  In an exclusive measurement of the reaction $\gamma d \to K^+ K^- p n$, a
narrow peak that can be attributed to an exotic baryon with strangeness $S=+1$
is seen in the $K^+n$ invariant mass spectrum. The peak is at $1.542\pm 0.005$
GeV/c$^2$ with a measured width of 0.021 GeV/c$^2$ FWHM, which is largely
determined by experimental mass resolution. The statistical significance of the
peak is $5.2 \pm 0.6 \sigma$. The mass and width of the observed peak are
consistent with recent reports of a narrow $S=+1$ baryon by other experimental
groups.
