# First measurement of the polarization observable $E$ and helicity-dependent cross sections in single $\pi^{0}$ photoproduction from quasi-free nucleons

## TL;DR

This study presents the first measurements of the polarization observable E and helicity-dependent cross sections in single π0 photoproduction from quasi-free nucleons, providing new insights into nucleon structure and reaction mechanisms.

## Contribution

It introduces the first experimental determination of the polarization observable E and helicity-dependent cross sections for single π0 photoproduction from bound nucleons, accounting for nuclear FSI effects.

## Key findings

- Nuclear FSI significantly modify absolute cross sections.
- The asymmetry E remains unaffected by FSI effects.
- Helicity-dependent cross sections for free neutrons are estimated using quasi-free data.

## Abstract

The double-polarization observable $E$ and the helicity-dependent cross sections $\sigma_{1/2}$ and $\sigma_{3/2}$ have been measured for the first time for single $\pi^{0}$ photoproduction from protons and neutrons bound in the deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the $\pi^0$ meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the $\pi^{0}N$ final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry $E$ since the $\sigma_{1/2}$ and $\sigma_{3/2}$ components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry $E$ measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons.