We present a measurement of the $\tau$-lepton mass using a sample of about 175 million $e^+e^- \to \tau^+\tau^-$ events collected with the Belle II detector at the SuperKEKB $e^+e^-$ collider at a center-of-mass energy of $10.579\,\mathrm{ Ge\kern -0.1em V}$. This sample corresponds to an integrated luminosity of $190\,\mathrm{ fb^{ -1}}$. We use the kinematic edge of the $\tau $ pseudomass distribution in the decay ${ \tau^-\to\pi^-\pi^+\pi^-\nu_\tau}$ and measure the $\tau$ mass to be $1777.09 \pm 0.08 \pm 0.11 \,\mathrm{ Me\kern -0.1em V\!/c^2}$, where the first uncertainty is statistical and the second systematic. This result is the most precise to date.
Categories: hep-ex