The Belle II Collaboration

04 August 2020

Abstract: The reconstruction of tau-pair production, $\epem \to \tau^{+}\tau^{-}$, from the subsequent 3-prong ($\tau^{+} \rightarrow \pi^{+} \pi^{-} \pi^{+} \bar{\nu}_{\tau}$) and 1-prong ($\tau^{-} \to \ell^{-} \bar{\nu}_{\ell} \nu_{\tau}$, $\tau^{-} \to h^{-} \nu_{\tau}$ or $\tau^{-} \to \pi^{-} \pi^0 \nu_{\tau}$) decays, is presented using 8.76~\invfb of $\epem $ collision data of {\belletwo} at the center-of-mass energy $\sqrt{s} = m_{\Upsilon(4S)}$. The pseudomass technique developed by the ARGUS experiment is used to measure the $\tau$-lepton mass $m_{\tau}$ in the 3-prong $\tau^{+} \to \pi^{+} \pi^{-} \pi^{+} \bar{\nu}_{\tau} $ decay, resulting in $m_{\tau} = 1777.28 \pm 0.75~{\rm (stat)} \pm 0.33 ~{\rm (syst)}\mevcc$.

Keyword(s): tau lepton ; tau mass