Deciphering the mechanism of near-threshold $J/\psi$ photoproduction

TL;DR

This paper suggests a new mechanism for near-threshold $J/-psi$ photoproduction involving $\u03bcla_c ar D^{(*)}$ intermediate states, which could impact understanding of proton mass and pentaquark states.

Contribution

It proposes a novel production mechanism for $J/-psi$ photoproduction via $\u03bcla_c ar D^{(*)}$ states, challenging the traditional vector-meson dominance model.

Findings

Hints of cusp structures at $\u03bcla_c ar D^{(*)}$ thresholds in cross section energy dependence

Implication of $J/-psi$-nucleon scattering lengths around 1 fm

Minor charm quark contribution to proton mass

Abstract

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 Letter, we provide a compelling hint for and propose measurements necessary to confirm a novel production mechanism via the $\Lambda_c \bar D^{(*)}$ intermediate states. In particular, there must be cusp structures at the $\Lambda_c \bar D^{(*)}$ 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.