Studies of baryon resonances with meson photoproduction at SPring-8

Meson photoproduction is a helpful tool for investigating baryon resonances due to many possibilities for meson-baryon combinations in the final-state, such as piN, etaN, omegaN, or multi-meson final states. Additionally, the use of a highly polarized photon beam enables the extraction of spin information of intermediate resonances.

The piN final states can couple to both I=1/2 states (N* resonances) and I=3/2 states (Δ resonances). On the other hand, the etaN and omegaN states can couple to only baryons with isospin 1/2, which is called an isospin filter. Furthermore, the eta meson contains ssbar components, and an omega meson has spin 1. These unique features make the eta- and omega-meson photoproduction promising for coulpling with resonances that are not well-coupled with the piN state.

Previous experiments have shown inconsistent measurements of the differential cross sections and a scarcity of data on photon beam asymmetries above 2 GeV. We studied photoproduction reactions of a neutral pion, eta, omega, eta-prime, and f0(980) meson on the proton at the LEPS2/BGOegg experiment using a GeV photon beam produced by the backward Compton scattering. This photon beam is highly linear polarized, and this polarization degree is more than 90 % in the highest energy region around the Compton edge.

We measured differential cross sections, photon beam asymmetries, and spin density matrix elements with high statistics and broad angular coverage by using a large acceptance calorimeter (BGOegg) and forward-angle charged-particle detectors.

This calorimeter can identify mesons that decay into multiple gammas, and its energy resolution is the world’s best among the experiments conducted in a similar energy range.

 In the eta photoproduction reaction, a bump-like enhancement of the differential cross sections was observed at backward angles. The strength of this enhancement increased as the eta emission angles became more backward, indicating the presence of high-spin nucleon resonances containing a significant ssbar component.

We report the photon beam asymmetries and spin density matrix elements in a wide polar angle range for the photon beam energy above 2 GeV for the first time, providing additional constraints to nucleon resonance studies at high energies.