Eur. Phys. J. A (2023) 59: 232
https://doi.org/10.1140/epja/s10050-023-01134-0

Regular Article - Experimental Physics

Measurement of polarization observables $\mathbf{T}$, $\mathbf{P}$, and $\mathbf{H}$ in ${\pi }^{\mathbf{0}}$ and $\eta$ photoproduction off quasi-free nucleons

¹ Department of Physics, University of Basel, Basel, Switzerland
² Helmholtz-Institut für Strahlen-und Kernphysik, University of Bonn, Bonn, Germany
³ II. Physikalisches Institut, University of Giessen, Giessen, Germany
⁴ Institut für Kernphysik, University of Mainz, Mainz, Germany
⁵ Joint Institute for Nuclear Research, Dubna, Russia
⁶ Department of Physics, Florida State University, Tallahassee, USA
⁷ Physikalisches Institut, University of Bonn, Bonn, Germany
⁸ SUPA School of Physics and Astronomy, University of Glasgow, Glasgow, UK
⁹ Institut für Experimentalphysik I, Ruhr University Bochum, Bochum, Germany
¹⁰ resent address: University of Liverpool, Liverpool, UK
¹¹ resent address: University of Hamburg, Hamburg, Germany
¹² resent address: Ruhr University Bochum, Bochum, Germany

^a nicolas.jermann@unibas.ch
^ch thoma@hiskp.uni-bonn.de

Received: 4 August 2023
Accepted: 21 September 2023
Published online: 17 October 2023

Abstract

The target asymmetry T, recoil asymmetry P, and beam-target double polarization observable H were determined in exclusive ${\pi }^0$ and $\eta$ photoproduction off quasi-free protons and, for the first time, off quasi-free neutrons. The experiment was performed at the electron stretcher accelerator ELSA in Bonn, Germany, with the Crystal Barrel/TAPS detector setup, using a linearly polarized photon beam and a transversely polarized deuterated butanol target. Effects from the Fermi motion of the nucleons within deuterium were removed by a full kinematic reconstruction of the final state invariant mass. A comparison of the data obtained on the proton and on the neutron provides new insight into the isospin structure of the electromagnetic excitation of the nucleon. Earlier measurements of polarization observables in the $\gamma p\to {\pi }^{0}p$ and $\gamma p\to \eta p$ reactions are confirmed. The data obtained on the neutron are of particular relevance for clarifying the origin of the narrow structure in the $\eta n$ system at $W=1.68\text{GeV}$. A comparison with recent partial wave analyses favors the interpretation of this structure as arising from interference of the $S_{11}(1535)$ and $S_{11}(1650)$ resonances within the $S_{11}$-partial wave.