Evidence for Narrow S=+1 Baryon Resonance in Photo-production from Neutron

The gamma n -> K+ K- n reaction on 12C has been studied by measuring both K+ and K- at forward angles. A sharp baryon resonance peak was observed at 1.54 +- 0.01 GeV with a width smaller than 25 MeV and a Gaussian significance of 4.6 sigma. The strangeness quantum number (S) of the baryon resonance is +1. It can be interpreted as a molecular meson-baryon resonance or alternatively as an exotic 5-quark state (uudd{s_bar}) that decays into a K+ and a neutron. The resonance is consistent with the lowest member of an anti-decuplet of baryons predicted by the chiral soliton model.