The $N{\pi }^0{\pi }^0$ decays of positive-parity $N^{*}$ and ${\mathrm{\Delta }}^{*}$ resonances at about 2 GeV are studied at ELSA by photoproduction of two neutral pions off protons. The data reveal clear evidence for several intermediate resonances: $\mathrm{\Delta }(1232)$, $N(1520)3/2^{-}$, and $N(1680)5/2^{+}$, with spin parities $J^P=3/2^{+}$, $3/2^{-}$, and $5/2^{+}$. The partial wave analysis (within the Bonn-Gatchina approach) identifies $N(1440)1/2^{+}$ and the $N(\pi \pi {)}_{S\text{wave}}$ (abbreviated as $N\sigma$ here) as further isobars and assigns the final states to the formation of nucleon and $\mathrm{\Delta }$ resonances and to nonresonant contributions. We observe the known $\mathrm{\Delta }(1232)\pi$ decays of $\mathrm{\Delta }(1910)1/2^{+}$, $\mathrm{\Delta }(1920)3/2^{+}$, $\mathrm{\Delta }(1905)5/2^{+}$, $\mathrm{\Delta }(1950)7/2^{+}$, and of the corresponding spin-parity series in the nucleon sector, $N(1880)1/2^{+}$, $N(1900)3/2^{+}$, $N(2000)5/2^{+}$, and $N(1990)7/2^{+}$. For the nucleon resonances, these decay modes are reported here for the first time. Further new decay modes proceed via $N(1440)1/2^{+}\pi$, $N(1520)3/2^{-}\pi$, $N(1680)5/2^{+}\pi$, and $N\sigma$. The latter decay modes are observed in the decay of $N^{*}$ resonances and at most weakly in ${\mathrm{\Delta }}^{*}$ decays. It is argued that these decay modes provide evidence for a 3-quark nature of $N^{*}$ resonances rather than a quark-diquark structure.

• Received 3 November 2014

DOI:https://doi.org/10.1103/PhysRevLett.114.091803