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

Abstract

Abstract Very recently, a new hadronic exotic state η 1(1855) at the invariant mass spectrum of η η ′ was observed by the BESIII Collaboration. According to its properties, such as the mass and decay width, the η 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 η 1(1855), a Reggeized model combined with the vector dominance model for η 1(1855) photoproduction on the proton target is presented. The η 1(1855) photoproduction is dominated by the t-channel vector mesons ρ, ω, and ϕ exchange. If the η 1(1855) is a s s ¯ g hybrid meson, the Primakoff effect also should be included. Our calculations show that the total cross section of the η 1(1855) production via γ p reaction can reach up to 0.276 pb for θ = 15° or 0.119 pb for θ = 0°, about 0.0297% or 0.0128% of the total cross section obtained by considering the η 1(1855) as an S-wave K K ¯ 1 ( 1400 ) molecule. We also find their line shapes are sizably different. If the η 1(1855) is a molecular state, the photoproduction of η 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 γ = 16.97 GeV, to observe the production of η 1(1855) if η 1(1855) is a s 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 η 1(1855).