The branching fraction of the ψ(2S) decay into τ+τ - has been measured for the first time using the BES detector at the Beijing Electron-Positron Collider. The result is Bττ=(2.710. 43±0.55)× 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 μ+ μ- 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 ψ(25) is determined to be (252±37) keV. ©2002 The American Physical Society.

In a sample of 7.8 million J/ψ decays collected in the Beijing Spectrometer, the process J/ψ→γηcis observed for five different ηcdecay channels: K+K-π+π-, π+π-π+π-, K±KS0π∓ (with KS0→π+π-), φφ (with φ→K+K-) and K+K-π0. From these signals, we determine the mass of ηcto be 2976.6±2.9 ±1.3 MeV. Combining this result with a previously reported result from a similar study using ψ(2S)→γηcdetected in the same spectrometer gives mηc=2976.3±2.3±1.2 MeV. For the combined samples, we obtain Γηc=11.0±8.1±4.1 MeV. ©2000 The American Physical Society.

The Beijing Spectrometer (BES) detector is a general purpose solenoid detector at the Beijing Electron Positron Collider in Beijing, China. The recent upgrade of the initial BES detector to the improved BES II detector is described. A new drift chamber, time-of-flight counter, luminosity monitor compatible with a low-β lattice was built as well as the vertex chamber.

Radiative decays of the radially excited charmonium resonance, Ψ(2S), into ππ, KK̄ and ηη final states have been measured in a sample of 4.02×106 Ψ(2S) events collected by the BES Collaboration. The branching ratios B(Ψ(2S)→γf 2(1270))=(2.12±0.19±0.32) × 10-4 and B(Ψ(2S)→γf0(1710))×B(f0(1710) →K+K-=(3.02±0.45±0.66)×10 -5 are obtained. When compared to the corresponding radiative J/Ψ decays, the observed Ψ(2S) radiative decay rates into γf 2(1270) and γf00(1710) are consistent with the prediction of the "12%" rule. © 2003 The American Physical Society.