First measurement of helicity-dependent cross sections in pi0-eta photoproduction from quasi-free nucleons

TL;DR

This study presents the first measurement of helicity-dependent cross sections in pi0-eta photoproduction from quasi-free nucleons, revealing identical contributions from helicity components and couplings for protons and neutrons, constraining nucleon resonance models.

Contribution

It provides the first experimental data on helicity-dependent cross sections in pi0-eta photoproduction, highlighting the equal electromagnetic couplings for protons and neutrons.

Findings

Helicity components contribute equally within uncertainties.

Proton and neutron couplings are identical.

Constraints on nucleon resonance contributions.

Abstract

The helicity-dependent cross sections for the photoproduction of $\pi^0\eta$ pairs have been measured for the first time. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS calorimeter. The experiment used a polarized deuterated butanol target and a circularly polarized photon beam. This arrangement allowed the $\sigma_{1/2}$ (photon and target spin antiparallel) and $\sigma_{3/2}$ (parallel spins) components to be measured for quasi-free production of $\pi^0\eta$ pairs off protons and neutrons. The main finding is that the two helicity components contribute identically, within uncertainties, for both participant protons and neutrons. The absolute couplings for protons and neutrons are also identical. This means that nucleon resonances contributing to this reaction in the investigated energy range have almost equal electromagnetic helicity couplings, $A_{1/2}^{n,p}$ and $A_{3/2}^{n,p}$. Identical couplings for protons and neutrons are typical for $\Delta$ resonances and identical $A_{1/2}$ and $A_{3/2}$ components are only possible for $J\geq 3/2$ states, which constrains possible contributions of nucleon resonances.