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 e⁺{e}^{{{-}}}→{{{τ}}}⁺τ^- was performed. Candidate events were identified by requiring that one {τ} decay via {τ}{→}e{ν}{ν}{}, and the other via {τ}{→}{μ}{ν}{ν}{}. The mass value, obtained from a fit to the energy dependence of the τ⁺{{{τ}}}^{{{-}}}$ cross section, is ${m}_{{{τ}}}$=1776.${9}_{{{-}}0.5}+0.4$±{}0.2 MeV.