Experimental study of $\eta$ meson photoproduction reaction at MAMI

TL;DR

This study provides new polarization and differential cross section data for $ ext{η}$ meson photoproduction on protons, resolving previous angular dependence issues and challenging existing theoretical models at energies up to 1.9 GeV.

Contribution

It offers comprehensive new experimental data and analysis that clarify previous ambiguities and test the validity of current theoretical models in $ ext{η}$ meson photoproduction.

Findings

Resolved the angular dependence problem of older $T$ data near threshold.

Found that existing models fail to reproduce new polarization data at higher energies.

Demonstrated the sensitivity of Legendre coefficients to nucleon resonance parameters.

Abstract

New data for the differential cross sections, polarization observables $T$, $F$, and $E$ in the reaction of $\eta$ photoproduction on proton from the threshold up to a center-of-mass energy of W=1.9 GeV are presented. The data were obtained with the Crystal-Ball/TAPS detector setup at the Glasgow tagged photon facility of the Mainz Microtron MAMI. The polarization measurements were made using a frozen-spin butanol target and circularly polarized photon beam. The results are compared to existing experimental data and different PWA predictions. The data solve a long-standing problem related the angular dependence of older $T$ data close to threshold. The unexpected relative phase motion between $s$- and $d$-wave amplitudes required by the old data is not confirmed. At higher energies, all model predictions fail to reproduce the new polarization data indicating a significant impact on our understanding of the underlying dynamics of $\eta$ meson photoproduction. Furthermore, we present a fit of the new data and existing data from GRAAL for $\Sigma$ asymmetry based on an expansion in terms of associated Legendre polynomials. A Legendre decomposition shows the sensitivity to small partial-wave contributions. The sensitivity of the Legendre coefficients to the nucleon resonance parameters is shown using the $\eta$MAID isobar model.