# Photoproduction of eta mesons from the neutron: cross sections and double polarization observable E

## TL;DR

This paper reports measurements of eta meson photoproduction from neutrons, revealing a narrow structure in the excitation function and a forward-backward asymmetry, providing insights into nucleon resonance behavior.

## Contribution

It presents new experimental data on eta photoproduction from neutrons, including angular distributions and polarization observables, confirming and extending previous findings.

## Key findings

- Confirmation of a narrow structure in neutron excitation function
- Observation of forward-backward asymmetry in angular distributions
- Identification of the structure's association with helicity-1/2 cross section

## Abstract

Photoproduction of $\eta$ mesons from neutrons} \abstract{Results from measurements of the photoproduction of $\eta$ mesons from quasifree protons and neutrons are summarized. The experiments were performed with the CBELSA/TAPS detector at the electron accelerator ELSA in Bonn using the $\eta\to3\pi^{0}\to6\gamma$ decay. A liquid deuterium target was used for the measurement of total cross sections and angular distributions. The results confirm earlier measurements from Bonn and the MAMI facility in Mainz about the existence of a narrow structure in the excitation function of $\gamma n\rightarrow n\eta$. The current angular distributions show a forward-backward asymmetry, which was previously not seen, but was predicted by model calculations including an additional narrow $P_{11}$ state. Furthermore, data obtained with a longitudinally polarized, deuterated butanol target and a circularly polarized photon beam were analyzed to determine the double polarization observable $E$. Both data sets together were also used to extract the helicity dependent cross sections $\sigma_{1/2}$ and $\sigma_{3/2}$. The narrow structure in the excitation function of $\gamma n\rightarrow n\eta$ appears associated with the helicity-1/2 component of the reaction.