Exclusive photoproduction of the cascade (Ξ) hyperons

We report on the first measurement of exclusive ${\Xi }^{-}(1321)$ hyperon photoproduction in $\gamma p\to K^{+}{K}^{+}{\Xi }^{-}$ for $3.2<E_{\gamma }<3.9$ GeV. The final state is identified by the missing mass in $p(\gamma ,K^{+}{K}^{+})X$ measured with the CLAS detector at Jefferson Laboratory. We have detected a significant number of the ground state ${\Xi }^{-}(1321){\frac{1}{2}}^{+}$ and have estimated the total cross section for its production. We also have strong evidence for the first excited state ${\Xi }^{-}(1530){\frac{3}{2}}^{+}$. Photoproduction provides a copious source of Ξ's. We discuss the possibilities of a search for the recently proposed ${\Xi }_5^{-}$ and ${\Xi }_5^{+}$ pentaquarks.

Figure 1

(Color online) The missing mass $m_X$ in the process $p(\gamma ,K^{+}{K}^{+})X$ for (a) the $g6a$ data set, and (b) the g6b data set. The plots have not been corrected for acceptance. The ground state ${\Xi }^{-}(1321){\frac{1}{2}}^{+}$ is clearly seen in both plots; the signal-to-background ratio for the g6a data set exceeds 10:1. The g6b data set shows evidence for the first excited state ${\Xi }^{-}(1530)$.

Figure 2

Consistency of the missing mass of the $K^{+}{K}^{+}$ system. Shown is the centroid of the peak in the $K^{+}{K}^{+}$ missing mass for $\gamma p\to K^{+}{K}^{+}X$ as a function of the incident photon energy. The vertical error represents one-half the bin width of the plot in Fig. 1. The horizontal line is the PDG value of the ${\Xi }^{-}$ mass of 1321.31 MeV.