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, (Formula presented) into (Formula presented) (Formula presented) and (Formula presented) final states have been measured in a sample of (Formula presented) (Formula presented) events collected by the BES Collaboration. The branching ratios (Formula presented) and (Formula presented) are obtained. When compared to the corresponding radiative (Formula presented) decays, the observed (Formula presented) radiative decay rates into (Formula presented) and (Formula presented) are consistent with the prediction of the “12%” rule. © 2003 The American Physical Society.

The branching fraction of the (Formula presented) decay into (Formula presented) has been measured for the first time using the BES detector at the Beijing Electron-Positron Collider. The result is (Formula presented) where the first error is statistical and the second is systematic. This value, along with those for the branching fractions into (Formula presented) and (Formula presented) 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 (Formula presented) is determined to be (Formula presented). © 2002 The American Physical Society.