Kinematic Reconstruction of Tau Leptons and Test for Lepton Universality in Charged Weak Interactions with the CMS Experiment

The Standard Model of Particle Physics (SM) postulates the universal coupling of the three lepton families to the weak current. The most precise measurement of lepton universality in W decays comes from the four experiments at the Large Electon-Positron Collider (LEP). If one compares the couplings of muons and tau leptons to the charged weak current, there is a discrepancy of nearly three standard deviations w.r.t. the SM expectation. There are models beyond the SM, which could explain the violation of lepton universality with new physics processes, if it is more than a statistical fluctuation. The Large Hadron Collider (LHC) offers a great opportunity to study decays of the charged-weak gauge bosons at very high event rates and at unmatched collision energies. This thesis presents an analysis strategy to test lepton universality with the Compact Muon Solenoid experiment (CMS) at the LHC. The analysis focusses on the decays of the W boson to particles of the second and third lepton family. For this purpose detector-simulated proton-proton events are used. The identification and reconstruction of tau leptons is a difficult task at the LHC. The reconstruction is often restricted by the limited precision of the commonly used collinear approximation. The application of a kinematic fit to particular tau-decay modes can improve the experimental resolution and provides an efficient background suppression. The development of such a fit with kinematic constraints derived from the topology of the decay tau-->3pi+nu_tau is described. The kinematic fit of tau leptons is not limited to the test for lepton universality, but can be deployed by various physics analyses in a broad energy range of the tau leptons. The event topology of W decays with leptonic final states is studied. Two event selections are developed: one for the W-->tau+nu and one for the W-->mu+nu decay. A common online selection is proposed, which is independent of the leptonic final state of the W bosons. The selection of W-->tau+nu events uses the developed kinematic fit of tau leptons. The selection of W-boson decays is done by a set of requirements on decay-specific event characteristics. Each of these criteria is introduced, and its impact on the selection of signal and background~processes is reviewed. The efficiencies and purities are derived, and statistical and systematic uncertainties are discussed. The sensitivity of CMS to lepton universality is estimated.