Evidence for a Narrow $S=+1$ Baryon 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.

Figure 1

(a) Vertex position ($vtz$) for $K^{+}{K}^{-}$ events along the photon-beam direction. Cut points to select the SC events or the ${\mathrm{L}\mathrm{H}}_2$ events are indicated by the arrows. (b) Invariant $K^{-}{K}^{+}$ mass distributions for the SC events (solid histogram) and the ${\mathrm{L}\mathrm{H}}_2$ events (dashed histogram). Cut points to exclude $\varphi$ contributions are indicated by the arrows.

Figure 2

(a) A scatter plot of $M{M}_{\gamma K^{+}}$ vs $M{M}_{\gamma K^{+}{\pi }^{-}}$ for $K^{+}{\pi }^{-}$ photoproductions at the SC. (b) The missing mass, $M{M}_{\gamma K^{+}}^c$, spectra for the $K^{+}{\pi }^{-}$ events from the SC (solid histogram) and for Monte Carlo events for the $\gamma n\to K^{+}{\Sigma }^{-}$ channel (dotted curve) calculated via Eq. (1). The dashed histogram shows the missing mass spectrum without the Fermi-motion correction, $M{M}_{\gamma K^{+}}$ the projection of the events in (a) on to the vertical axis.

Figure 3

(a) The $M{M}_{\gamma K^{+}}^c$ spectrum [Eq. (2)] for $K^{+}{K}^{-}$ productions for the signal sample (solid histogram) and for events from the SC with a proton hit in the SSD (dashed histogram). (b) The $M{M}_{\gamma K^{-}}^c$ spectrum for the signal sample (solid histogram) and for events from the ${\mathrm{L}\mathrm{H}}_2$ (dotted histogram) normalized by a fit in the region above $1.59\mathrm{G}\mathrm{e}\mathrm{V}/c^2$.