The Beijing Electron–Positron Collider II (BEPC II) is a Chinese electron–positron collider, a type of particle accelerator, located in Beijing, People's Republic of China. It has been in operation since 2008 and has a circumference of 240.4 m.
It was intended as a charm factory and continues the role CLEO-c detector. The center of mass energy can go up to 4.6 GeV with a design luminosity of 1033 cm−2·s−1. Operations began in summer 2008 and the machine has run at multiple energies.
The BES III (Beijing Spectrometer III) is the main detector for the upgraded BEPC II.
BES III uses a large superconducting solenoid to provide a 1-tesla magnetic field, and also features a helium gas-based tracking chamber and an electromagnetic calorimeter using 6240 caesium iodide crystals.