Antidecuplet candidate ${\Xi }^{--}(1862)$ as a heptaquark with the overlap of two antikaons and a nucleon

I study the very recently discovered $S=-2$ resonance ${\Xi }^{--}(1886)$ by the NA49 Collaboration at the CERN SPS. I find that repulsion excludes the ${\Xi }^{--}$ as a $ddss\overline{u}$ $s$-wave pentaquark. I explore the ${\Xi }^{--}$ as a heptaquark, equivalent to a $N+K+K$ linear molecule, with positive parity and total isospin $I=3/2$. I find that the kaon-kaon repulsion is canceled by the attraction existing in the kaon-nucleon channels. In our framework this state is easier to bind than the ${\Theta }^{+}$ described by a $\pi +K+N$ Borromean bound state. The remaining $I=1/2$ doublet and $I=1$ triplet of the exotic antidecuplet are also studied, and the coupling to $p$-wave decay channels is briefly addressed.

Figure 1

The Jacobi coordinates of the heptaquark.

Figure 2

The masses of the narrow exotic antidecuplets, in the heptaquark and pentaquark scenarios, are shown in a three-dimensional strangeness-flavor-mass plot. The ${\Theta }^{+}$ isosinglet is a $K-\pi -N$ molecule. The isodoublet $N^{*}$ is a $K-\overline{K}-N$ molecule. The isotriplet ${\Sigma }^{*}$ is a $\overline{K}-N$ pentaquark. The ${\Xi }_5^{*}$ isoquadruplet is a $\overline{K}-N-\overline{K}$ molecule.