Prof. Alessandro Strumia, National Institute of Chemical Physics and Biophysics (Tallinn, Estonia)
Prof. Martti Raidal, University of Tartu,
Dr. Oxana Smirnova, Lund University, (Sweden)
Prof. Rein-Karl Loide, Tallinn University of Technology,
The standard model of particle physics describes known matter and fundamental interactions and has been experimentally verified to a great level of accuracy. However, it is generally believed that the SM is not a complete theory, but needs to be extended. One of the clear indications of physics beyond the standard model are nonzero neutrino masses, that are extremely small in comparison to the masses of other particles. The Large Hadron Collider (LHC) is the worlds largest particle collider that works at higher energies than ever achieved in previous experiments and tries to find answers to the most fundamental questions in modern physics like the origin of mass, dark matter or nonzero neutrino masses. The LHC is situated in the 27 km long circular tunnel and is composed of a ring of magnets that store two counter-rotating proton beams. The proton collisions are measured and further analyzed by several particle detectors that are situated at the collision points of the proton beams. The data collected by one of the large general-purpose experiments - Compact Muon Solenoid (CMS) - were used in the two analyses that are summarized in this thesis. Firstly, the measurement of the cross section of a W- boson decaying to a τ-lepton and a neutrino, is one of the known standard model processes. The measurement confirms the standard model and is a contribution to τ-lepton physics in CMS. The search for a doubly charged Higgs boson that decays to τ-leptons tests new beyond the standard model physical theory, that is related to the neutrino mass generation mechanism. Analyzed data was found to be in good agreement with the standard model prediction and a new lower bound on the doubly charged Higgs boson mass was set, which considerably improves the previous measurements.