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 paper, we provide a compelling hint for and propose measurements necessary to confirm a novel production mechanism via the varLambda _c bar{D}^{(*)} intermediate states. In particular, there must be cusp structures at the varLambda _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.
Highlights
Verse gets most of its mass that can be translated to how the proton and neutron—the fundamental ingredients of all kinds of nuclei in the universe—acquire their masses
If this is the dominant mechanism, the near-threshold J/ψ photoproduction cross section would provide the link to the J/ψ p elastic scattering amplitude at low energies which is fundamentally important because the J/ψ p scattering length can be related to the nucleon matrix element of two-gluon operators and thereby to the trace anomaly contribution to the nucleon mass
(i) the J/ψ attached to the photon is highly off-shell while the J/ψ p scattering length is defined for the on-shell scattering amplitude; (ii) the Λ+c D 0 threshold is only 116 MeV above the J/ψ p threshold, rendering the contribution from the Λc Dchannel potentially sizeable and making the relation between the photoproduction cross section and the trace anomaly contribution to the nucleon mass even more obscure
Summary
Another fundamental issue of nonperturbative QCD is that it is still unclear how the hadron spectrum is organized. Exotic hadrons such as multiquark states are being sought experimentally and studied theoretically using. The rich physical implications related to the photoproduction of the J/ψ off the proton provide a strong motivation to revise the assumptions underlying the VMD approach, and identify its possible caveats. We propose a new coupled-channel (CC) mechanism for the near-threshold J/ψ photoproduction which is not directly related to the nucleon matrix element of the gluonic operator since the J/ψ p final state is produced through the nearby open-charm channels Λc Dand Λc D ∗, see Fig. 2. We demonstrate that the data recently measured at GlueX can be quantitatively understood using this mechanism with reasonable parameters.
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