Speaker
Description
Meson photoproduction is a helpful tool for studying baryon resonances thanks to many possibilities of final-state meson-baryon combinations, e.g., $\pi N$, $\eta N$, $\omega N$, or multi meson final states.
In addition, the spin information of intermediate resonances can also be obtained by utilizing the high polarization of a photon beam.
The $\pi N$ final states can couple to both I = 1/2 states ($N^{*}$ resonances) and I = 3/2 states ($\Delta$ resonances).
On the other hand, the $\eta N$ and $\omega N$ states can couple to only baryons with isospin 1/2, which is called an isospin filter.
Moreover, an eta meson contains $s\bar{s}$ components, and an omega meson has spin 1.
From these features, the $\eta$- and $\omega$-meson photoproduction are expected to offer an attractive capability of coupling with the resonances that do not couple well with the $\pi N$ state.
The differential cross sections measured in past experiments were inconsistent with each other, and the data of photon beam asymmetries were scarce above 2 GeV.
We studied photoproduction reactions of a $\pi^{0}$, $\eta$, and $\omega$ 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 $\gamma$'s, and its energy resolution is the world’s best among the experiments conducted in a similar energy range.
A bump-like enhancement of differential cross sections can only be seen at backward angles in the $\eta$ photoproduction reaction.
Its strength increases as the $\eta$ emission angles become more backward.
This enhancement indicates the existence of high-spin nucleon resonances that contain a large $s\bar{s}$ 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.
