JLab and J-PARC for the J/{\ensuremath{\psi}} Production at the Threshold

Abstract

New threshold measurements for $\gamma~p\to p~J/\psi \to p~(\mu^+\mu^-)$ by 007 and $\gamma^\ast~p\to p~J/\psi \to p~(e^+e^-)$ by CLAS12 allow us to extend the previous phenomenological determination of the J/\ensuremath{\psi}-proton scattering length, $\alpha_{J/\psi p}$, using GlueX threshold data for $\gamma~p\to p~J/\psi \to p~(e^+e^-)$. The agreement between all three J/\ensuremath{\psi} data sets shows no indication of systematic differences between methodologies. Furthermore, perturbative QCD predictions support the phenomenological determination of heavy vector meson-nucleon scattering lengths. The role of the nucleon form factor, $F_N$, is discussed, and a possible correction to the phenomenological scattering length for heavy vector meson photoproduction is calculated using the pole form of $F_N$. Upcoming J-PARC threshold measurements of the reaction $\pi^-~p\to n~J/\psi \to n~(l^+l^-)$ will help to evaluate the possible role of heavy pentaquark, $P_c$, states in low-energy $J/\psi$ production and the effects caused by the nucleon form factors.