Measurement of the mass of the τ lepton

The mass of the \ensuremath{\tau} lepton has been measured at the Beijing Electron-Positron Collider using the Beijing Spectrometer. A search near threshold for ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$\ensuremath{\rightarrow}${\mathrm{\ensuremath{\tau}}}^{+}$${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}}$ was performed. Candidate events were identified by requiring that one \ensuremath{\tau} decay via \ensuremath{\tau}\ensuremath{\rightarrow}e\ensuremath{\nu}\ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{}, and the other via \ensuremath{\tau}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\nu}\ensuremath{\nu}\ifmmode\bar\else\textasciimacron\fi{}. The mass value, obtained from a fit to the energy dependence of the ${\mathrm{\ensuremath{\tau}}}^{+}$${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}}$ cross section, is ${\mathit{m}}_{\mathrm{\ensuremath{\tau}}}$=1776.${9}_{\mathrm{\ensuremath{-}}0.5}^{+0.4}$\ifmmode\pm\else\textpm\fi{}0.2 MeV.