Deciphering the mechanism of near-threshold J/ψ photoproduction

The photoproduction of the J/psi off the proton is believed to deepen our understanding of various physics issues. On the one hand, it is proposed to provide access to the origin of the proton mass, based on the QCD multipole expansion. On the other hand, it can be employed in a study of pentaquark states. The process is usually assumed to proceed through vector-meson dominance, that is the photon couples to a J/psi which rescatters with the proton to give the J/psi p final state. In this paper, we provide a compelling hint for and propose measurements necessary to confirm a novel production mechanism via the Lambda cD() intermediate states. In particular, there must be cusp structures at the Lambda cD() thresholds in the energy dependence of the J/psi photoproduction cross section. The same mechanism also implies the J/psi -nucleon scattering lengths of order 1 mfm. Given this, one expects only a minor contribution of charm quarks to the nucleon mass.