The anti-decuplet candidate Xi--(1862) as a heptaquark with the overlap of two anti-kaons and a nucleon

Abstract

I study the very recently discovered S=-2 resonance Xi--(1886) by the NA49 collaboration at the CERN SPS. This resonance was already predicted, with a mass close to 1.9 GeV, in a recent publication mostly dedicated to the S=1 resonance Theta+(1540). To confirm this recent prediction, I apply the same standard quark model with a quark-antiquark annihilation constrained by chiral symmetry. This method also explained with success the repulsive hard core of nucleon-nucleon, kaon-nucleon exotic scattering, and the short range attraction present in pion-nucleon and pion-pion non-exotic scattering. I find that repulsion excludes the Xi-- as a dd ss u/bar 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 cancelled 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 anti-decuplet are also studied, and the coupling to p-wave decay channels is briefly addressed.