Differential cross sections and photon beam asymmetries of $\eta$ photoproduction on the proton at $E_\gamma$ = 1.3-2.4 GeV

TL;DR

This study measures differential cross sections and photon beam asymmetries for $ ext{η}$ photoproduction on protons at energies 1.3-2.4 GeV, revealing a bump structure linked to high-spin resonances and providing new data to constrain nucleon resonance models.

Contribution

It presents new experimental measurements of $ ext{η}$ photoproduction observables at high energies, highlighting a bump structure and photon asymmetries not explained by existing analyses.

Findings

Confirmation of a bump at W=2.0-2.3 GeV at backward angles.

Photon beam asymmetries measured for the first time above 2.1 GeV.

Existing partial wave analyses do not reproduce the observed results.

Abstract

We have carried out exclusive measurements for the photoproduction of an $\eta$ meson from a proton target with an egg-shaped calorimeter made of BGO crystals (BGOegg) and forward charged-particle detectors at the SPring-8 LEPS2 beamline. The differential cross sections and photon beam asymmetries of the $\gamma p \to \eta p$ reaction are measured in a center-of-mass energy ($W$) range of $1.82$-$2.32$ GeV and a polar angle range of $-1.0 < \cos{\theta^{\eta}_{\mathrm{c.m.}}} < 0.6$. The reaction is identified by selecting a proton and two $\gamma$'s produced by an $\eta$-meson decay. The kinematic fit method was employed to select the reaction candidate with the confidence level larger than $1$\%. A bump structure at $W$ = $2.0$-$2.3$ GeV in the differential cross section is confirmed at extremely backward $\eta$ polar angles, where the existing data are inconsistent with each other. This bump structure is likely associated with high-spin resonances that couple with $s\bar{s}$ quarks. The results of the photon beam asymmetries in a wide $\eta$ polar angle range are new for the photon beam energies exceeding $2.1$ GeV. These results are not reproduced by the existing partial wave analyses. They provide an additional constraint to nucleon resonance studies at high energies.