BESIII

BESIII (Beijing Spectrometer III) is a general-purpose particle detector for the Beijing Electron–Positron Collider II (BEPCII) at the Institute of High Energy Physics in Beijing, China. It is designed to study physics in the tau-charm energy region (center-of-mass energies from 2 to 4.6 GeV) using electron-positron collisions. The detector began data taking in 2008 and BEPCII has achieved high luminosity, enabling large data samples at resonance, notably J/psi, psi(2S), and psi(3770).

The detector is a cylindrical, multi-purpose apparatus comprising a main drift chamber for charged-particle tracking, a time-of-flight system for particle identification, an electromagnetic calorimeter for energy measurement, and a muon counter, all contained within a superconducting solenoid providing about 1 tesla. The combination provides good momentum, vertex, and energy resolution over a wide solid angle.

BESIII's physics program covers charm and tau physics, charmonium spectroscopy, and hadron spectroscopy. It has produced precise measurements of D meson decays, studied tau decays, and contributed to the study of charmonium and charmonium-like states, including observations related to charged Z states such as Zc(3900). The data samples from J/psi and psi(2S) decays enable high-statistics studies of light hadrons and tests of quantum chromodynamics, while data at the psi(3770) resonance enables precise absolute branching fractions for D mesons.

BEPCII and BESIII have contributed to the global understanding of strong interaction dynamics in the tau-charm region and continue to provide data for particle physics analyses.