Photoproduction of meson pairs: First measurement of the polarization observable Is

The polarization observable Is, a feature exclusive to the acoplanar kinematics of multi-meson final states produced via linearly polarized photons, has been measured for the first time. Results for the reaction γ→p→pπ0η are presented for incoming photon energies between 970 MeV and 1650 MeV along with the beam asymmetry Ic. The comparably large asymmetries demonstrate a high sensitivity of Is to the dynamics of the reaction. The sensitivity of these new polarization observables to the contributing partial waves is demonstrated by fits using the Bonn-Gatchina partial wave analysis.