Threshold Photo-production of J/psi Mesons

With the advent of higher energies at Jefferson Lab, the study of charmonium becomes possible. The threshold production of J/Ψ meson photo-production on hydrogen is ∼ 8.2 GeV, thus with a 8+ GeV beam, the elementary γ-J/Ψ cross section can be measured. Threshold charm production on a nucleus can give information on the J/Ψ-N interaction. The standard method to extract this cross section has been to measure the nuclear dependence of J/Ψ production. The majority of these A-dependent J/Ψ production experiments have been measured at high energy, while the only near-threshold experiment was performed using 20 GeV photons. This 20 GeV SLAC experiment measured σJ/#Psi#N = 3.6 ± 0.8 ± 0.5 mb[1]; whereas theory predicts this cross section to be higher, about 7 mb [2]. It is unclear whether the SLAC determination of σJ/#Psi#N corresponds to the physical σJ/#Psi#N, due to the fact that at these energies the J/Ψ may still be formed outside the nucleus [3] [4]. A measurement of the nuclear dependence of threshold J/Ψ photo-production may resolve this issue