A mesurement of the mass of the τ lepton using new methods to study semi-invisible decays

Measuring the mass of particles whose decay products cannot be detected poses a significant challenge due to the complexity of reconstructing these decays and measuring various parameters. However, studying processes involving undetectable particles is crucial as it enables us to delve deeper into familiar decays involving energy loss, such as Standard Model processes involving neutrinos. Additionally, it provides an opportunity to test models associated with physics beyond the Standard Model that can be generated in leptonic colliders. In this study, the mass of the tau lepton was determined by comparing three different methods for decays with semi-invisible final states. Specifically, the measurement focused on the decay τ − → π −ντ (signal). Among the three methods employed, the most accurate result was obtained using the Mmin method, yielding a tau lepton mass value of Mτ = 1777.06 ± 0.44 MeV. The measurement utilized official Monte Carlo data provided by the Belle II collaboration, specifically from the MC13a campaign conducted until 2020, with an integrated luminosity of 100fb−1 .