Photoproduction of the doubly-strange Ξ hyperons

We report on the first measurement of exclusive Ξ− and Ξ0 photoproduction. The Ξ− states are produced in the reaction γp→K+K+Ξ−, and the Ξ0 states in γp→K+K+π−Ξ0. Identification is made by the unique mass measured as the missing mass of the K+K+ (or K+K+π−) system using the CLAS detector at the Thomas Jefferson National Accelerator Facility. A systematic study of the excited Ξ spectrum improves our understanding of the N∗ and Δ∗ states, since the Ξ∗ states are related to them by SU(3) flavor symmetry. At the highest energies available at Jefferson Lab, we begin to find evidence for known excited Ξ− states in the photoproduction process, and possibly new states at 1770 and 1860 MeV, although we do not have enough statistics to draw a strong conclusion. A search for the Ξ5−−(1862) pentaquark state seen by NA49 is made using the process γp→K+K+π+X, but the result is inconclusive for lack of statistics.