Evidence for a NarrowS=+1Baryon Resonance in Photoproduction from the Neutron

The $\gamma n\to K^{+}{K}^{-}n$ reaction on ${}^{12}\mathrm{C}$ has been studied by measuring both $K^{+}$ and $K^{-}$ at forward angles. A sharp baryon resonance peak was observed at $1.54\pm 0.01\mathrm{G}\mathrm{e}\mathrm{V}/c^2$ with a width smaller than $25\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ 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 five-quark state ($uudd\overline{s}$) that decays into a $K^{+}$ and a neutron. The resonance is consistent with the lowest member of an antidecuplet of baryons predicted by the chiral soliton model.