First Measurement of the Branching Fraction of the Decay psi(2S) --> tau tau

Abstract

The branching fraction of the psi(2S) decay into tau pair has been measured for the first time using the BES detector at the Beijing Electron-Positron Collider. The result is $B_{\tau\tau}=(2.71\pm 0.43 \pm 0.55) \times 10^{-3}$, 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(2S) is determined to be $(252 \pm 37)$ keV.