Limits for the Central Production of ${\Theta }^{+}$ and ${\Xi }^{--}$Pentaquarks in 920-GeV pA Collisions

We have searched for ${\Theta }^{+}$(1540) and ${\Xi }^{--}$(1862) pentaquark candidates in proton-induced reactions on C, Ti, and W targets at midrapidity and $\sqrt{s}=41.6\mathrm{G}\mathrm{e}\mathrm{V}$. In $2\times {10}^8$ inelastic events we find no evidence for narrow ($\sigma \approx 5\mathrm{M}\mathrm{e}\mathrm{V}$) signals in the ${\Theta }^{+}\to p{K}_S^0$ and ${\Xi }^{--}$$\to$${\Xi }^{-}$${\pi }^{-}$ channels; our 95% C.L. upper limits (UL) for the inclusive production cross section times branching fraction $\mathcal{B}d\sigma /dy{|}_{y\approx 0}$ are $(4–16)\mu b/N$ for a ${\Theta }^{+}$ mass between 1521 and 1555 MeV, and $2.5\mu b/N$ for the ${\Xi }^{--}$. The UL of the yield ratio of ${\Theta }^{+}/\Lambda (1520)<(3–12)\%$ is significantly lower than model predictions. Our UL of $\mathcal{B}{\Xi }^{--}/\Xi (1530{)}^0<4\%$ is at variance with the results that have provided the first evidence for the ${\Xi }^{--}$.

Figure 1

Signals of (a) ${\Xi }^{-}\to \Lambda {\pi }^{-}$ and ${\overline{\Xi }}^{+}\to \overline{\Lambda }{\pi }^{+}$ (all targets),and (b) $\Lambda (1520)\to {\mathrm{p}\mathrm{K}}^{-}$ and $\overline{\Lambda }(1520)\to \overline{\mathrm{p}}{\mathrm{K}}^{+}$ (Ctarget).

Figure 2

The ${\mathrm{p}\mathrm{K}}_S^0$ invariant mass distributions: (a) data from the $p+C$ collisions and the background estimate (continuous line);(b) deduced UL(95%) for the $p+C$ inclusive cross section at midrapidity; the dashed lineshows our 95% C.L. sensitivity; (c),(d) same as (a) but requiring (c) a trackmultiplicity of $<10$, and (d) in addition a ${\mathrm{K}}^{-}$ particle in the event. The arrowsmark the masses of 1521, 1530, and 1555 MeV.

Figure 3

The $\Xi \pi$ invariant mass distributions: (a) data from the $p+C$ collisions in indicated neutral and doubly charged channelsand the background estimates (continuous lines); (b)sum of all four $\Xi \pi$ spectra with the background subtracted, and (c) deduced UL(95%)for the $p+C$ inclusive cross section at midrapidity. The dashed lineshows our 95% C.L. sensitivity. The arrows mark the mass of 1862 MeV.