# Helicity-dependent cross sections and double-polarization observable E in eta photoproduction from quasi-free protons and neutrons

## TL;DR

This study measured helicity-dependent cross sections and the double-polarization observable E in eta photoproduction from quasi-free protons and neutrons, revealing spin-dependent structures and supporting interference models of partial waves.

## Contribution

It provides the first detailed polarization measurements of eta photoproduction on quasi-free nucleons, clarifying the spin dependence of observed narrow structures.

## Key findings

- Narrow structure appears only in reactions with antiparallel photon and nucleon spins.
- Results support interference of P11 and S11 partial waves in explaining the structure.
- Good agreement with Bonn-Gatchina model predictions.

## Abstract

Precise helicity-dependent cross sections and the double-polarization observable $E$ were measured for $\eta$ photoproduction from quasi-free protons and neutrons bound in the deuteron. The $\eta\rightarrow 2\gamma$ and $\eta\rightarrow 3\pi^0\rightarrow 6\gamma$ decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz. A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were detected with the electromagnetic calorimeters Crystal Ball and TAPS, which covered 98\% of the full solid angle. The results show that the narrow structure observed earlier in the unpolarized excitation function of $\eta$ photoproduction off the neutron appears only in reactions with antiparallel photon and nucleon spin ($\sigma_{1/2}$). It is absent for reactions with parallel spin orientation ($\sigma_{3/2}$) and thus very probably related to partial waves with total spin 1/2. The behavior of the angular distributions of the helicity-dependent cross sections was analyzed by fitting them with Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which uses an interference of $P_{11}$ and $S_{11}$ partial waves to explain the narrow structure.