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

Abstract

In an exclusive measurement of the reaction $\gamma d\to K^{+}{K}^{-}pn$, 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.

5 pages, 5 figures