$J/\psi$ photoproduction near threshold and signals for the hidden charm pentaquarks

TL;DR

This study models near-threshold $J/a$ photoproduction, highlighting the significance of open charm channels and $P_c$ pentaquark states, and suggests experimental measurements to clarify underlying mechanisms.

Contribution

It introduces a comprehensive model including open charm channels and $P_c$ states to explain differential cross sections and threshold effects in $J/a$ photoproduction.

Findings

Open charm channels improve data description at thresholds.

Structures at $\u039b_car{D}^{(*)}$ thresholds are explained as cusp effects.

$P_c$ production is suppressed, consistent with lack of direct signals.

Abstract

We study the $J/\psi$ photoproduction $\gamma p\to J/\psi p$ near threshold to investigate the role played by the open charm channels and possible signals from the hidden charm $P_c$ pentaquark states. With the diffractive mechanism described by a Pomeron exchange model extrapolated from high energies to the lower energy region, it shows that the differential cross sections compared with the recent results from the GlueX Collaboration have apparent deficits in the large scattering angles. The inclusion of the open charm channels and intermediate $P_c$ states can significantly improve the descriptions of the differential cross section data, in particular, at the energy regions of the $\Lambda_c\bar{D}^{(*)}$ thresholds. This can explain the structures observed by the GlueX Collaboration at the $\Lambda_c\bar{D}^{(*)}$ thresholds as the open charm CUSP effects. Given that these $P_c$ pentaquark states are hadronic molecules dynamically generated by the $\Sigma_c\bar{D}^{(*)}$, we find that the production of these $P_c$ states should be suppressed at leading order since their couplings to both $\gamma p$ and $J/\psi p$ are through loop transitions. This can explain why no obvious signals for the $P_c$ states are observed by the present datasets. In order to further disentangling the role played by $s$-channel mechanisms, we also investigate the polarized beam asymmetry which shows sensitivities to the open charm threshold effects and interferences from the $P_c$ productions. Experimental measurement of this observable at the Jefferson Laboratory is strongly encouraged.