The BES collaboration at the Beijing Electron Positron Collider (BEPC) has observed a clear signal for a narrow enhancement in the ppbar mass distribution near the 2mp threshold in the process J/Ψ→γppbar. The peak, which has a statistical significance of more than 16 σ, was found in the analysis of 58 million J/Ψ events. No corresponding enhancement has been seen in J/Ψ→ppbarπ0 decays.
The peak can be fit with either a S-wave (0-+) or a P-wave (0++) Breit-Wigner resonance function. For the S-wave fit, the mass is below 2mp at 1859+3-10 (stat)+5-25 (sys) MeV/c2 and the width is less than 30 MeV/c2 at 90% CL. For the P-wave fit, the mass is 1876.4±0.9 MeV/c2 with a width of 4.6±1.8(stat) MeV/c2. The acceptance-corrected photon angular distribution is consistent with that expected for a resonance with JPC= 0-+ or 0++.
Although there were indications of ppbar mass threshold enhancements in the J/Ψ→γppbar process from earlier experiments, including MARK-III at SLAC and DM2 at Orsay, the limited statistics in the data samples meant no firm conclusions could be drawn. The high statistics acquired with BES-II, however, has excluded the interpretation of the effect as being due to known particles, such as the η(1760). The high statistics also allow the mass and width of the state to be well determined, that is, below 2mp (S-wave) or at 2mp (P-wave). The mass and unexpectedly narrow width of the new resonance suggest it could be interpreted as a “deuteron-like” spin 0 proton-antiproton bound state (baryonium), with a zero baryon number.
Further reading
J Z Bai et al. 2003 Phys. Rev. Lett. 91 022001.
