Measurement of the beam-helicity asymmetry $I^{\odot}$ in the photoproduction of $\pi^0$-pairs off the proton and off the neutron

TL;DR

This study measures beam-helicity asymmetries in neutral pion pair photoproduction off protons and neutrons, revealing similar asymmetries for both and validating quasi-free measurements for neutron studies.

Contribution

It provides the first detailed comparison of asymmetries off free and quasi-free nucleons, demonstrating the reliability of quasi-free measurements for neutron observables.

Findings

Excellent agreement between free and quasi-free proton results.

Similar asymmetries observed for reactions off protons and neutrons.

Results align with theoretical models like Two-Pion-MAID and Bonn-Gatchina.

Abstract

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of neutral pion pairs in the reactions $\vec{\gamma}p\rightarrow p\pi^0\pi^0$ and $\vec{\gamma}d\rightarrow (n)p\pi^0\pi^0$, $\vec{\gamma}d\rightarrow (p)n\pi^0\pi^0$ off free protons and off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. Circularly polarized photons were produced from bremsstrahlung of longitudinally polarized electrons and tagged with the Glasgow magnetic spectrometer. Decay photons from the $\pi^0$ mesons, recoil protons, and recoil neutrons were detected in the 4$\pi$ covering electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. After kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons. This demonstrates that the free-nucleon behavior of such observables can be extracted from measurements with quasi-free nucleons, which is the only possibility for the neutron. Contrary to expectations, the measured asymmetries are very similar for reactions off protons and neutrons. The results are compared to the predictions from the Two-Pion-MAID reaction model and (for the proton) also to the Bonn-Gatchina coupled channel analysis.