Measurement of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>τ</mml:mi></mml:math>-lepton mass with the Belle II experiment

Abstract

We present a measurement of the $\ensuremath{\tau}$-lepton mass using a sample of about 175 million ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\tau}}^{+}{\ensuremath{\tau}}^{\ensuremath{-}}$ events collected with the Belle II detector at the SuperKEKB ${e}^{+}{e}^{\ensuremath{-}}$ collider at a center-of-mass energy of 10.579 GeV. This sample corresponds to an integrated luminosity of $190\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$. We use the kinematic edge of the $\ensuremath{\tau}$ pseudomass distribution in the decay ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ and measure the $\ensuremath{\tau}$ mass to be $1777.09\ifmmode\pm\else\textpm\fi{}0.08\ifmmode\pm\else\textpm\fi{}0.11\text{ }\text{ }\mathrm{MeV}/{c}^{2}$, where the first uncertainty is statistical and the second systematic. This result is the most precise to date.