Even Parity $\Theta$-Pentaquark and Stable Strange Nuclear Matter

The newly discovered exotic $\Theta$ baryon of mass 1540 MeV (and very small width) truly has a very low mass, if it is a pentaquark system of even parity. A schematic model in which the coherent interaction of $u\bar s$ and $d\bar s$ pairs leads to a very large residual (non-confining) attractive interaction is introduced. This collective vibrational model accounts for the mass and small decay width to the $KN$ channel, but yields a significant coupling to the virtual $K^*N$ channel. The schematic model predicts an attractive $\Theta$-nucleon interaction strong enough to bind a $\Theta$ particle to a nucleus in a state that is stable against decay via strong interactions. The discovery of $\Theta$-nuclei could be a definitive proof that the $\Theta$ parity is even.