Precision Measurement of the Mass of the $\tau$ Lepton

An energy scan near the $\tau$ pair production threshold has been performed using the BESIII detector. About $24$ pb${}^{-1}$ of data, distributed over four scan points, was collected. This analysis is based on $\tau$ pair decays to $ee$, $e\mu$, $eh$, $\mu \mu$, $\mu h$, $hh$, $e\rho$, $\mu \rho$ and $\pi \rho$ final states, where $h$ denotes a charged $\pi$ or $K$. The mass of the $\tau$ lepton is measured from a maximum likelihood fit to the $\tau$ pair production cross section data to be $m_{\tau }=(1776.91\pm 0.12_{-0.13}^{+0.10}$) MeV/$c^2$, which is currently the most precise value in a single measurement.