Search for Exotic Baryons at HERMES

The prediction of narrow exotic baryon resonances based on the chiral soliton model has triggered an intensive search for the exotic members of an antidecuplet with spin 1/2. In this antidecuplet all three vertices are manifestly exotic. The lightest exotic member lying at its apex, named the $\Theta^+$, is predicted to have a mass near 1530 Mev and a narrow width. Its existence is currently the subject of considerable controversy. Experimental evidence for a second exotic member of the antidecuplet came from the reported observation of a $S$ =-2, $Q$=-2 baryon resonance in proton-proton collisions at $\sqrt{s}$=17.2 GeV. An experimental search for the $\Theta^{+}(1530)$ and $\Xi^{--}(1862) $ resonances has been performed by the HERMES collaboration using the decay modes $pK_s$ and $\Xi^- \pi^-$, respectively. While evidence for a peak at 1530 MeV with a statistical significance of $4\sigma$ is observed that can be interpreted as the $pK_s$ decay of the $\Theta^{+}$, no evidence for the $\Xi^{--}(1862)$ is found. The absence of a peak in the $\Xi^- \pi^-$ spectrum near 1862 MeV allows only an estimate of an upper limit for quasi-real photoproduction of the $\Xi^{--}(1862)$. Systematic studies and future plans will be discussed as well.