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

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 $1.542\pm 0.005\mathrm{G}\mathrm{e}\mathrm{V}/c^2$ with a measured width of $0.021\mathrm{G}\mathrm{e}\mathrm{V}/c^2$ 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.

Figure 1

Missing mass spectrum for the $\gamma d\to p{K}^{+}{K}^{-}X$ reaction, after timing cuts to identify the charged particles and the coincident photon, which shows a peak at the neutron mass. There is a small, broad background from misidentified particles and other sources. The inset shows the neutron peak with a tighter requirement on the timing between the proton and kaons.

Figure 2

A rescattering diagram that could contribute to the exclusive reaction mechanism that produces the ${\Theta }^{+}$ and an energetic proton through final state interactions. Note that the ${\Theta }^{+}$ is produced independently of the secondary scattering.

Figure 3

Invariant mass of the $K^{+}{K}^{-}$ system (top) and the $p{K}^{-}$ system (bottom) showing peaks at the mass of known resonances. These resonances are removed in the analysis by placing cuts on the peaks shown. Results for the number of counts ($N$), the mass ($M$), and the widths ($\sigma$) from fits are also given.

Figure 4

Invariant mass of the $n{K}^{+}$ system, which has strangeness $S=+1$, showing a sharp peak at the mass of $1.542\mathrm{G}\mathrm{e}\mathrm{V}/c^2$. A fit (solid line) to the peak on top of the smooth background (dashed line) gives a statistical significance of $5.8\sigma$. The dotted curve is the shape of the simulated background. The dash-dotted histogram shows the spectrum of events associated with $\Lambda (1520)$ production.

Figure 5

Spectra of the $(n{K}^{+})$ invariant mass for different cuts: (a) no cut on the $K^{+}$ momentum; (b) tight cut on proton-kaon vertex time. See the text for details.