This paper will discuss the design and construction of BESIII [1], which is designed to study physics in the τ-charm energy region utilizing the new high luminosity BEPCII double ring e + ecollider [2]. The expected performance will be given based on Monte Carlo simulations and results of cosmic ray and beam tests. In BESIII, tracking and momentum measurements for charged particles are made by a cylindrical multilayer drift chamber in a 1 T superconducting solenoid. Charged particles are identified with a time-of-flight system based on plastic scintillators in conjunction with dE/dx (energy loss per unit pathlength) measurements in the drift chamber. Energies of electromagnetic showers are measured by a CsI(Tl) crystal calorimeter located inside the solenoid magnet. Muons are identified by arrays of resistive plate chambers in the steel magnetic flux return. The level 1 trigger system, Data Acquisition system and the event filter system based on networked computers will also be described.

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The decay channel J/ψ → γπ + π − η ′ is analyzed using a sample of 5.8 × 10 7 J/ψ events collected with the BESII detector. A resonance, the X(1835), is observed in the π + π − η ′ invariant mass spectrum with a statistical significance of 7.7σ. A fit with a Breit-Wigner function yields a mass M = 1833.7 ± 6.1(stat) ± 2.7(syst) MeV/c 2 , a width Γ = 67.7 ± 20.3(stat) ± 7.7(syst) MeV/c 2 and a