Diquarks and Exotic Spectroscopy

We propose that the recently discovered ${\ensuremath{\Theta}}^{+}$ baryon is a bound state of four quarks and an antiquark, containing two highly correlated $ud$ pairs. If so, the ${\ensuremath{\Theta}}^{+}$ has positive parity, and it lies in an near-ideally mixed $\mathrm{S}\mathrm{U}(3{)}_{f}$ ${\overline{\mathbf{10}}}_{f}\ensuremath{\bigoplus}{\mathbf{8}}_{f}$. The Roper resonance and the ${P}_{11}(1710)$ fit naturally into this classification. We predict an isospin $3/2$ multiplet of $\ensuremath{\Xi}$'s ($S=\ensuremath{-}2$) with ${J}^{\ensuremath{\Pi}}={\frac{1}{2}}^{+}$ around 1750 MeV. A search for manifestly exotic ${\ensuremath{\Xi}}^{+}$ and ${\ensuremath{\Xi}}^{--}$ in this mass range could provide a sharp test of our proposal. We predict that charm and bottom analogs of the ${\ensuremath{\Theta}}^{+}$ may be stable against strong decays.