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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.

The decay J/ψ → ωpp has been studied, using 225.3 × 10 6 J/ψ events accumulated at BESIII. No significant enhancement near the pp invariant-mass threshold (denoted as X(pp)) is observed. The upper limit of the branching fraction B(J/ψ → ωX(pp) → ωpp) is determined to be 3.9 × 10 −6 at the 95% confidence level. The branching fraction of J/ψ → ωpp is measured to be B(J/ψ → ωpp) = (9.0 ± 0.2 (stat.) ± 0.9 (syst.)) × 10 −4 . 124The investigation of the near-threshold pp invariant 125 mass spectrum in other J/ψ decay modes will be helpful 126 in understanding the nature of the observed structure. 127The decay J/ψ → ωpp restricts the isospin of the pp 128 system, and it is helpful to clarify the role of the pp in the return iron yoke of the superconducting magnet. 174The position resolution is about 2 cm. 175The optimization of the event selection and the es- 247The branching fraction of J/ψ → ωpp is calculated 248 according to :(1) where N obs is the number of signal events determined Breit-Wigner function :Here, q is the momentum of the proton in the pp rest where N obs is the number of signal events, and L is the Author's Copy where σ sys. is the total systematic uncertainty which will 299 be described in the next section. The upper limit on the 300 product of branching fractions is B(J/ψ → ωX(pp) → 301 ωpp) < 3.9 × 10 −6 at the 95% C.L.. 302An alternative fit with a Breit-Wigner function includ-for X(pp) is performed. Here, f FSI is the Jülich FSI cor- between data and MC simulation is 2% per charged track. 323The systematic uncertainty from PID is 2% per proton 324(anti-proton). 325The photon detection systematic uncertainty is studied efficiency difference is about 1% for each photon [32, 33]. 329Author's Copy Near-threshold pp invariant-mass spectrum. The signal J/ψ → ωX(pp) → ωpp is described by an acceptanceweighted Breit-Wigner function, and and signal yield is consistent with zero. The dotted line is the shape of the signal which is normalized to five times the estimated upper limit. The dashed line is the non-resonant contribution described by the function f (δ) and the dashed-dotted line is the non ωpp contribution which is estimated from ω sidebands. The solid line is the total contribution of the two components. The hatched area is from the sideband region.Here, 3% is taken as the systematic error for the efficien- ciency between data and MC is 3%, and is taken as the 338 systematic uncertainty caused by the kinematic fit. 339As described above, the yield of J/ψ → ωpp is de- The signal J/ψ → ωX(pp) → ωpp is described by an acceptanceweighted Breit-Wigner function, and and signal yield is consistent with zero. The dashed line is the non-resonant contribution fixed to a phase space MC simulation of J/ψ → ωpp and the dashed-dotted line is the non ωpp contribution which is estimated from ω sidebands. The solid line is the total contribution of the two components. The hatched area is from a phase space MC simulation of J/ψ → ωpp.sented by Figure.

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