Measurement of the mass of the τ lepton

The mass of the τ lepton has been measured at the Beijing Electron-Positron Collider using the Beijing Spectrometer. A search near threshold for ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$→${\mathrm{\tau }}^{+}$ ${\mathrm{\tau }}^{\mathrm{-}}$ was performed. Candidate events were identified by requiring that one τ decay via τ→eνν¯, and the other via τ→μνν¯. The mass value, obtained from a fit to the energy dependence of the ${\mathrm{\tau }}^{+}$ ${\mathrm{\tau }}^{\mathrm{-}}$ cross section, is ${\mathit{m}}_{\mathrm{\tau }}$=1776.$9_{\mathrm{-}0.5}^{+0.4\mathrm{}}$±0.2 MeV.