Three-Body Nature of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:mrow><mml:mo>*</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi mathvariant="normal">Δ</mml:mi><mml:mo>*</mml:mo></mml:msup></mml:math>Resonances from Sequential Decay Chains

The $N{\ensuremath{\pi}}^{0}{\ensuremath{\pi}}^{0}$ decays of positive-parity ${N}^{*}$ and ${\mathrm{\ensuremath{\Delta}}}^{*}$ 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: $\mathrm{\ensuremath{\Delta}}(1232)$, $N(1520)3/{2}^{\ensuremath{-}}$, and $N(1680)5/{2}^{+}$, with spin parities ${J}^{P}=3/{2}^{+}$, $3/{2}^{\ensuremath{-}}$, and $5/{2}^{+}$. The partial wave analysis (within the Bonn-Gatchina approach) identifies $N(1440)1/{2}^{+}$ and the $N(\ensuremath{\pi}\ensuremath{\pi}{)}_{S\text{ }\text{wave}}$ (abbreviated as $N\ensuremath{\sigma}$ here) as further isobars and assigns the final states to the formation of nucleon and $\mathrm{\ensuremath{\Delta}}$ resonances and to nonresonant contributions. We observe the known $\mathrm{\ensuremath{\Delta}}(1232)\ensuremath{\pi}$ decays of $\mathrm{\ensuremath{\Delta}}(1910)1/{2}^{+}$, $\mathrm{\ensuremath{\Delta}}(1920)3/{2}^{+}$, $\mathrm{\ensuremath{\Delta}}(1905)5/{2}^{+}$, $\mathrm{\ensuremath{\Delta}}(1950)7/{2}^{+}$, 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 $N(1440)1/{2}^{+}\ensuremath{\pi}$, $N(1520)3/{2}^{\ensuremath{-}}\ensuremath{\pi}$, $N(1680)5/{2}^{+}\ensuremath{\pi}$, and $N\ensuremath{\sigma}$. The latter decay modes are observed in the decay of ${N}^{*}$ resonances and at most weakly in ${\mathrm{\ensuremath{\Delta}}}^{*}$ decays. It is argued that these decay modes provide evidence for a 3-quark nature of ${N}^{*}$ resonances rather than a quark-diquark structure.

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