Helicity-dependent cross sections for the photoproduction of $\pi^0$ pairs from nucleons

TL;DR

This study measures helicity-dependent cross sections for $\pi^0$ pair photoproduction from nucleons, revealing the reaction's helicity and isospin structure and significantly impacting theoretical models of nucleon resonances.

Contribution

It provides the first measurement of helicity- and isospin-dependent observables for $\pi^0$ pair photoproduction, offering new insights into nucleon resonance contributions.

Findings

First measurement of helicity-dependent cross sections for $\pi^0$ pairs

Data significantly impact and refine reaction model predictions

Reveals the helicity and isospin structure of the reaction

Abstract

The double-polarization observable $E$ and helicity-dependent cross sections $\sigma_{1/2}$, $\sigma_{3/2}$ have been measured for the photoproduction of $\pi^0$ pairs off quasi-free protons and neutrons at the Mainz MAMI accelerator with the Crystal Ball/TAPS setup. A circularly polarized photon beam was produced by bremsstrahlung from longitudinally polarized electrons and impinged on a longitudinally polarized deuterated butanol target. The reaction products were detected with an almost $4\pi$ covering calorimeter. The results reveal for the first time the helicity- and isospin-dependent structure of the $\gamma N\rightarrow N\pi^0\pi^0$ reaction. They are compared to predictions from reaction models in view of nucleon resonance contributions and also to a refit of one model that predicted results for the proton and for the neutron target. The comparison of the prediction and the refit demonstrate the large impact of the new data.