Diquarks and Exotic Spectroscopy

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

Figure 1

Quark content of representative members of the $(q^4\overline{q}){\overline{\mathbf{10}}}_f+{\mathbf{8}}_f$.

Figure 2

Relative masses of states in the ideally mixed $(q^4\overline{q}){\overline{\mathbf{10}}}_f\oplus {\mathbf{8}}_f$, compared with the mass hierarchy in the chiral soliton model.