l$ is either an electron or muon and $\tau_{h}$ stands for hadronic tau decays. After combining all the decay channels in SM Higgs boson to di-tau search and in particular using the new anti-electron discriminator, an excess of observed events with respect to the background-only hypothesis is found. The statistical significance of this excess corresponds to 3.2 standard deviations at a mass of 125 GeV. The measured signal strength is compatible with the SM Higgs signal expectation. This result constitutes an evidence for a coupling between the scalar boson and leptons.Finally, a new strategy in the search for the MSSM Higgs bosons is presented in this thesis. It combines both the novel tau isolation technique and a new refined categorization based on the $\tau_{h}$ transverse momentum. With respect to the previous CMS public result (2013), the sensitivity of the analysis presented in this document is improved by more than 30%. This is equivalent to an increase of the total integrated luminosity by a factor 3 to 4. The interpretation of the model independent upper limits on the cross-section times branching ratio of the production process shows no significant excess over the Standard Model backgrounds.
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https://pastel.archives-ouvertes.fr/tel-01089488
Contributor : Ivo Nicolas Naranjo Fong <>
Submitted on : Monday, December 1, 2014 - 6:42:01 PM
Last modification on : Wednesday, March 27, 2019 - 4:40:29 PM
Long-term archiving on : Monday, March 2, 2015 - 1:37:59 PM

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Ivo Nicolas Naranjo Fong. Tau lepton reconstruction and search for Higgs bosons decaying to tau pairs in the CMS experiment at the LHC.. High Energy Physics - Experiment [hep-ex]. Ecole Polytechnique, 2014. English. ⟨tel-01089488⟩

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