# Insight into the narrow structure in {\boldmath{$\eta$}}-photoproduction on the neutron from helicity dependent cross sections

## TL;DR

This study measures helicity-dependent cross sections in eta photoproduction on neutrons, revealing that a previously observed narrow structure is linked to a specific spin-1/2 nucleon resonance, supporting the existence of a narrow P11 resonance.

## Contribution

It provides new experimental data on helicity-dependent cross sections for eta photoproduction on neutrons, clarifying the nature of a narrow structure and supporting the resonance hypothesis.

## Key findings

- The narrow structure appears only in $\sigma_{1/2}$, indicating a spin-1/2 resonance.
- Nucleon resonances involved are $N1/2^-$ ($S_{11}$) and $N1/2^+$ ($P_{11}$).
- Legendre coefficients agree with models predicting a narrow P11 resonance.

## Abstract

The double polarization observable $E$ and the helicity dependent cross sections $\sigma_{1/2}$ and $\sigma_{3/2}$ were measured for $\eta$ photoproduction from quasi-free protons and neutrons. The circularly polarized tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost $4\pi$ detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from $\eta\rightarrow 2\gamma$ and $\eta\rightarrow 3\pi^0$. The results show that the narrow structure previously observed in $\eta$ photoproduction from the neutron is only apparent in $\sigma_{1/2}$ and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that contribute to this partial wave in $\eta$ production are only $N1/2^-$ ($S_{11}$) and $N1/2^+$ ($P_{11}$). Furthermore, the extracted Legendre coefficients of the angular distributions for $\sigma_{1/2}$ are in good agreement with recent reaction model predictions assuming a narrow resonance in the $P_{11}$ wave as the origin of this structure.