Revealing the inner structure of the newly observed η ₁(1855) via photoproduction

Abstract

Very recently, a new hadronic exotic state $\eta_1(1855)$ at the invariant mass spectrum of $\eta\eta^{'}$ was observed by the BESIII Collaboration.
According to its properties, such as the mass and decay width, the $\eta_1(1855)$ have been suggested to be a compact multi-quark state, a hadron
molecule, or a hybrid meson. In order to distinguish the various interpretations of the $\eta_1(1855)$, a Reggeized model combined with the vector
dominance model for $\eta_1(1855)$ photoproduction on the proton target is presented. The $\eta_1(1855)$ photoproduction is dominated by the $t$-channel
vector mesons $\rho$, $\omega$, and $\phi$ exchange. If the $\eta_1(1855)$ is a $s\bar{s}g$ hybrid meson, the Primakoff effect also should be included.
Our calculations show that the total cross section of the $\eta_1(1855)$ production via $\gamma{}p$ reaction can reach up to 0.276 pb for $\theta=15^{\circ}$
or 0.119 pb for $\theta=0^{\circ}$, about 0.0297$\%$ or 0.0128$\%$ of the total cross section obtained by considering the
$\eta_1(1855)$ as an $S$-wave $K\bar{K}_1(1400)$ molecule. We also find their line shapes are sizably different. If the $\eta_1(1855)$ is a molecular state,
the photoproduction of $\eta_1(1855)$ near the threshold offers a nice place to test its molecular nature. However, it should be better to take high energy,
at least above $E_{\gamma}=16.97$ GeV, to observe the production of $\eta_1(1855)$ if $\eta_1(1855)$ is a $s\bar{s}g$ hybrid meson. These results can be measured
in the GlueX experiment or Electron-Ion Collider in China to test the nature of the $\eta_1(1855)$.