First measurement of helicity-dependent cross sections in π⁰η photoproduction from quasi-free nucleons

The helicity-dependent cross sections for the photoproduction of π⁰η 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 σ_1/2 (photon and target spin antiparallel) and σ_3/2 (parallel spins) components to be measured for quasi-free production of π⁰η 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 Δ resonances and identical A_1/2 and A_3/2 components are only possible for J≥3/2 states, which constrains possible contributions of nucleon resonances.