title: Three-body nature of N(*) and Δ(*) resonances from sequential decay chains.
abstract: The Nπ^{0}π^{0} decays of positive-parity N^{*} and Δ^{*} 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: Δ(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(ππ)_{S wave} (abbreviated as Nσ here) as further isobars and assigns the final states to the formation of nucleon and Δ resonances and to nonresonant contributions. We observe the known Δ(1232)π decays of Δ(1910)1/2^{+}, Δ(1920)3/2^{+}, Δ(1905)5/2^{+}, Δ(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^{+}π, N(1520)3/2^{-}π, N(1680)5/2^{+}π, and Nσ. The latter decay modes are observed in the decay of N^{*} resonances and at most weakly in Δ^{*} decays. It is argued that these decay modes provide evidence for a 3-quark nature of N^{*} resonances rather than a quark-diquark structure.
authors: Helmholtz-Institut für Strahlen- und Kernphysik, Friedrich-Wilhelms Universität, 53115 Bonn, Germany.
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authors: Helmholtz-Institut für Strahlen- und Kernphysik, Friedrich-Wilhelms Universität, 53115 Bonn, Germany.
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authors: Helmholtz-Institut für Strahlen- und Kernphysik, Friedrich-Wilhelms Universität, 53115 Bonn, Germany.|II. Physikalisches Institut, Justus-Liebig-Universität Gießen, 35392 Gießen, Germany.
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authors: Helmholtz-Institut für Strahlen- und Kernphysik, Friedrich-Wilhelms Universität, 53115 Bonn, Germany.|Petersburg Nuclear Physics Institute, Gatchina, Leningrad district 188300, Russia.
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authors: Helmholtz-Institut für Strahlen- und Kernphysik, Friedrich-Wilhelms Universität, 53115 Bonn, Germany.|Petersburg Nuclear Physics Institute, Gatchina, Leningrad district 188300, Russia.
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