Observation of an Exotic $S=+1$ Baryon in Exclusive Photoproduction from the Deuteron

An exclusive measurement of the reaction $\gamma d \to K^+ K^- p n$ is reported from the CLAS collaboration at Jefferson Lab. A narrow peak, which can be attributed to an exotic baryon with strangeness $S=+1$, is seen in the $K^+n$ invariant mass at $1542\pm 5$ MeV/c$^2$ with a measured width of 21 MeV FWHM that is consistent with the instrumental resolution of the CLAS detector. The statistical significance of the peak is $5.3 \pm 0.5 \sigma$, for a Gaussian peak shape on top of a smooth background. This result is consistent with inclusive measurements of a narrow $S=+1$ baryon reported by other experimental groups.