Measurement of the mass of the $\tau$-lepton and an upper limit on the mass difference between $\tau^+$ and $\tau^-$

Abstract

The mass of the $\tau$ lepton has been measured in the decay mode $\tau \to 3\pi \nu_\tau$ using a pseudomass technique. The result obtained from $414 \mathrm{fb}^{-1}$ of data collected with the Belle detector is $M_\tau = (1776.61\pm 0.13 {(stat.)} \pm 0.35 {(sys.)})$ MeV/$c^2$. The upper limit on the relative mass difference between positive and negative $\tau$ leptons is $|M_{\tau^+}-M_{\tau^-}|/M_{\tau} < 2.8 \times 10^{-4}$ at 90% confidence level.