First measurement of the branching fraction of the decay $\psi \mathrm{}\left(2S\right)\mathrm{}\to {\tau }^{+}{\tau }^{-}$

The branching fraction of the $\psi \mathrm{}\left(2S\right)\mathrm{}$ decay into ${\tau }^{+}{\tau }^{-}$ has been measured for the first time using the BES detector at the Beijing Electron-Positron Collider. The result is $B_{\tau \tau }=(2.71\mathrm{}\pm 0.43\pm 0.55)\times {10}^{-3},$ where the first error is statistical and the second is systematic. This value, along with those for the branching fractions into $e^{+}{e}^{-}$ and ${\mu }^{+}{\mu }^{-}$ of this resonance, satisfy well the relation predicted by the sequential lepton hypothesis. Combining all these values with the leptonic width of the resonance, the total width of the $\psi \mathrm{}\left(2S\right)\mathrm{}$ is determined to be $(252\mathrm{}\pm 37)\mathrm{keV}.$