  The measurements of J/psi decays into photon, proton and antiproton show a
strong enhancement at the proton-antiproton threshold not seen in the decays
into neutral pion, proton and antiproton. What is the nature of this
enhancement? A natural interpretation can be performed in terms of a classical
model of nucleon-antinucleon interactions based on G-parity transformation. The
observed proton-antiproton structure is the consequence of the strong
attraction in the singlet S-wave state related predominantly to pion exchanges.
Similar attractions generate near threshold: an isospin-zero virtual (or
quasi-bound) state in singlet S-wave, an isospin-one quasi-bound state in
triplet P-wave with total angular momentum one and an isospin-zero resonance in
triplet P-wave with total angular momentum zero. These P-wave structures find
support in the antiproton-atomic data.
