Neutral Higgs boson and Z boson decays into pairs of tau leptons with the CMS detector

Abstract

In this thesis, analyses of Z boson and neutral Higgs boson decays into pairs of tau leptons at the Compact Muon Solenoid (CMS) experiment at the CERN Large Hadron Collider (LHC) are studied. A measurement of the $Z / \gamma^* \rightarrow \tau \tau$ cross section in proton-proton collisions at a centerof-mass energy of 13 TeV is presented, using 2015 proton-proton collision data corresponding to an integrated luminosity of $2.3fb^{-1}$. The $Z / \gamma^* \rightarrow \tau \tau$ decay is an important standard model process constituting the dominant irreducible background in Higgs boson searches in the di-tau final state. The product of the cross section and branching fraction is measured to be $\sigma(pp \rightarrow Z / \gamma^* + X)B(Z / \gamma^* \rightarrow \tau \tau) = 1848 ± 12 (\text{stat}) ± 67 (\text{syst+lumi})$ pb, in agreement with the standard model expectation. In addition, the reconstruction and identification efficiency and the energy scale of tau lepton decays to hadron+$\nu_\tau$ final states are obtained. A search for additional neutral Higgs bosons decaying to pairs of tau leptons is presented, using 2016 proton-proton collision data recorded at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of $35.9fb^{-1}$. The search is performed in the context of the minimal supersymmetric extension of the standard model (MSSM). No significant deviations above the expected standard model background are observed and model-independent upper limits are set on the product of the cross section and the branching fraction. Furthermore, exclusion contours in the $m_A-\tan \theta$ plane for selected MSSM benchmark scenarios are provided. Finally, upgrade plans that are foreseen to prepare the CMS detector for the high luminosity LHC era are discussed, where special emphasis is put on the identification performance of tau lepton decays to hadron+$\nu_tau$ final states.