Data on IsIs and IcIc in γ→p→pπ0π0 reveal cascade decays of N(1900)N(1900) via N(1520)πN(1520)π

Photoproduction of two neutral pions off the proton is studied using linearly polarized photons, and the polarization observables IsIs and IcIc are measured for the first time. These two observables are unique to multi-meson final states; they characterize correlations between the linear photon polarization and the direction of outgoing single particles in photoproduction of three-body final states. The IsIs and IcIc distributions suggest that, in the 1.8 to 2.0 GeV mass region, the N(1520)3/2−πN(1520)3/2−π intermediate state is reached with reaction dynamics consistent with a dominant JP=3/2+JP=3/2+ wave. These data are included in the Bonn–Gatchina (BnGa) partial wave analysis which is based on a large variety of data; the analysis confirms a significant contribution from the reaction chain γp→N(1900)3/2+→N(1520)3/2−π0→pπ0π0γp→N(1900)3/2+→N(1520)3/2−π0→pπ0π0.