Étude des leptons taus et recherche de nouvelle physique avec le détecteur CMS

Two data analyses are presented in this masters thesis. The first one is a study of the CMS Hadrons Plus Strips (HPS) algorithm, which reconstructs and identifies tau leptons that decay into a tau neutrino and one or more hadrons. Those are the so-called hadronic taus (τh). The second analysis is a search for a new neutral massive resonance in the τ +τ − decay channel, in the particular nal state τµτh where one of the tau leptons is a hadronic tau and the other decays into a muon and two neutrinos. The data used in this thesis were collected in the year 2015 by the CMS detector at the LHC collider. The data were collected in proton-proton collisions at a center-of-mass energy √ s = 13 TeV, and correspond to an integrated luminosity of 2.2 fb−1 . The rst part of this thesis is motivated by the essential role the HPS algorithm efficiency plays in all CMS data analyses that study nal states containing one or more hadronic taus, such as new physics searches in the τ +τ − channel and the measure of the Standard Model scalar boson coupling to tau leptons. The study of the tau lepton reconstruction and identification efficiency is especially important because this is the first time it is done with high energy (√ s = 13 TeV) data and the new CMS identification and reconstruction tools. The HPS algorithm efficiency was first determined in the simulations, and the ratio r of the efficiency in the data over the efficiency in the simulations was calculated, using the Z → τµτh/Z → µµ ratio method. This method determines the value of r by a simultaneous adjustment of event yields in two data regions, one of them rich in Z → τµτh events and the other rich in Z → µµ events. Three values of r were calculated, one for each working point of the HPS algorithm. The values r = 0.929 ± 0.055, r = 0.924 ± 0.058 and r = 0.911 ± 0.053 were found for, respectively, the loose, medium and tight working points. The second analysis presented in this thesis is justified by the existence of numerous new physics models which predict the existence of a new neutral massive resonance. These models were built in order to solve some of the open problems left by the Standard Model, such as the nature of the dark matter components and the unification of gravity to the three other fundamental forces. The signal used as a reference in this second analysis is the Sequential Standard Model (SSM) Z' boson. The τµτh invariant mass spectrum was studied and no sign of new physics was found : the number of data events is in agreement with Standard Model predictions. A statistical method was used in other to determine both the expected and observed 95% confidence limits for the SSM Z' boson mass, taking into account the results obtained previously for the HPS algorithm efficiency. The expected limit is 1.37 ± 0.13 TeV, and the observed limit is 1.18 TeV.