First Results and Prospects for $\tau$ Lepton Physics at Belle~II

Abstract

Belle II has a broad $\tau$ physics program, in particular in searches for lepton flavour and lepton number violations. Belle II profits from the relatively large $\tau$-pair production rate in the low background environment of $e^+e^-$ -collisions at 10.58 GeV. Up to mid-2021 Belle II collected a sample corresponding to $214~\text{fb}^{-1}$ of data. We present a first measurement of the $\tau$ mass, the prospects for the $\tau$-lifetime measurement, and review the overall $\tau$ lepton physics program of Belle II.