Observation of an Exotic Baryon with $S=+1$ in Photoproduction from the Proton

The reaction $\gamma p\to {\pi }^{+}{K}^{-}{K}^{+}n$ was studied at Jefferson Laboratory using a tagged photon beam with an energy range of 3–5.47 GeV. A narrow baryon state with strangeness $S=+1$ and mass $M=1555\pm 10\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ was observed in the $n{K}^{+}$ invariant mass spectrum. The peak’s width is consistent with the CLAS resolution ($\mathrm{F}\mathrm{W}\mathrm{H}\mathrm{M}=26\mathrm{M}\mathrm{e}\mathrm{V}/c^2$), and its statistical significance is $(7.8\pm 1.0)\sigma$. A baryon with positive strangeness has exotic structure and cannot be described in the framework of the naive constituent quark model. The mass of the observed state is consistent with the mass predicted by the chiral soliton model for the ${\Theta }^{+}$ baryon. In addition, the $p{K}^{+}$ invariant mass distribution was analyzed in the reaction $\gamma p\to K^{-}{K}^{+}p$ with high statistics in search of doubly charged exotic baryon states. No resonance structures were found in this spectrum.

Figure 1

The missing mass distributions in the reaction $\gamma p\to {\pi }^{+}{K}^{-}{K}^{+}X$ for the three different runs [runs (a), (b), and (c)] and the combined data spectrum [(d)]. The mass resolutions are 10, 11, and $19\mathrm{M}\mathrm{e}\mathrm{V}/c^2$ for runs (a), (b), and (c), respectively.

Figure 2

The $n{K}^{+}$ invariant mass spectrum in the reaction $\gamma p\to {\pi }^{+}{K}^{-}{K}^{+}(n)$. The neutron was measured from the missing 4-momentum.

Figure 3

The diagram in (a) shows a possible production mechanism for the ${\Theta }^{+}$, which could be a decay product of an intermediate baryon resonance. The three diagrams in (b)–(d) are background processes in the reaction $\gamma p\to {\pi }^{+}{K}^{-}{K}^{+}(n)$. All background processes have a $K^{+}$ going in the forward direction in the center-of-mass system.

Figure 4

The $n{K}^{+}$ invariant mass spectrum in the reaction $\gamma p\to {\pi }^{+}{K}^{-}{K}^{+}(n)$ with the cut $\cos {\theta }_{{\pi }^{+}}^{*}>0.8$ and $\cos {\theta }_{K^{+}}^{*}<0.6$. ${\theta }_{{\pi }^{+}}^{*}$ and ${\theta }_{K^{+}}^{*}$ are the angles between the ${\pi }^{+}$ and $K^{+}$ mesons and photon beam in the center-of-mass system. The background function we used in the fit was obtained from the simulation. The inset shows the $n{K}^{+}$ invariant mass spectrum with only the $\cos {\theta }_{{\pi }^{+}}^{*}>0.8$ cut.

Figure 5

The $n{K}^{+}{K}^{-}$ invariant mass spectrum calculated from the missing mass off the ${\pi }^{+}$ in the reaction $\gamma p\to {\pi }^{+}{K}^{-}{K}^{+}(n)$ with the cuts $\cos {\theta }_{{\pi }^{+}}^{*}>0.8$ and $\cos {\theta }_{K^{+}}^{*}<0.6$. ${\theta }_{{\pi }^{+}}^{*}$ and ${\theta }_{K^{+}}^{*}$ are the angles between the ${\pi }^{+}$ or $K^{+}$ mesons and photon beam in the center-of-mass system. These events have $M(K^{+}n)$ between 1.54 and $1.58\mathrm{G}\mathrm{e}\mathrm{V}/c^2$. The shape of the background curve was obtained from the simulation as discussed in the text. The inset shows the $n{K}^{+}{K}^{-}$ invariant mass spectrum for all other events in Fig. 4.