Thomas Kraetzschmar

05 September 2021
22nd Particles and Nuclei International Conference

Abstract: The Belle~II experiment is a substantial upgrade of the Belle detector and will operate at the SuperKEKB energy-asymmetric $e^+e^-$-collider. The design luminosity of the machine is $8~\times~10^{35}~\text{cm}^{2}\text{s}^{-1}$ and the Belle~II experiment aims to record $50~\text{ab}^{-1}$ of data, a factor of 50 more than its predecessor. From February to July 2018, the machine has completed a commissioning run and main operation of SuperKEKB has started in March 2019. Belle II has a broad $\tau$ physics program, in particular in searches for lepton flavour and lepton number violations, benefiting from the large cross section of the pair wise $\tau$ lepton production in $e^+e^-$ -collisions. We expect that after 5 years of data taking, Belle II will be able to reduce the upper limits on lepton flavour and lepton number violating $\tau$ decays by an order of magnitude. Any experimental observation of lepton flavour or lepton number violations in $\tau$ decays constitutes an unambiguous sign of physics beyond the Standard Model, offering the opportunity to probe the underlying new physics. We also present a first measurement of the $\tau$-mass and the prospects for the $\tau$-lifetime measurement. In this talk we will review the $\tau$ lepton physics program of Belle~II.