Search for Charged Higgs Bosons Decaying to a Tau Lepton and a Neutrino with the CMS Experiment

The Standard Model of particle physics is the most successful and precise theoreticaldescription of fundamental physics so far. The discovery of the Higgs boson in 2012by the ATLAS and CMS experiments at the LHC provided strong evidence for theEnglert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism, explaining how elementaryparticles gain their masses in the Standard Model. However, the Standard Model isknown to be an incomplete description of nature, as it cannot explain the origin ofdark matter, neutrino masses or the observed matter–antimatter asymmetry. Thereforemore general models with an extended Higgs sector are actively being studied. Modelswith at least two Higgs doublets predict the existence of electrically charged Higgsbosons. The observation of charged Higgs bosons would provide direct evidence fornew physics and guide the way towards a more comprehensive theory.In this thesis, a search is presented for charged Higgs bosons decaying into a tau leptonand a neutrino, based on proton-proton collision events recorded by the CMS experiment in 2016 at a center-of-mass energy of 13 TeV. The amount of data correspondsto an integrated luminosity of 35.9 fb−1 . The search targets the hadronic final statewith a hadronically decaying tau lepton, missing momentum due to neutrinos, andadditional jets from an associated top quark decay.This analysis contains multiple methodological improvements with respect to theprevious CMS results on the same search channel. The particle identification algorithms and selection criteria are optimized for good performance under challengingluminosity conditions. Categorization of events based on tau lepton helicity is used toenhance sensitivity. The background from events with jets misidentified as tau leptonsis estimated from data, whereas the background from genuine-tau events is estimatedfrom simulation. This thesis also presents a new version of the tau embedding method,which allows the estimation of the genuine-tau background using single-muon events.The transverse mass of the tau-neutrino system is reconstructed. As the data agreewith the background-only hypothesis, upper limits are derived for the charged Higgsboson production rate. The search covers signal hypotheses from 80 GeV to 3 TeV, andfor the first time in CMS, the hypotheses with the charged Higgs boson mass closeto the top quark mass are scanned. For maximal signal sensitivity, the results arecombined with those from the leptonic final states of the same search channel. Thecombined result is interpreted in the context of the Minimal Supersymmetric StandardModel.