Search for prompt heavy neutral lepton decays into tau leptons with the ATLAS detector

According to the Standard Model, neutrinos should not have mass, and therefore neutrino oscillations are not explained. Neutrino Minimal Standard Model is a theory that would explain the mass of the neutrinos (active neutrinos) and their oscillations by the prediction of three unobserved, sterile neutrinos, usually called heavy neutral leptons. These sterile neutrinos would, in principle, mix with the neutrinos described by the Standard Model. Some projects are studying the interaction of the heavy neutral lepton with electrons and muons using data from the ATLAS experiment. The channel with tau neutrinos is yet not explored, but it could be possible. In this thesis, a prompt heavy neutral lepton generated from a $W$ boson manifesting itself in a three lepton final state involving taus is studied using simulated data from the ATLAS collaboration. Getting the sensitivity of ATLAS to this process will tell if the exclusion limits set by other experiments can be extended. It has been found that for a specific channel involving mixing with the muon and the tau flavour, the mixing angles ATLAS is sensitive to are within the exclusion limits of previous searches.