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.

Apart from the charmful decay channels of Y (4260), the charmless decay channels of Y (4260) also provide us a good platform to study the nature and the decay mechanism of Y (4260). In this paper, we propose to probe the structure of Y (4260) through the charmless decays Y (4260) → V P via intermediate D1D+c.c. meson loops, where V and P stand for light vector and pseudoscalar mesons, respectively. Under the molecule ansatz of Y (4260), the predicted total branching ratio BRV P for all Y (4260) → V P processes are about (0.34 +0.32 −0.23 )% to (0.75 +0.72 −0.52 )% with the cutoff parameter α = 2 ∼ 3. Numerical results show that the intermediate D1D +c.c. meson loops may be a possible transition mechanism in the Y (4260) → V P decays. These predicted branching ratios are the same order to that of Y (4260) → Z + c (3900)π − , which may be an evidence of D1D molecule and can be examined by the forthcoming BESIII data in the near future.

Using a sample of 1.06 × 10 8 ψ ′ decays collected by the BESIII detector, χ c0 and χ c2 decays into π 0 π 0 and ηη are studied. The branching fraction results are Br(χ c0 → π 0 π 0 ) = (3.23 ± 0.03 ± 0.23 ± 0.14) × 10 −3 , Br(χ c2 → π 0 π 0 ) = (8.8 ± 0.2 ± 0.6 ± 0.4) × 10 −4 , Br(χ c0 → ηη) = (3.44 ± 0.10 ± 0.24 ± 0.2) × 10 −3 , and Br(χ c2 → ηη) = (6.5 ± 0.4 ± 0.5 ± 0.3) × 10 −4 , where the uncertainties are statistical, systematic due to this measurement, and systematic due to the branching fractions of ψ ′ → γχ cJ , respectively. The results provide information on the decay mechanism of χ c states into pseudoscalars.

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To validate the molecular description of the observed Z b (10610)/Z b (10650) and Z c (3900)/Z c (4025), it is valuable to investigate their counterparts, denoted as Z (′) QV in this work, and the corresponding decay modes. In this work, we present an analysis of the Z (′) QV using flavor symmetry. We also use the effective Lagrangian based on the heavy quark symmetry to explore the rescattering mechanism and calculate the partial widths for the isospin conserved channels Z (′) QV → η Q V . The predicted partial widths are of an order of MeV for Z QV → η Q V , which correspond to branching ratios of the order of 10 −2 ∼ 10 −1 .For Z ′ QV → η Q V , the partial widths are a few hundreds of keV and the branching ratios are about 10 −3 . Future experimental measurements can test our predictions on the partial widths and thus examine the molecule description of heavy quarkoniumlike exotic states.