Three-body nature of $N^{\bf *}$ and $Δ^*$ resonances from sequential decay chains

Abstract

The $Nπ^0π^0$ 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 $N(ππ)_{\rm S-wave}$ (abbreviated as $Nσ$ here) as further isobars, and assigns the final states to the formation of nucleon and $Δ$ resonances and to non-resonant 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.

4 pages, 3 figures, to be published in Phys. Rev. Lett