BEPCII/BESIII are a major upgrade of BEPCI/BESII at the BEPC accelerator for the studies of light and charm quarks physics and τ -charm physics with the highest accuracy achieved until now. The designed peak luminosity of the double-ring e+e−. collider, BEPCII, is 1033cm−2s−1 at about 3.7 GeV.The BESIII detector with a geometrical acceptance of 93% of 4π, consists of the following main components :
- 1) A drift chamber(MDC) equipped with 6796 signal wires and 21884 field wires arranged in a small cell configuration with 43 layers working in a gas mixture of He(40%) and C3H8(60%). The single wire resolution on average is 135 μm and the momentum resolution for charged particles in a 1 T magnetic field is 0.5% at 1 GeV;
- 2) An electromagnetic calorimeter(EMC) made of 6240 CsI(Tl) crystals arranged in a cylindrical shape plus two endcaps. The energy resolution is 2.5% in the barrel and 5% in the endcaps at 1.0 GeV; the position resolution is 6mm in the barrel and 9mm in the endcaps at 1.0 GeV; photon energy resolution at BESIII is much better than that at BESII and comparable to those at CLEO and Crystal Ball; the peaking time of the energy deposit in the EMC is recorded with a precision of 50 ns . Such a time stamp can be used to identify the energy deposit in coincidence with the beam-crossing, hence significantly reduce the background;
- 3) A Time-Of-Flight system(TOF) for particle identification made of 176 pieces, 2.4m long plastic scintillators arranged as a cylinder with two layers for the barrel, 5cm thick each, and 96 fan-shaped, 5 cm thick, plastic scintillators for two endcaps; time resolution is about 80 ps in the barrel, and about 110 ps in the endcaps, corresponding to a K/π separation of more than 2σ level up to about 1.0 GeV;
- 4) A muon chamber system(MUC) made of 1000m2 of Resistive Plate Chambers(RPC) arranged in 9 layers in the barrel and 8 layers in the endcaps. The position resolution is about 2 cm.