Single $\pi^0$ Production Off Neutrons Bound in Deuteron with Linearly Polarized Photons

TL;DR

This study measures the photon beam asymmetry in neutral pion production off neutrons within deuterons at energies 390-610 MeV, providing new data to test and refine partial-wave analyses and isospin symmetry predictions.

Contribution

First measurement of photon beam asymmetry for $ ightarrow$pi^0$ production off neutrons in deuterons at these energies, enabling improved understanding of resonance contributions.

Findings

Data agree with existing partial-wave analyses after revision.

New measurements constrain resonance models and isospin symmetry.

Comparison highlights areas for theoretical improvement.

Abstract

The quasifree $\overrightarrow{\gamma} d\to\pi^0n(p)$ photon beam asymmetry, $\Sigma$, has been measured at photon energies, $E_\gamma$, from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the first time. The data were collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49 to 148$^\circ$. In this kinematic region, polarization observables are sensitive to contributions from the $\Delta (1232)$ and $N(1440)$ resonances. The extracted values of $\Sigma$ have been compared to predictions based on partial-wave analyses (PWAs) of the existing pion photoproduction database. Our comparison includes the SAID, MAID, and Bonn-Gatchina analyses; while a revised SAID fit, including the new $\Sigma$ measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict $\pi^0n$ photoproduction observables, based on fits to published data in the channels $\pi^0p$, $\pi^+n$, and $\pi^-p$.