NEPAL · NEw Physics searches with tAu Leptons

If the Standard Model (SM) of particle physics succeeds in describing the behaviour of fundamental constituents of matter and their interactions observed experimentally, it is unable to solve the most important riddles of our time such as the nature of the dark matter or the origin of the matter-antimatter asymmetry of the Universe. Manifestations of physics beyond the SM are extensively searched for, in particular through heavy flavour decays that are rare or forbidden in the SM. In this domain, final states involving electrons and muons are widely studied while channels involving tau leptons are much less known because of their challenging reconstruction. The interest of decays involving tau leptons is also dramatically reinforced by the recent anomalies reported in tests of lepton flavour universality violation and rare B decays, suggesting a special role of the third family. In particular, in the presence of physics beyond the SM, lepton flavour violating tau decays and rare B decays into tau leptons could be just below the current experimental limits.With the NEPAL project, I propose to build a team of analysts that will exploit the worlds largest B and tau samples recorded in the clean environment of an electron/positron machine by the Belle II experiment. The full detector operation will start end 2018 and aims at recording five times more statistic than the total previous flavour-factory experiments by 2020, and a final dataset of 50 inverse attobarns by 2025. Thanks to the development of a common analysis framework, sophisticated machine learning techniques for signal selections, the use of a full event interpretation and the reconstruction of 95% of tau decays, my team will search for more than thirty lepton flavour violating tau decays and rare B decays into tau leptons. This will allow to set the worlds best limits in the best possible timescale, reshaping the landscape of searches for physics beyond the Standard Model.